WO2007033684A1

WO2007033684A1 - Process and furnace for heat treatment of poultry eggshells, and eggshell product

Info

Publication number: WO2007033684A1
Application number: PCT/DK2006/050048
Authority: WO
Inventors: Frederik Møller PEDERSEN; Thomas BØGNER; Keen Nielsen
Original assignee: Sanovo Environmental Solutions A/S
Priority date: 2005-09-26
Filing date: 2006-09-26
Publication date: 2007-03-29
Also published as: EP1928616A1

Abstract

A process for heat treatment of poultry eggshells originating from an in dustrial egg-processing line, in which heat for the heat treatment is gen- erated at least partially by combustion of egg trays. The heat treatment converts at least a part of the calcium carbonate of the eggshells into calcium oxide. The invention also relates to an eggshell product, which may be obtained from said process, and an eggshell heat treatment fur¬ nace, which may be used for carrying out said process.

Description

A process for heat treatment of poultry eggshells, an eggshell product, and an eggshell heat treatment furnace

Background In that part of the industry, in which poultry eggs are used in the production of e.g. human food or possibly animal feed, i.e. the egg- processing industry, there is a great demand for disposal of waste products from the eggs, especially in the form of remaining eggshells and egg trays. Many egg-processing lines handle about 1 million eggs per day.

The eggshell constitutes approximately 9 to 10% of the weight of an egg. At an average weight of 65 g per egg 1 million eggs produce 5.9 to 6.5 tonnes of eggshells per day, which is a considerable amount that many facilities have to dispose of every day. Poultry eggshells mainly consist of calcium carbonate with other trace minerals. Organic material, in the form of a protective membrane, is found on the inside of the eggshell. The membrane is usually more strongly attached to the shell and thus remains with the shell during egg-breaking. The eggshells are often contaminated with faecal bacteria, originating from hen. Faecal bacteria from hen are under suspicion of being the cause of diseases like avian flue, and the eggshells should thus be handled with great precaution. This is a significant problem and represents a danger of disease spreading when the eggshells are handled and transported to other locations.

When stored without further treatment, biological decaying processes start very quickly, i.e. within hours, especially when stored at ambient temperatures or above. The biological decay produces a distinct odour of sulphurous compounds, i.e. "rotten egg". Thus, only a very short storage time can be allowed when there are no measures for cooling the eggshells.

Eggs are most often stored and transported in paper-based egg trays. A paper-based tray holds a plurality of eggs, typically 24, 30 or 36. A paper-based tray of 30 eggs weighs approximately 60 g; 1 million eggs thus produce approximately 2000 kg of egg trays per day (dry basis). Paper-based trays are made of recycled paper, and the paper may have been recycled several times before ending up as egg tray material. As with eggshells, egg trays are often contaminated with faecal bacteria and thus represent an equivalent risk of disease spreading.

In the prior art several methods for the disposal/recycling of eggshells and egg trays from the egg-processing industry have been proposed. One possibility for tray handling is collection without compact- ing and reuse in the original form. This is, however, costly, requires extensive handling and holds disease spreading risks. Presently, egg trays from the egg-processing industry are therefore primarily disposed of by handing them over to a local disposal company leaving the faith of the trays in their hands. In some cases it has been reported that the trays have been reused. However, most often the trays are deposited on a landfill, or incinerated in waste combustion plants together with other general waste materials.

Generally considered these solutions are undesirable. And when potentially contaminated material leaves the facility there exists a poten- tial risk of the egg-processing industry being the source of disease- spreading.

Eggshells give rise to problems similar to the problems related to egg trays.

After being separated from the egg white and egg yolk, white, yolk and membrane remaining on the eggshells are usually removed from the eggshells in order to make the shells more suitable for recycling, see e.g. US 6,176,376. Following this, the eggshells are typically sterilized by means of low temperature heat treatment (up to 150⁰C), after which they may be added to e.g. animal feed, food or medicinal compositions, such heat treatment typically being carried out on the Sa- novo eggshell drying system disclosed in greater detail below. Methods directed to the production of medicinal and food products require that the facilities be approved as drug and/or food producers. Since obtaining such approval imposes the fulfilment of considerable requirements on the facility, none of these methods have widely been adapted in the egg- processing industry.

Currently, only a few methods of handling eggshells from the egg product industry are employed by the industry. In one case a screw conveyor carries the eggshells to a storage facility. From the storage facility the shells are loaded onto a means of transportation, e.g. a lorry. The shells are then driven to a farmer and spread on farmland as a calcium carbonate fertilizer, cf. also WO 2004/105912. This procedure is considered the most common way of disposing eggshells throughout the world. This of cause involves the earlier described potential risks of disease spreading, as the eggshells undergo no kind of bacteria inhibiting or reducing steps. Spreading the shells on farmland is comparably uncomplicated but includes transport costs, which are to be covered by the egg product manufacturer. As a rule, the farmer neither pays for nor receives payment for receiving the shells. As a consequence, this solution is an overall expense for the egg product manufacturer.

WO 2004/105912 A2 discloses a process of calcining eggshells in order to obviate eggshell disposal problems and to alleviate environ- mental concerns. The calcination is carried out at specific temperature and time intervals in order to produce a product with substantial amounts of specific metal oxides, i.e. zinc oxide (350 to 600⁰C), magnesium oxide (600 to 1000⁰C) and calcium oxide a.k.a. quicklime (1000 to 1200⁰C). The eggshell product containing calcium oxide allegedly pos- sesses anti-microbial activity. The product can be used as a food additive, anti-microbial agent and fertilizer. The eggshells are heat-treated in a heating apparatus, i.e. a furnace that may comprise means for moving the eggshells through its length, e.g. a screw or a conveyor. Calcium oxide has a number of other profitable applications known, such as neu- tralization, wastewater treatment and in the manufacture of concrete in the construction industry.

Summary of the invention It is the object of the present invention to provide a solution to the disposal of bulk eggshells and egg trays and optionally other byproducts from the egg-processing industry, which lessens the accumulated environmental strain from the industrial processing of eggs. In order to meet this object the present invention in a first aspect relates to a process for heat treatment of poultry eggshells originating from an industrial egg-processing line, in which heat treatment of the eggshells is carried out at a temperature in the range of 700 to 1200⁰C for a period of time sufficient to convert at least a part of the cal- cium carbonate of the eggshells into calcium oxide; and heat for the heat treatment is generated at least partially by combustion of fuel. According to the present invention this process is characterized in that the fuel comprises paper-based egg trays originating from the industrial egg- processing line and optionally a supplemental fuel. This process of the present invention results in considerable environmental advantages. Eggshells, egg trays and optionally other byproducts are decontaminated by the combustion, thus overcoming the decease-spreading problems associated with faecal bacteria originating from poultry. And both eggshells and egg trays of an incoming batch of eggs are utilised to obtain a product, which is of value and is environmentally desirable. The utilization of the calorific power of the egg trays reduces the overall energy consumption during production of calcium oxide since it reduces the need for combustion fuels such as oil and natural gas. Surprisingly, in many cases the calorific power of an amount of egg trays is sufficient to provide the desired conversion of eggshells from a corresponding amount of eggs in an incoming batch of eggs. In many cases, supplementary fuel such as natural gas is only required as a pilot fuel, i.e. for initiating the combustion of the egg trays. The accumulated environmental strain from the process according to the inven- tion is thus considerably lessened compared to the known processes.

It has surprisingly been found that any mixing of heat-treated eggshells and egg tray combustion residues during the process does not lessen the quality of the resultant eggshell product; for some applica- tions the product is even improved. It is thus not necessary to keep the eggshells and egg trays separated during the heat treatment of the eggshells, and being mixed with egg tray combustion residues does not destroy the eggshell product. Also, the present invention makes it possible to convert eggshells and egg trays into one valuable product such that no solid waste material remains from the processing of eggshells and egg trays.

Furthermore, the need for a heat source powered by expensive fuels with a negative environmental impact, such as fossil fuels, is re- duced or eliminated.

The process according to the invention may easily be implemented in a production line where up to for example 1 million eggs are being processed every day. In the process the eggshells and the egg trays may be processed with approximately the same speed as they are provided from the preceding process steps, which reduces the problem with nuisance from the "rotten eggs" odour. The need for transportation of the eggshells out of the production facility is eliminated when the process is implemented in the production line.

In a preferred embodiment of the process according to the first aspect of the invention the main part of the fuel is in the form of paper- based egg trays. Alternatively, one or more supplementary fuels may constitute a major part of the fuel.

In a particular embodiment of the process according to the first aspect of the invention the eggshells are heat-treated in a high tempera- ture zone of a furnace, and the egg trays are combusted in an egg tray combustion zone. Thereby a shorter retention time of the eggshells is obtained and thus a more continuous flow of the shells in the production line is achieved without any substantial need for a buffer storage tank for the eggshells and/or refrigeration of the stored eggshells in order to avoid the formation of "rotten egg" odour.

In another particular embodiment the process further comprises the step of heating a fluid with excess heat from the resultant eggshell product and/or the combustion, the fluid preferably being used for proc- ess heating, such as pre-heating of the eggshells, pasteurization of the separated egg white and egg yolk and/or egg white and/or egg yolk powder production. In this particular embodiment the heat produced from the fuel is at least partially recycled to the production line reducing the need for an external heat supply in the production line.

In another particular embodiment said supplemental fuel comprises natural gas, wood pallets, wood pellets and/or bio fuels. Most often egg trays are stacked on wood pallets that may be used as a production by-product fuel with advantages similar to using egg trays. In another particular embodiment said eggshells are heat- treated for a period in the range of 5 to 40 minutes, preferably 10 to 20 minutes. A processing time of less than 40 minutes is particularly suitable for the applicability of the process in an egg-processing line handling in the order of 1 million eggs per day without accumulating sub- stantial amounts of eggshells.

In a second aspect the invention provides an eggshell product comprising more than 50% (w/w) calcium oxide originating from poultry eggshells and less than 50 % residues from combusted paper-based egg trays. The eggshell product of the present invention possesses a higher reaction rate for reaction with water than the known eggshell product of calcium oxide. This is at least partly due to residues from combustion of the egg trays mixed with the heat-treated eggshells. The eggshell product according to the invention has a more greyish colour. If the product is used as a substitution for a conventional quicklime product in the manufacture of concrete, the concrete will be given a more greyish colour. This may be interesting for aesthetic purposes.

In a particular embodiment the eggshell product is mixed with water to form a product comprising hydrated calcium oxide. Hydrated calcium oxide is a widely used construction material and is used for ex- ample in concrete.

In a third aspect the invention relates to an eggshell heat treatment furnace comprising: an eggshell heat treatment chamber operating with a high temperature zone within a temperature range of 700 to 1200⁰C in order to convert at least a part of the calcium carbonate of the eggshells into calcium oxide; and an eggshell transporter disposed in said high temperature zone of said furnace. According to the present invention the eggshell heat treatment furnace is characterized by further comprising: an eggshell inlet of said furnace leading to said eggshell transporter, said eggshell transporter conveying eggshells in said high temperature zone while being heat-treated; an egg tray inlet extending to an egg tray transporter disposed in said furnace, said egg tray transporter conveying egg trays in said furnace while being combusted; and at least one heat-treated eggshell and egg tray residue transporter conveying heat-treated eggshells and/or egg tray residue out of said eggshell treatment furnace.

The eggshell heat treatment furnace provides the advantages and effects mentioned above in connection with the first aspect of the invention.

In a preferred embodiment of the eggshell heat treatment furnace according to the invention said eggshell transporter and said egg tray transporter are different transporters. They may also be designed as a common transporter so that eggshells and egg trays are carried on one and the same transporter through the furnace.

The furnace can be designed so that the eggshells enter the furnace from the same side as the egg trays, but preferably the egg trays enter the furnace from one of the other sides of the furnace, such as an opposite side or an adjacent side, as this allows to convey said eggshells and said egg trays in different directions within said furnace.

In a preferred embodiment the eggshell heat treatment furnace forms part of an egg-processing plant comprising an egg-loading apparatus for removing eggs from egg-holding units, an egg-breaking apparatus for receiving eggs from the egg-loading apparatus and for separating egg white and egg yolk from eggshells.

Figures

Examples of embodiments and processes of the invention are described in further detail in the following with reference to the highly schematic drawings, in which

Figures IA to ID are highly schematic illustrations of different embodiments of an eggshell heat treatment furnace according to the in- vention suitable for carrying out a process according to the invention;

Figures IE to IG are front, side and back elevations, respectively, of an embodiment of an eggshell heat treatment furnace according to the invention, suitable for carrying out a process according to the invention; Figure 2 is an illustration of steps performed in an embodiment of an egg-processing plant comprising an eggshell heat treatment furnace according to Figure IE;

Figure 3 is a diagram illustrating a process according to the invention, which can be carried out on an egg-processing plant pertaining to Figure 2; and

Figures 4 and 5 are graphs depicting the reactivity with water of eggshell products according to the present invention.

Detailed description of the invention The egg product industry/egg-processing industry is defined as industry involving industrial poultry egg-breaking and separation of egg yolk and egg white from shells. The term "industrial" refers to processing lines handling from 10⁴ to 10⁷ or more eggs per day.

Production/processing line in the context of the present inven- tion contemplates an egg loader, breaker etc. However, a production/processing facility or plant may comprise one or more production lines. In the context of the present invention the capacity of about one million eggs a day refers to one production/processing line. In this context production and processing may be used interchangeably. Quicklime, burnt chalk, calcium oxide and calcinated calcium carbonate may be used interchangeably. Calcination is the process whereby a solid is heated below its melting point in order to create a state of thermal decomposition or phase transition other than melting. Reactions that may occur while heating a solid to below its melting point include thermal dissociation and thermal recrystallization. Dissociation of calcium carbonate (CaCOs) to calcium oxide (CaO) and carbon dioxide (CO₂) is a reaction that is favoured under high temperature conditions, and is an endothermic process.

By-product/waste product in the context of the present invention refers to the remains when a major part of egg yolk and white has been removed. Thus, by-products/waste products comprise, but are not limited to, eggshells and other egg residues such as yolk and white resi- dues, egg trays, wood pallets, eggs that have been unintentionally broken and the like.

The description of the present invention relates to hen's eggs, but might as well be applied to poultry eggs with any other origin.

Figure IA shows a schematically represented example of an eggshell heat treatment furnace generally designated 12 according to the third aspect of the invention, the furnace being usable for carrying out a process according to the first aspect of the invention. The process according to the first aspect of the invention may produce an eggshell product according to the second aspect of the invention. The furnace 12 of Figure IA comprises an eggshell heat treatment chamber comprising a primary chamber 1 into which fuel in the form of egg trays and optionally supplemental fuels are fed. In this chamber 1 the primary combustion of the fuel takes place. The eggshell heat treatment chamber further comprises a secondary chamber 2 which forms a high-temperature zone into which eggshells to be heat-treated are fed. A secondary combustion of the fuel fed into the first chamber 1 may be performed in the second chamber 2. Furthermore, the second chamber 2 ensures a holding time for the exhaust gas of two seconds at 850⁰C in order to comply with environmental regulations. The eggshell heat treatment furnace 12 comprises a chimney 5 communicating with the second chamber 2 through which the exhaust gas of the combustion exits.

A heating unit 3, e.g. in the form of a natural gas burner, is provided in the secondary chamber or high-temperature zone 2. This second heating unit 3 may serve at least the following purposes: initiating the combustion of the fuel present in the primary chamber 1 when firing up the furnace 12 (i.e. acting as a pilot fuel), raising the tempera- ture of the exhaust gas to the specified regulation temperature (i.e. at least 850⁰C) and ensuring that heat treatment of the eggshells is carried out at the chosen temperature at all times. In another embodiment the heating unit 3 is positioned in the primary chamber. In yet another embodiment no such heating unit is provided. In Figure IB an alternative embodiment of the eggshell heat treatment furnace according to the invention is shown, wherein the eggshell heat treatment furnace 12 comprises a heat exchanger 4, through which the exhaust gas from the heating chamber(s) passes, heating an array of fluidum-filled pipes. Having passed the heat exchanger 4, the at least partially cooled exhaust gas leaves the eggshell heat treatment furnace 12 through the chimney 5.

As shown by means of arrows in Figures IA and IB the eggshells to be heat-treated are fed through an eggshell inlet 2a positioned at the left periphery of the furnace 12 into the secondary chamber 2. The eggshell inlet 2a leads to an eggshell transporter 6 positioned in a high temperature zone, which is coincident with the secondary chamber 2. The eggshell transporter 6 extends to an eggshell outlet 2b from which the heat-treated eggshells are lead out of the furnace 12. The eggshell transporter may be in the form of a moving hearth, a moving grate, a screw conveyor or any other suitable conveyor such as a stationary transporter making use of gravity, such as a chute. The speed at which the eggshells move through the eggshell heat treatment furnace 12 is preferably adjusted to allow complete or at least partial conversion of eggshell calcium carbonate into calcium oxide. As is also shown by arrows in Figures IA and IB the fuel, i.e. egg trays and any supplementary fuel, is fed through an egg tray inlet Ia positioned at a lower position at the left periphery of the furnace 12 into the primary chamber 1. The egg tray inlet Ia leads to an egg tray transporter 6a positioned in the primary chamber 1. The egg tray transporter 6a extends to an egg tray outlet Ib from which the at least partially combusted egg trays are lead out of the furnace 12. The egg tray transporter 6a is preferably in the form of a moving grate, but may be in the form of any other suitable conveyor such as the ones mentioned in the above. The moving grate may be shaped like a flight of steps where each or every other step can move horizontally by means of e.g. a hydraulic piston. The moving action provides transportation of the fuel through the eggshell heat treatment furnace 12, and the combusted ma- terial can be collected at the lower end of the steps, e.g. by means of a screw conveyor (not shown), preferably positioned with its transport direction perpendicular to the transport direction of the egg tray transporter 6a. Alternatively, the egg tray transporter 6a (and/or the eggshell transporter 6) extends out of the furnace 12 for collection outside the furnace 12. The speed at which the steps move, and hence the speed at which the material moves through the eggshell heat treatment furnace 12, is adjusted to allow substantially full combustion of the fuel. The described configuration of the transporters 6 and 6a allows practically continuous operation and inlet and outlet of materials. Depending on the configuration of the eggshell transporter 6, some heat-treated eggshells may fall down onto the egg tray transporter 6a. After exiting the furnace the heat-treated eggshells may be mixed with the egg tray residues.

A third and a fourth embodiment of an eggshell heat treatment furnace according to the invention are shown in Figures 1C and ID, re- spectively. In these embodiments egg tray inlet Ia and egg tray outlet Ib as well as transportation of the fuel are positioned equivalently as in the above-described embodiments.

In the embodiment of Figure 1C eggshell inlet 2a is also provided at an equivalent position. The eggshell transporter 6a is, however, of a configuration, which allows the heat-treated eggshells to fall down onto the egg tray transporter 6a. This may be achieved by means of equivalent transporters as mentioned in connection with the embodiments of Figures IA and IB, the transporters 6a of the present embodi- ments, however, terminating at a point which provides the dropping of heat-treated eggshells onto the egg tray transporter 6a before reaching the right periphery of the furnace 12. Thus, the heat-treated eggshells and the combusted fuel will be mixed on the egg tray transporter 6a be- fore leaving the furnace 12.

By contrast, in the embodiment of Figure ID the eggshell inlet 2a is provided at the right periphery of the furnace 12. Since the fuel inlet Ia is provided at the left periphery of the furnace 12, this has the advantage that more room is available for installing for example the eggshell inlet 2a, transporting tubes or the like for conveying the eggshells from the egg production to the furnace 12 and potential eggshell pre-treatment devices such as an eggshell centrifuge. Further, the longitudinal extent of the eggshell transporter 6 may be shortened, still achieving that the heat-treated eggshells fall down at a point of the egg tray transporter 6a at which the egg trays and potential supplemental fuel has undergone an essentially complete combustion.

Yet other configurations of eggshell heat treatment furnaces may be thought of within the scope of the present invention, the issue however remaining to facilitate transportation of heat from the combus- tion of fuel to eggshells. In case of a desire to avoid any mixture of eggshells and burnt fuel, a partitioning wall may, as an example, separate the primary and secondary chambers. In a presently preferred embodiment no such wall is present since the inventors of the present invention have shown that mixture of eggshells and egg tray residues do not lower the quality of the resultant eggshell product.

Figure IE to IG show three different views of a presently preferred, more concrete embodiment of the eggshell heat treatment furnace according to the third aspect of the invention drawn in scale 1:50. Figure IE shows the furnace 12 shown from the front, Figure IF shows the furnace 12 from the side, and Figure IG shows the furnace from the back. In all three figures, in order to make inside elements of the furnace 12 discernible the part of the casing (or periphery 19) of the furnace 12 facing the observer has been omitted from the drawing. The furnace 12 has, in the illustrated embodiment, a height H of approx. 5.7 m, a length L of approx. 3.2 m and a width W of approx. 2.2 m.

The principle structure of the furnace 12 of this embodiment corresponds to the embodiment illustrated in Figure ID, the furnace of the present embodiment being likewise provided with a primary chamber 1 and a secondary chamber 2. However, furnace 12 is mounted with a heat exchanger principally corresponding to the one illustrated in Figure IB. The heat exchanger is embodied as a steam boiler 4 situated above the second chamber 2 and can produce steam at a pressure of 12 bars and a temperature of 188°C. The output capacity is approx. 800 kW. No dividing walls are provided between the first chamber 1 and the second chamber 2, and neither between the second chamber 2 and the steam boiler 4. The floor of the first chamber 1 provides an egg tray transporter in the form of a moving step grate 6a comprising six steps oscillating in the back-to-front-direction of the furnace 12. The moving step grate 6a communicates with an egg tray inlet Ia in the form of an auger feeder, and with a common egg tray and eggshell outlet Ib, 2b at the bottom of the furnace at a front position of the furnace 12. The last step of the moving step grate 6a drops the resultant eggshell product through the outlet Ib, 2b. The outlet Ib, 2b leads to an eggshell product transporter in the form of an auger conveyor 25 located in a mounting base of the furnace 12 with its axis normal to the back-to-front-direction of the fur- nace 12. The auger conveyor 25 conveys the eggshell product to storage or to further transportation.

The eggshells are loaded onto the hearth, which is in the form of a chute 6 installed in the after-burning chamber. Like in the description of Figure ID an eggshell inlet 2a (in the form of two auger feeders) enters from the opposite periphery of the furnace 12 compared to the egg tray inlet Ia and thus above fourth step of the moving grate 6a counted from the back of the furnace 12. The eggshell inlet 2a drops the eggshells on a radiation arch 20 or on radiation arches 20, and from this position onwards subsequently supplied eggshells push previously supplied eggshells onto the chute 6, the chute 6 being supported on the radiation arch 20 or radiation arches 20. The chute 6 has a slope of approximately 30° allowing the heat-treated eggshells to slide by gravity to the end of the chute 6, the at least partially heat-treated eggshells falling onto the at least partially combusted egg trays in order to be mixed with the egg tray ash before being retrieved. The chute 6 has a flat surface area of approx. 1.54 m². The angle of the chute 6 can be varied in order to accommodate different retention times of the incoming eggshells on the chute 6. A further radiation arch 21 is also provided in the second chamber 2 at the opposite periphery of the furnace 12.

A supplemental fuel can be provided, e.g. in the form of a natural gas burner 3, which is situated so that the flame from the burning gas is directed towards the hearth or chute 6. This allows for direct regu- lation of the temperature of eggshells being transported on the hearth. In this embodiment the secondary chamber 2 also functions as an afterburning chamber as required by environmental regulations, cf. the description of the plant below. The flue gas exits from the steam boiler 4 through chimney 5. The side plates or casing forming the periphery 19 of the furnace 12 are hollow in order to provide a water-cooling system circulating water through tubes 22 provided on the outside of the furnace 12 as is known to the skilled person. Primary combustion air is provided through channels 23 at the bottom sides of the furnace 12, secondary combus- tion air is supplied through tubes 24 secured to the inner sides at the bottom of the furnace 12.

Although it is generally preferred to include a high-temperature zone in which the eggshells are heat-treated and a lower zone of lower temperature in which the egg trays are combusted (as is the case in the above-described embodiments in the form of the secondary chamber), this feature is not necessarily essential in all embodiments within the scope of the invention. As an example, the eggshells and egg trays may be mixed before essentially complete combustion of the egg trays and even before the heat treatment of the eggshells is initiated and/or before combustion of the egg trays is initiated. However, such mixture before combustion or at an early stage of the combustion tends to slow down the calcination process and, as a consequence, the heat treatment tem- perature should be raised and/or the heat treatment period should be extended in order to achieve a level of calcination comparable to the situation in which mixing occurs following combustion of at least 50% of the egg tray material.

In a preferred embodiment the eggshell heat treatment furnace forms part of an egg-processing plant. An embodiment of such an egg- processing plant applying a furnace 12 as described in relation to Figure IE to IG is shown highly schematically in Figure 3. In the embodiment shown the egg-processing plant comprises an egg-loading apparatus 9 communicating with an egg-breaking apparatus 10, a centrifuge 11, and an eggshell heat treatment furnace 12. The furnace 12 is preferably in the form of one of the embodiments of Figures IA to IE. The egg- processing plant of Figure 3 is adapted such as to be able to implement a process according to the first aspect of the invention. The egg- processing plant may comprise one or more conveyor belts, egg-loading apparatuses, egg-breaking apparatuses, mixing apparatuses etc., see below Example 1. Said apparatuses may comprise any means for carrying out the individual process steps as described below; as an example the breaking apparatus may comprise a number of pairs of twin-cups and breaking knives. Also, the egg-processing plant may be supple- mented with one or more separate processing lines, e.g. a processing line for the further processing of egg white and egg yolk. One of the resulting products from the processes carried out at the egg-processing plant may be an eggshell product according to the third aspect of the invention. Details of the above-mentioned different embodiments can be combined into further embodiments, and details of the embodiments can be designed in other manners within the scope of the present invention. Heat treatment of eggshells according to the process of the first aspect of the invention may take place in an eggshell heat treatment furnace embodied according to either of Figures IA to IE.

A presently preferred example of a process line applying a presently preferred embodiment of the process according to the first aspect of the invention carried out on an egg-processing plant as described in the above is described below in Example 1.

In a preferable embodiment the egg trays are size-reduced into smaller pieces, e.g. by cutting, shredding, tearing and/or any other suitable method known to the skilled person, before being combusted. This has a practical reason when the trays are being transported from the egg loader, where the eggs are removed from the tray, to the eggshell heat treatment furnace in which the eggshells are being heat-treated. When size-reduced, the egg trays take up less room and may be transported by a broader spectrum of transporting means such as by means of an air flow like blowing or suction. Therefore, in a particular embodiment the trays are size-reduced into smaller pieces, e.g. approximately having a width of 10 cm and a length of 25 cm, more preferred the width is approximately 5 cm, and most preferred the length is similarly 5 cm. In an embodiment the egg trays are size-reduced to pieces having an average largest dimension in the range from 0.1 cm to 10 cm, preferably in the range from 2 cm to 6 cm. In addition to facilitating transportation of the egg tray material, the smaller size obtained by the size-reduction also increases the surface area of a given amount of egg tray material and thus the ability of the material to combust or incinerate more read- ily. Like the egg trays, the wood pallets may be reduced in size to optimize the process.

In another particular embodiment the heat-treated eggshells are mixed with the at least partly combusted fuel. This mixing operation may be carried out in a furnace 12 as described in the above, or it may be carried out when the said products have left the furnace. The said mixing may fulfil at number of purposes. One purpose is to simplify the process, this being obtained since an eggshell heat treatment furnace needs only comprise a single outlet and since the longitudinal extent of an eggshell transporter within the furnace can be shortened (see above). Another purpose is to obtain a different eggshell product that has properties different from an eggshell product not containing egg tray residues. The eggshell product according to the invention may obtain a greyish colour. If the product is used as a substitution for a conventional quicklime product in the manufacture of concrete, the concrete may also obtain a more greyish colour. The inventors of the present invention have shown that the reactivity of a product comprising both calcinated eggshells and egg tray combustion residues is significantly improved compared to a product of solely calcinated eggshells without egg tray residues. One purpose of said mixing is to avoid having to dispose of egg trays in themselves, used egg trays being a product of little commercial interest. When mixing calcinated eggshells and combusted fuel, the solid egg tray combustion residues form part of a valuable product. The point at the processing line at which the fuel and eggshells are being mixed may depend on the desired thoroughness of mixing. The mixing may be performed using any mixing apparatus known in the art, e.g. either manually or mechanically operated, positioned inside or outside the furnace, and choosing said mixing apparatus is within the skills of the skilled person.

Mixing within the furnace offers a simple embodiment. Mixing in the furnace may simply be performed by letting the heat-treated eggshells fall down upon the combusted fuel or fuel being combusted, as is described with reference to Figures 1C to IE. In an alternative embodiment egg tray ashes are mixed with at least partially untreated eggshells before the untreated eggshells enter the furnace or inside the furnace. Thereby, the mixture of egg tray ashes and untreated eggshells are heat-treated in the furnace.

In yet another presently preferred embodiment the ratio of fuel to eggshells is in the range of 1: 1 to 1:3. The ratio, at which the eggshells are mixed with the fuel, should at least ensure that enough heating energy is provided to obtain the desired end product, e.g. conversion of at least a substantial part of the calcium carbonate into calcium oxide. Furthermore, the fuel should be at least substantially combusted during the retention time in the furnace. It is within the skills of the skilled person to monitor and control the temperature in order to supply sufficient amounts of fuel to the furnace in order to achieve the desired end prod- uct.

According to the invention the fuel to be used in order to heat- treat the eggshells comprises egg trays originating from the industrial processing line. In theory, one tray of approximately 60 grams corresponds to 30 shells of 6 to 6.5 g per shell. This provides a ratio of fuel to eggshells of about 1:3 by weight. Combusting egg trays and eggshells at a ratio corresponding to 1 tray per 30 eggshells ensures that the incoming starting materials are being consumed at the same rate. The calorific value of 1 egg tray provides enough heat for completely calcinating the shells of 30 hen's eggs. In this embodiment no supplemental fuel is re- quired and, if mixed, eggshells and egg trays from the egg-processing are being converted into a single, valuable product.

In an embodiment the fuel comprises a supplemental fuel such as wood pallets, natural gas and/or wood pellets.

A supplemental fuel is wood pellets that can be considered a bio fuel. One advantage in using wood pellets as a supplemental fuel is that the combustion of the pellets produces a residue that may form part of the resulting eggshell product. But due to the very low ash content of wood pellets, an eggshell end product may comprise as little as about 0.10 % (w/w) ashes from the fuel if wood pellets are used as the pri- mary source of fuel.

In a presently preferred embodiment a mixture of egg trays and wood pellets may be used as fuel. This ensures enough heating energy to convert the eggshell calcium carbonate into calcium oxide and at the same time the supplied egg trays are being consumed. The mixture of the egg shell and egg tray starting material for feeding to the furnace may advantageously comprise a ratio w/w of egg tray and egg shell material in the range of 1:0.5 to 1:2, preferably about 1: 1.7 in order to utilize the full calorific energy in the egg trays for heat treatment of the eggshells while taking use of all the egg shell and egg tray materials originating from the egg-processing.

During combustion, the temperature inside the eggshell heat treatment furnace is preferably continuously monitored. The tempera- ture is preferably not to exceed 1200⁰C in order to avoid sintering of the eggshells, similarly the temperature is preferably not to decrease below 700⁰C since below this temperature the calcination process is very slow. From the temperature measurements the control system of the eggshell heat treatment furnace may calculate the necessary amounts of fuel re- quired, and control the operation of individual transport devices from the eggshell and fuel source(s) or storages to ensure the desired mixture. The operation of the transport devices may be controlled either by start/stop action or by frequency inverter adjustable motors on the transport devices or using any means known in the art for operating transport devices.

The combustion process, which can take place in the primary chamber 1 of Figures IA to IE, is in most cases a continuous process, in which eggshells and combustion fuel is continuously supplied to the eggshell heat treatment furnace and combustion products are continuously extracted from the eggshell heat treatment furnace. This ensures a steady and constant excess heat supply to the production facility. The continuous combustion process may take place 24 h per day or less, e.g. intermittent operation during the day, or with a duration of e.g. 20 h per day, depending on local conditions. A discontinuous batch combustion process may in some cases also be relevant, but the remaining description focuses on a continuous process, where the materials are added to the eggshell heat treatment furnace in smaller fractions.

In a particular embodiment the heat treatment of the eggshells is carried out at a temperature within the range of 900⁰C to 1050⁰C, more preferred from 950⁰C to 1000⁰C and most preferred about 975°C. Such temperature intervals form a good compromise between converting calcium carbonate of the eggshells into carbon oxide at a relatively high rate without having to use expensive, very heat-resistant equip- ment (e.g. manufactured in ceramics) or exposing cheaper equipment (e.g. manufactured from steel) to excessive thermal wear.

The period in which the heat treatment of the eggshells is carried out should be as short as possible, however preferably ensuring that a major part of the eggshells have been converted into quicklime. The said period also depends on the temperature at which heat treatment is carried out. Thus, the preferred length of the period depends on how well the eggshells are exposed to heat, e.g. the thickness of the eggshell layer on the eggshell transporter. In a particular embodiment said heat treatment is carried out in a period in the range of 5 to 40 minutes, preferably 30 to 35 minutes (975°C, layer of eggshells 3 to 4 cm). In some embodiments 10 to 20 minutes is preferred (975°C, layer of eggshells about 2 cm). Shorter treatment times may result in large parts of the eggshells not having been calcinated; longer treatment times may result in sintering of a substantial part of the eggshell amounts. The preferred treatment time also depends on the pressure present in the treatment region.

The temperature and the treatment time control the transformation of eggshell calcium carbonate into calcium oxide. The present in- ventors have shown that a temperature of 900⁰C to 1000⁰C for a period in the range of 5 to 40 minutes is sufficient to provide the conversion of about 100% of the calcium carbonate. The degree of conversion may further depend on the size of the eggshells. The smaller the eggshell pieces, the faster the conversion. Other temperature and time combina- tions may be possible, but the temperature should preferably not exceed 1200⁰C, as this will cause sintering and reduced reactivity of the obtained eggshell product. If the temperature is in the range from 1000⁰C to 1200⁰C the heat balance is somewhat unfavourable due to poor utilization of the calorific value, so in a presently most preferred embodi- ment of the invention the temperature in the eggshell heat treatment furnace is in the range of 900⁰C to 1000⁰C and preferably in the range of 950⁰C to 975°C.

Extending the treatment time to more than approximately 40 minutes inevitably increases the residence time in the eggshell heat treatment furnace significantly, a larger furnace being needed in order to process all eggshells from an ordinary egg-processing plant. Similarly, increased treatment time increases the risk of sintering and, hence, re- duced reactivity of the resultant eggshell product. Therefore, in a particular embodiment the shells are being treated for less than 40 minutes, e.g. 20 to 40 minutes, preferably less than 30 minutes, e.g. 20 to 30 minutes.

The processing steps of the invention are preferably performed on-site at the egg-processing facility, preferably as part of operation of the egg-processing plant; however, the processing steps can also be performed elsewhere. The latter can result in the disadvantage of not having potential disease loaded material leaving the production facilities.

An eggshell product comprising at least 50% calcium oxide is obtainable by means of the described process as the ashes from the egg trays in the case of 30 eggs per tray constitute approximately 13% (w/w) of the final product, if combusted completely. Some egg tray material may not be completely combusted, in which case the fuel residue part of the eggshell product can constitute as much as 50% of the resul- tant eggshell product. If the egg trays are supplemented by wood pallets and/or by other sources of fuel, such as wood pellets or bio fuel, the content of ashes may be reduced, as e.g. wood pellet ashes may only constitute about 0.10 percent by weight of an obtained eggshell product if used as the sole fuel. In a preferred embodiment the eggshell product comprises in the range of 5 to 15 %, preferably about 13% (w/w), of ashes originating from the fuel.

In some embodiments more or less egg tray material is used per amount of eggshell material. This depends on the kind of egg trays used and, in general, on how much egg tray material is available for the combustion.

A present the following embodiment is the most preferred. A process line for complete industrial processing of continuously arriving eggs in trays and applying of the process according to the first aspect of the invention is illustrated in Figure 3. An eggshell heat treatment furnace according to the above-described embodiment of Figure IE to IG is used to heat-treat the eggshells and is designed to treat the full output rate of both eggshells and egg trays from the process line. The process line is implemented on the exemplary embodiment of an egg-processing plant described in the above with reference to Figure 2.

As an example of the amounts of eggshells and egg trays processed it can be mentioned that the line - corresponding to a feed of approx. 1 million eggs per day - may result in approx. 2500 kg per day of waste cardboard egg trays and approx. 6500 kg per day of egg shells (wet basis).

Eggs are typically delivered in trays of 30 pieces. The egg trays are delivered on wood pallets. From the pallets the trays are transferred in stacks of 6 to 9 egg trays per lift to a first conveyor belt. From the first conveyor belt the egg tray stacks may be loaded automatically onto a second conveyor belt. From the first or second conveyor belt the egg tray stacks are positioned automatically at so-called egg loaders.

The egg loader has the function of lifting the eggs out of the egg tray and onto an egg conveyor, transporting the individual eggs (step I of Figure 2). Thus, at this point the eggs will be separated from the egg trays. The egg loader operates in three steps. In the first step, one egg tray, including 30 eggs, is lifted from the stack of egg trays. In the same step a suction head grips each individual egg. In the second step, the tray is released while the suction heads continue to hold onto the eggs. From this step the eggs and egg tray are thus separated from each other and the trays continue in a separate line, which will be described below. The egg loader operates automatically. Often a production facility houses more than one egg loader and the loaders then operate in parallel. In the third step (step III of Figure 2) the eggs are loaded onto an egg conveyor belt, where eggs are positioned for feeding to subsequent egg breaking. When the eggs are loaded into the egg-breaking machine, eggs are placed individually in an egg breaking unit which can be associated with an underlying cup capable of receiving the contents of the broken egg and possibly capable of separating yolk from egg white. When the egg is located in the egg-breaking unit the egg is held on the eggshell by two gripping devices at locations on opposite sides of an egg breaking knife. The egg breaking knife causes the eggshell to break in two parts when it hits the eggshell and the knife is activated to open the egg, thus causing the liquid egg components of the egg to be released, and by gravity flow away from the opened shell for further processing. The eggshell membrane and residues from the liquid egg components may remain attached to the shell, cf. step II of Figure 2.

The two eggshell parts are now released from the gripping device and fall by gravity to some sort of transporting system. This may be in the form of a pneumatic vacuum transporting system or a mechanical screw conveyor. In both cases the eggshells will be further reduced in size due to the forces acting on the shells during transportation.

The eggs may also be boiled before separating yolk and white from the shell by pealing the egg.

The separating step (step I of Figure 2) of the process may proceed as outlined above. The eggshells are after being separated from the yolk and white (step II of Figure 2) transported to a means for reducing residual liquid from the shells (step III of Figure 2) in the form of a centrifuge. After centrifugation the moisture content of the eggshells is approx. 17%. Undergoing the centrifugation step the eggshells will be further reduced in size, i.e. to a size of approx. 5 x 5 mm. This may en- sure a higher degree of conversion into quicklime during a set time period. Reducing the liquid content of the eggshells is not necessary, albeit preferred because treatment times may be reduced.

The eggshells are preferably stored in a buffer storage tank in the form of two silos. The total storage capacity is approx. 8 m³ corre- sponding to the amount of eggshells generated during 8 hours. The eggshells are transported from the storage silos into the furnace via cast iron augers. The eggshells may be stored for a shorter period in the silos prior to entering the eggshell heat treatment furnace (step IV of Figure 3). However, without cooling means said storage should preferably not occur over a period of time that will cause "rotten egg" formation. A buffer storage tank is not necessary, albeit preferred because it enables the process to run for example 24 h per day although eggshells are only produced for example 8 h per day.

The egg trays, which were previously separated from the eggs, are delivered in stacks of 1 to 8 trays from the production line. Egg material adhering to the egg trays is removed manually. The egg trays are directed to a shredder by means of a chute or, in case of furnace failure, to a compactor. The shredder can be of type MPS 2100 from TIM Envipro of Denmark. The shredder may be driven by a diesel combustion engine by an electric motor. The shredder may be operated continuously during production on the egg breaking line, i.e. approx. 8 hours per day. They are shredded into smaller parts (step IA in Fig. 2) and are transported to another buffer silo, and from the silo to the eggshell heat treatment furnace in which they are combusted (step V of Figure 2). The shredded tray material may if desired be stored in the silo for later combustion. The pneumatic transport system may comprise a Multiair 2000 system from Kongskilde Maskinfabrik A/S of Denmark. The pneumatic transport system is operated continuously during operation of the shredder, i.e. approx. 8 hours per day. The pneumatic transport system is connected to the shredder by means of a feed inlet funnel, situated beneath the shredder outlet. Combustion of the trays provides heat for heat treatment, i.e. calcination, of the eggshells. Sizing the trays permits use of alternative means of transporting the egg trays, i.e. suction tubes. Similarly, wood pallets used as supplemental fuel may be reduced in size.

Eggshell and egg tray storage silos are situated outdoors. The egg tray silo is insulated to prevent freezing of the wet cardboard material. Both silos may be of the make Justsen Energiteknik A/S of Den- mark. From the eggshell and egg tray silos eggshells and egg trays are fed to the furnace by means of a series of auger conveyors. The furnace, including augers, may be of the make Justsen Energiteknik A/S of Denmark. The natural gas burner in the second chamber of the furnace is used as the sole supplemental fuel (i.e. the sole fuel besides egg trays) in order to achieve an effective combustion and conversion of the eggshell calcium carbonate to calcium oxide and/or dispose of further by- products from the egg-processing. In the second chamber of the furnace the temperature of the smoke gas or flue gas must at all times be maintained at 850⁰C for a minimum of two seconds to ensure complete destruction of e.g. bacteria and viruses. The temperature is ensured by means of the natural gas burner, which is always activated if the tem- perature of the flue gas drops below 850⁰C.

Moist eggshells are generated during an 8 h working day and stored in the silo. From the storage, the eggshells are fed continuously to the hearth and are heat-treated in the furnace. The feed rate of the centrifuged, moist eggshells is approx. 255 kg/h, and the furnace is kept operating for 24h per day. The eggshells are loaded continuously onto the hearth of the furnace, forming a layer on the hearth with varying thicknesses of 1 to 5 cm. With a specific mass of approx. 1000 kg/m³, this results in a mean residence time of the eggshells on the hearth of approx. 10 minutes. The rate of egg tray material fed is maintained at a constant rate of approx. 98 kg/h, corresponding to the full rate of egg trays generated from the full-scale production line. The eggshells are heat-treated on the chute at a temperature, which is monitored and maintained between 900 and 1000⁰C by adjusting the amount of natural gas com- busted in the furnace.

The shredded egg tray material is fed through the egg tray inlet onto the highest grate step of the moving grate of the furnace. During combustion the moving step grate transports the material downwards towards the egg tray residue and heat-treated eggshell transporter lo- cated at the lowermost step at the opposite periphery of the furnace.

The eggshells are loaded onto the hearth or chute and sled on the chute by gravity, falling onto the moving grate. In the furnace the egg tray combustion residues, i.e. ashes, are thus mixed with the egg- shell residues (step VI of Figure 2). It is noted that embodiments in which ashes and eggshell residues are not mixed or are mixed outside the furnace lie within the scope of the present invention. Eggshell products produced according to the invention are normally not intended for human or animal consumption.

From the eggshell heat treatment furnace the residual ash and eggshell product fraction is carried out of the eggshell heat treatment furnace by means of a screw conveyor and are cooled (step VII of Figure 2). Alternatively, other means of out-transport may be used, and making this choice is within the skills of the skilled person. From the eggshell heat treatment furnace the ash/eggshell residue fraction is transported to a storage facility (not shown in Figure 2), preferably by means of a screw conveyor, pneumatic transport or similar. The storage facility may comprise a silo executed in e.g. concrete, fibreglass or stainless steel. The storage facility is preferably water tight and sealed from the surroundings, as the high reactivity of the quicklime will draw moisture from the atmosphere with resulting reduced reactivity. If stored dry the quicklime/ash has an almost indefinite shelf lifetime. The storage facility may be fitted with means for loading the quicklime ash directly onto a tank lorry, or the limestone ash may be packed in e.g. plastic bags or barrels at the egg-processing facility.

Means for reducing the size of the eggshell quicklime particles may be situated either at the inlet or the outlet of the storage tank. Size reduction can be performed to a specified size distribution, depending on marked demands or may even be omitted.

The process further comprises the step of heating a fluid, i.e. water, with excess heat from the combustion process, the heated fluid being used for other process purposes. This is done by means of the steam boiler as is described in the above. The flue gas from the combus- tion in the furnace passes the steam boiler and transfers heat to it. After exiting the boiler the flue gas is cooled further in a so-called economiser, reducing the flue gas temperature to approx. 180⁰C while simultaneously pre-heating the boiler feed water. As a consequence an optimized exploitation of the products and by-products in the egg product industry provides reduced disposal costs, minimized heating expenses and an additional profit of the produced quicklime compared to traditional handling of waste products from the egg product industry. Fluid, i.e. water, heat exchanging with the resulting product and/or the furnace may subsequently be recirculated to the product facility, e.g. for the pasteurisation of the egg yolk and whites. In doing so the cost for heat supply to the egg processing can be reduced. The heated fluid may be used for other purposes, such as preheating of the shells before entering the eggshell heat treatment furnace or for sterilizing eggshells at a lower temperature in a parallel production line. The steam produced in the boiler may also be utilised for preheating of atmospheric air for drying of egg white powder. Also, steam is supplied to the existing steam distribution system at the egg product manufacturer and is used for many purposes, like e.g. cleaning. Further, excessive heat may be supplied to the local district heating grid in return of a fee. The district heating grid has the function of emergency cooling. As an alternative, excess heat may be released to the surroundings.

The plant comprises a flue gas cleaning system. From the economiser the flue gas is directed to a series of flue gas cleaning steps for reduction of the dust concentration and optional chemical cleaning. The gas cleaning consists of a multi cyclone for coarse dust removal followed by a bag filter for reduction of dust concentration below 10 mg/Nm³. Activated carbon and/or lime may be injected in the flue gas prior to the bag filter for chemical removal of dioxins and/or acidic gas- ses. After filtering the flue gas is led to a steel stack of 26 m height and discharged to the atmosphere. The flue gas quality is continuously monitored by measuring the concentration of O₂, CO, NO_x, TOC, H₂O and dust along with values of mass flow and temperature. The measuring point is located after flue gas cleaning and prior to the flue gas entering the steel stack. All measurements are accomplished by means of equipment supplied by CK. Environment A/S of Denmark.

Dust collected in the multi cyclone and bag filter is recovered and mixed with the residues collected from the moving grate in order to produce the ultimate eggshell product. The eggshell product is transported by means of a flexible screw conveyor to a storage silo. The flexible screw conveyor is similar to the type offered by MHJ Agroteknik A/S of Denmark, and the transport distance is approx. 50 m. The storage silo has a volume of approx. 70 m³ with a diameter of approx. 3 m and a height of approx. 11 m. The silo is mounted on a steel structure, allowing a tank truck to drive underneath for loading. Eggshell product falls by gravity and is, by means of a loading bellow, transferred to a tank truck for transportation to a site for use. Such a site may be a construction site where the eggshell product is used for the manufacture of concrete.

The entire process line is controlled and regulated by a central control system equipped with a touch screen panel for visual display and interaction. The control system can be supplied by Industri Tech A/S of Denmark. A separate sub control panel controlling the operation of the shredder and pneumatic transport system is situated at the egg breaking process line.

The following, non-limiting, calculation of heat and mass balances for the furnace demonstrates how heat developed during combus- tion of egg trays can be utilised to calcinate limestone in eggshells, and quantifies the amount, if any, of additional fuel theoretically needed.

For the purposes of the present calculations heat requirements are estimated as follows, but naturally other temperatures can be used at the calcinations, and also the ambient temperature may vary. However for the calculations a single calcination temperature has to be used, and for simplicity it is chosen as 1000⁰C:

- Heating moist eggshells to 100⁰C

- Evaporating moist in eggshells and egg trays at 100⁰C

- Heating dry eggshells from 100⁰C to 1000⁰C - Calcining eggshells at 1000⁰C

- Heating combustion air and smoke gas

The main energy consumption lies in the calcining process, as will be demonstrated below, so the main part of the energy from combustion should be available above 1000⁰C.

Some of the heat is recovered as the calcinated eggshells cool down before leaving the furnace. In this calculation it is assumed that eggshells and ash leaves the furnace at 300⁰C. It is also assumed that steady-state prevails. The amounts of eggshells and egg trays used in the calculations are selected as follows:

Kg/h

Eggshells, 17% moist 256 Eggshells, dry basis 212

Egg trays, 10% moist 250

Egg trays, dry basis 278

This corresponds to the typical values in a 10⁶ eggs per day egg product factory.

The amount of combustion air and the excess air coefficient is for the sake of calculations set to 1,829 Nm³/h (dry basis). A part of the combustion air is preheated to improve the furnace efficiency. Otherwise the temperature of the combustion air and all components entering the furnace is assumed ambient.

The combustion value of egg trays from one producer of egg trays, Hartmann of Denmark, is 17,000 kJ/kg. With the above given hourly amount of egg trays the combustion power input can thus be calculated as 1,181 kW. Under the assumption that only energy above 1000⁰C can be utilised for the calcining process, the adiabatic combustion temperature can be estimated by using the method described in G. Cerbe and HJ. Hoffmann "Einfϋhrung in die Thermodynamic, 10th edition, chapter 9. Hereby the adiabatic combustion temperature is determined to 1,396°C, from which it is expected that 90% can be utilised. The adiabatic combustion temperature is calculated in a similar manner for natural gas, which in this example is used as supplemental fuel. Thus, the adiabatic combustion temperature for natural gas is 1,570⁰C from which it is also expected to utilise 90%.

Calculations of heat consumed are based on the following values:

Specific heat capacities of substances Air 1.007 kJ/kg

CO₂ 49.5 kJ/kg

Smoke gas from grate 1.62 kJ/kg

Smoke gas from natural gas 1.7 kJ/kg

Smoke gas from furnace 1.51 kJ/kg Trays (paper) 1.2 kJ/kg

Natural gas (methane) 2.1 kJ/kg

Water vapour 2.1 kJ/kg

CaCO₃, 5 - 120⁰C 88.16 kJ/kg

CaCO₃, 5 - 100O⁰C 114.20 kJ/kg

The chemical reaction of calcination is as follows:

CaCO₃ + heat → CaO + CO₂

The reaction enthalpy is calculated form the following values: Enthalpy of formation

CaCO₃ (s) -1,206.9 kJ/mol CaO (s) -635.09 kJ/mol CO₂ (g) -393.51 kJ/mol

Thus, the enthalpy of reaction can be calculated as

-635.09 - 393.51 - (-1,206.9) = 178.3 kJ/mol or 105.15 kW

It is assumed that 100% of the limestone in the eggshells is cal- cinated. Since the amount of limestone in the egg trays is not quantified this is not included in the calculation.

It is assumed that wet eggshells at ambient temperature are loaded onto the hearth. Thus aside from the entropy for reaction also the entropy for heating the moist eggshells and evaporating the moisture has to be supplied to the hearth at more than 1000⁰C. The total required amount of heat on the hearth is therefore:

Heating 212 kg/h dry CaCO₃: 67.01 kW

Heating and evaporating 44 kg/h water: 55.49 kW

Calcining 212 kg/h CaCO₃: 105.15 kW

Total energy demand at >1000°C: 227.65 kW

From the adiabatic combustion temperature it is calculated, that of the total 1,181 kW available from combustion of the trays 227.29 kW is available over 1000⁰C. Thus, the remaining 0.36 kW is supplied via combustion of supplemental fuel. This can be achieved by combustion of approx. 0.1 m³/h natural gas, consuming 2 Nm³/h of combustion air. The emitted smoke gas can be summarised as follows:

Emitted smoke gas

From combusted egg trays 1,971 Nm³/h

Moist from eggshells 52 Nm³/h CO2 from eggshells 47 Nm³/h

Moist from egg trays 35 Nm³/h

From combusted natural gas 2 Nm³Zh

Total 2,107 NmVh

The overall combustion plant heat balance is as follows:

Thermal input

Egg trays 1,181 kW

Natural gas 1 kW

Total fuel 1,182 kW Eggshells -228 kW

Remaining thermal input 954 kW

Thermal output as steam production

Boiler Efficiency 0.80 Steam output 763 kW

With the above example it is thus demonstrated, that the heat developed during combustion of egg trays can be used to calcinate CaCU₃ in eggshells. In the above example the heat is essentially sufficient for complete calcination in a typical ratio between eggshells and egg trays and only a marginal amount of supplemental fuel is required. It also appears that a slight increase in furnace efficiency or the use of egg shells that are slightly less wet that estimated in the calculations al- lows the process to run continuously without use of supplemental fuel. It is clear to persons skilled in the art that changing the ratio between egg trays and eggshells will influence the amount of supportive fuel necessary.

Also, the example demonstrates, that a significant amount of excess heat is generated which can be used for e.g. steam or hot water production. Again persons skilled in the art will recognize that the ratio between eggshells and egg trays will influence the amount of excess heat produced in the combustion.

The following description is of non-limiting examples where the following common procedure is applied:

A number of eggshell samples were heat-treated using an electric furnace as the heat source. The furnace was a CSF 1200 from Car- bolite Furnaces. The thermostat on the oven was set to 1000⁰C. The temperature was measured with a digital thermometer and recorded regularly, showing a resulting treatment temperature varying between approx. 960⁰C to 1010⁰C. Each sample was heated for 30 minutes to ensure complete calcination. The samples were heated on a steel tray. The samples were placed on the tray, which was then inserted into the preheated oven. After the reaction time of 30 minutes the tray including the sample was removed from the oven and allowed to cool off before weighing. The steel tray had a capacity of 150 g of eggshells and larger samples were thus treated in fractions of 150 g.

After weighing, similar fractions were mixed and grinded in a mortar. The quality of the quicklime product was tested for reactivity with water. The test was performed according to European and Danish Standard DS/EN 459-2, a standard procedure for determining slaking curves for quicklime.

Quicklime is very reactive with water while developing heat, and the heat generation can be measured as the temperature increase in water with added quicklime.

According to the standard procedure 150.0 g burnt lime from eggshells was added to 600 g of water in a capped DEWAR container equipped with a mechanic stirrer and a digital thermometer. Immedi- ately after the burnt lime was added to the water, the time measurement was started, and after 30 seconds the temperature of the mixture was registered and again every minute up until ten minutes. After ten minutes the temperature was registered every two minutes.

From the recorded data the temperature is represented graphi- cally as a function of slacking time in Figure 4. From the resulting curve the time, which had elapsed from the time measurement was started until a temperature of 60⁰C was reached, was determined. This time is known as T₆₀ and is the basis of comparing quicklime qualities; the smaller the T₆₀, the better the quality.

Example 1: Heat treatment of eggshells with external heat source.

Eggshells dried at approx. 200⁰C were used for the experiment. A total of 750 g of eggshells were heat-treated in five fractions of 150 g according to the above-described procedure.

The grinded product was homogeneous bright white with a few darker particles. The dark particles are assumed to originate from the steel tray. The total weight after heat treatment is 313.5 g resulting in a yield of approx. 47.7%. This is less than the theoretical 56% at full calcination and is probably due to a relatively large content of organic material in the dried eggshells used for the experiment.

The slacking curve for the product was determined twice for the resultant heat-treated eggshell product according to the above- mentioned procedure. The results are presented in Table 1 and the temperature is shown as a function of reaction time in Figure 4.

From the results T60 values of 2 minutes and 2 minutes 6 seconds are determined. It appears from Figure 4 that the slacking curves are essentially similar as is the T60 values.

Example 2: Heat treatment of eggshells partially using egg travs as the heat source.

Egg trays were placed in the furnace underneath the eggshells in order to at least partially produce the heat for calcination by combustion of the egg trays. Full-scale egg trays and eggshells were treated approximately in a ratio corresponding to the amounts generated from the production line, i.e. 1 tray per shells from 30 eggs.

The ratio was therefore calculated as follows:

Egg trays 2338 kg/day Eggshells 4070 kg/day* *Dry eggshells.

This corresponds to a ratio by weight of:

Egg trays 1 Eggshells 1.74

This ratio was also maintained in the experiment, and thus a total of 391 g of eggshells and 224.1 g of egg trays were treated in this experiment, following the earlier outlined procedures.

The two materials were kept separate throughout treatment, and slacking was done with the combusted eggshells only. The weight of the eggshells after combustion was 166 g resulting in a yield of approx. 42.5%. This is again less than the theoretical 56% at full calcination.

The slacking curve was determined according to the above- mentioned procedure. The results are presented in Table 2, and the temperature is shown as a function of reaction time in Figure 4.

From the results a T60 value of 2 minutes and 21 seconds was determined. As can be seen in Figure 4, the slacking curve and the T60 value is essentially identical to the slacking curves and T60 values de- termined when not using egg trays as fuel. Thus, it has been shown that using egg trays as fuel for calcination of eggshells does not significantly affect the slacking characteristics of the quicklime product.

Example 3: Combustion of eggshells mixed with egg tray ashes. Eggshells and egg tray ashes were mixed and treated together.

First, a specific amount of egg trays were ignited and combusted separately outside the furnace in a construction normally used to fire up charcoal for a garden grill. Approx. 14.4% of the egg trays remained in the form of ashes. In order to maintain the earlier calculated ratio between egg trays and eggshells 500 g of eggshells were mixed thoroughly with 41.36 g combusted egg trays. A total of 455.1g of this mixture was heat-treated according to the above procedure. The weight of the mixture after heat treatment was 241.1 g resulting in a yield of approx. 53%. The yield in this experiment is believed to be higher due to the additional content of egg tray ashes.

The slacking curve was determined according to the above- mentioned procedure. The results are presented in Table 3, and the temperature is shown as a function of reaction time.

From the results a T60 value of 21 seconds was determined. As can be seen in Figure 4 the slacking curve is surprisingly steeper when egg trays are mixed with the eggshells. As a consequence the T60 value is also significantly lower than for the product produced from eggshells alone. Thus, a more reactive and thus better quicklime quality was achieved when combusting and mixing eggshells and egg trays.

Without the whish to be bound by any specific theory, minerals in the egg tray ashes are believed to improve the hydrophilic nature of the quicklime, thus allowing the water to enter more quickly into the material and provide a faster chemical reaction.

The above described improved reactivity, by mixing egg trays and eggshells, is confirmed by another set of experiments in which the reactivity of quicklime produced from eggshells only was compared with quicklime produced from a mixture of eggshells and egg trays as described above. However, in these experiments the eggshells were heated to 110⁰C for 24 h before calcination to combust any organic material still attached to the shells. Also calcination was performed at approx. 900⁰C instead of at approx. 1000⁰C. The yield was calculated to 53% after calcination of eggshells alone, which is close to the theoretical value of approx. 55%. Assuming that the ash from the egg trays was fully combusted prior to calcination, a yield of 53% could also be calculated for the mixture of eggshells and egg trays.

The slacking curve was determined in the samples according to the above-mentioned procedure. The results are presented in Table 4, and the temperature is shown as a function of reaction time Figure 4.

From the results T60 values of 8 minutes 42 seconds and 1 minute 55 seconds were determined. This was significantly poorer than in the above-mentioned experiments, but nevertheless confirms the im- proved reactivity with egg trays added to the product. The slower reactivity may be caused by a number of reasons, e.g. the lower calcination temperature, poorer grinding, or the fact that the samples were stored under unfavourable conditions for several weeks before the slacking test was performed. However, the generally poorer reactivity in the later experiment is not believed to be representative.

Example 4: Combustion of eggshells and egg travs separately and mixing the combustion products

Eggshells and egg trays are combusted independently, cooled of, grinded and mixed in a specific ratio. The slacking curve is determined for the mixture.

Eggshells are combusted at 1000⁰C for 30 minutes in the electrical furnace as described above. Egg trays are ignited and combusted in the above described contraption and the ash is similar to the above description. The combustion residues are grinded thoroughly in a mortar.

For determination of the slacking curve 150 g of mixture is produced by mixing thoroughly 19.3 g of egg tray ash with 130.7 g of combusted eggshells. This ratio corresponds to 14.4% ash from 2338 kg of egg trays and 56% ash from 4070 kg of eggshells. The ash content of the eggshells is the theoretical value if all eggshell limestone is calcinated.

The slacking curve is shown in figure 4 from which the T60 can be determined to 1 minute 24 seconds. The experiment is carried out under the same conditions as the previous experiments shown in figure 4.

Thus is it shown, that T60 is improved when compared to the pure eggshell product, however, the improvement is lesser than when eggshells and egg trays are mixed during combustion. However, the egg trays are combusted under highly uncontrolled condition and if combusted under more controlled condition as e.g. in the furnace in example 1, a further improvement is expected.

From the experiments it is concluded that that a quicklime product can be produced from eggshells, with reactivity, expressed as T60, at least comparable with the commercial reference. Mixing the eggshell based quicklime with ashes from combusted cardboard egg trays generally improves the reactivity. However, if mixed during combustion the improvement is more significant.

The fuel used for the heat treatment of the eggshells comprises paper-based egg trays, and the eggshell product may contain residues from the combustion of the egg trays. Other fuels than egg trays can be utilized, such as wood pellets or a bio fuel of another kind, e.g. straw. Bio fuel includes organic matter, and the advantageous product properties obtained by including residues from combustion of egg trays in the product may also be obtained by including in the product residues from combustion of said bio fuel as a supplement to or substitute for residues from the combustion of the egg trays.

Claims

P A T E N T C L A I M S

1. A process for heat treatment of poultry eggshells originating from an industrial egg-processing line, in which heat treatment of the eggshells is carried out at a temperature in the range of 700 to 1200⁰C for a period of time sufficient to convert at least a part of the calcium carbonate of the eggshells into calcium oxide; and heat for the heat treatment is generated at least partially by combustion of fuel, c h a r a c t e r i z e d in that the fuel comprises paper-based egg trays originating from the industrial egg-processing line and optionally a supplemental fuel.

2. A process according to claim 1, wherein the main part of the cumulative calorific value of the fuel is provided from paper-based egg trays.

3. A process according to claim 1 or 2, wherein the eggshells are heat-treated in a high temperature zone of a furnace, and the egg trays are combusted in an egg tray combustion zone.

4. A process according to claim 3, wherein the eggshells enter said furnace from one side of said furnace, and said egg trays enter said furnace from an opposite side or an adjacent side of said furnace such as to convey said eggshells and said egg trays in different directions in said furnace.

5. A process according to any of the previous claims, wherein the process comprises the step of mixing residues of the combusted egg trays with the heat-treated eggshells to form an eggshell product.

6. A process according to claim 5, wherein the eggshells and residues of the combusted egg trays are mixed during the heat treat- ment of the eggshells.

7. A process according to any of the preceding claims, wherein the egg trays before combustion have been size-reduced into smaller pieces, preferably approximately less than 10 cm in width and less than 25 cm in length, and more preferably so that the egg trays are size- reduced to pieces having an average largest dimension in the range from 2 cm to 6 cm.

8. A process according to any of the preceding claims, wherein the ratio by weight of fuel to eggshells is in the range of 1:1 to 1:3.

9. A process according to any of the preceding claims, wherein said supplemental fuel comprises natural gas, wood pallets, wood pellets and/or bio fuel.

10. A process according to any of the preceding claims, wherein the heat treatment of the eggshells is carried out at a temperature in the range of 900-1050⁰C, more preferably 950-1000⁰C, most preferred about 975°C.

11. A process according to any of the preceding claims, wherein said eggshells are heat-treated for a period in the range of 5 to 40 minutes, preferably 10 to 20 minutes.

12. A process according to any of the preceding claims, wherein it comprises the step of separating egg yolk and egg white from the eggshells before heat treatment of the eggshells.

13. A process according to any of the preceding claims further comprising the step of heating a fluid with excess heat from the resul- tant eggshell product and/or from the combustion of the fuel, the fluid preferably being used for process heating, such as pre-heating of the eggshells, pasteurization of the separated egg white and egg yolk and/or egg white and/or egg yolk powder production.

14. A process according to any of the preceding claims, wherein prior to the heat treatment the liquid content of eggshells is reduced, preferably by centrifugation.

15. A process according to any of the preceding claims, wherein the eggshells prior to the heat treatment are stored in a buffer storage tank.

16. A process according to any of the preceding claims, wherein the resultant eggshell product is mixed with water to form a product comprising hydrated calcium oxide.

17. An eggshell product comprising more than 50% (w/w) cal- cium oxide originating from poultry eggshells and in the range of 0.10 to 25% (w/w) residues from combusted paper-based egg trays or bio fuel, and optionally a supplemental fuel.

18. An eggshell heat treatment furnace comprising: an eggshell heat treatment chamber operating within a temperature range of 700 to 1200⁰C in a high temperature zone in order to convert at least a part of the calcium carbonate of the eggshells into calcium oxide; and an eggshell transporter disposed in said high temperature zone of said furnace; said eggshell heat treatment furnace being c h a r a c t e r i z e d by further comprising: an eggshell inlet of said furnace leading to said eggshell transporter, said eggshell transporter conveying eggshells in said high tem- perature zone while being heat-treated; an egg tray inlet extending to an egg tray transporter in said furnace, said egg tray transporter conveying egg trays or in said furnace while being combusted; and at least one heat-treated eggshell and egg tray residue trans- porter conveying heat-treated eggshells and/or egg tray residue out of said eggshell heat treatment furnace.

19. An eggshell heat treatment furnace according to claim 18, wherein said eggshell transporter and said egg tray transporter are different transporters.

20. An eggshell heat treatment furnace according to claim 19, wherein said eggshell inlet enters said furnace from one side of said furnace, and said egg tray inlet enters said furnace from another side, such as an opposite side of said furnace, such as to convey said eggshells and said egg trays in substantially opposite directions in said furnace.

21. An eggshell heat treatment furnace according to claim 19 or

20, wherein said eggshell transporter is in the form of an eggshell hearth, e.g. an eggshell chute, and said egg tray transporter is in the form of an egg tray moving grate or the like.

22. An eggshell heat treatment furnace according to any of claims 19 to 21, wherein the heat-treated eggshell transporter and the egg tray transporter are converging such as to mix heat-treated eggshells and egg tray combustion residues, e.g. to form an eggshell prod- uct according to claim 12 or 13.

23. An eggshell heat treatment furnace according to claim 22, wherein the at least one heat-treated eggshell and egg tray residue transporter is separate from said eggshell transporter and said egg tray transporter, e.g. in the form of a screw conveyor.

24. An eggshell heat treatment furnace according to any of claims 19 to 23 further comprising a device for combustion of a supplemental fuel, e.g. a natural gas burner.

25. An eggshell heat treatment furnace according to any of claims 19 to 24 forming part of an egg-processing plant comprising an egg-loading apparatus for removing eggs from egg-holding units, an egg-breaking apparatus for receiving eggs from the egg-loading apparatus and for separating egg white and egg yolk from eggshells.

26. An eggshell heat treatment furnace according to claim 25, wherein said plant further comprises a centrifuge for removing at least part of the remaining liquid components of the eggs.