GB2167542A - Furnace for agricultural waste - Google Patents

Abstract

There is disclosed a furnace (10) for burning relatively bulky and compressed material, such as bales of straw. The furnace comprises a chamber (11) through which the material passes and having an inlet (12) at one end and a combustion space at its opposite end, restraining bars (14) arranged at the opposite end of the chamber (11) to be engaged by the bales, a pusher (15) for moving the bales through the chamber (11) so as to press continuously against the restraining bars (14) whereby combustion takes place while the chamber is substantially filled with material, first nozzles (21) arranged to blow one or more jets of primary combustion air into a lower portion of an end face (26) of the material, and a baffle (22) arranged across an upper part of the combustion space so as to cause partial recirculation of burning and burnt gas. <IMAGE>

Description

SPECIFICATION Furnace This invention relates to a furnace.

The invention has been developed primarily, though not exclusively, with a view to providing a furnace for controlled rate continuous combustion of waste product, and especially agricultural waste product. A major agricultural waste product, as a by-product in cereal growing, is straw which is normally burnt in the field or used as bedding for cattle, pigs and horses. However, straw-burning in the field is becoming more restricted owing to harmful consequences.

Straw is usually tightly packed into bales, in order to facilitate storage and handling, but baling is expensive, and farmers therefore require there to be an economic purpose for such an expense.

Some farmers use straw burners to dispose of some part of their straw production, such burners providing a source of heat for domestic use or for keeping livestock warm in winter. Bearing in mind that straw bales are relatively densely packed, existing designs of straw burners, which work on the batch firing principle, are not able to provide complete combustion of the bales. This results in emission of soot and only partly burnt "volatiles", which are often tarry and which tend to adhere to the cooler parts of the furnace wall, the flue and other relatively cool regions. Furthermore, the incomplete combustion of straw is damaging to the environment and gives rise to emission of soot, especially soon after a new charge of bales has been inserted.Also, incomplete combustion of straw bales results in an undesirable solid residue which is (a) not completely burnt, (b) cannot readily be discharged from the furnace, and (c) is not very useful for distribution on the land as a fertiliser.

Also, bearing in mind that the amount of straw currently burnt on the fields per annum in Great Britain is believed to be equivalent to about four million tons of coal, straw could be a very valuable fuel source if a means could be found for efficient combustion of straw.

Therefore, there exists a clear need to provide a design of furnace which is able to consume, by partial or complete combustion, agricultural waste and especially straw more efficiently than has hitherto been possible.

According to one aspect of the the invention there is provided a furnace for burning relatively bulky and compressed material, such as bales of agricultural waste product or straw, the furnace comprising: a chamber through which the material is to pass and having an inlet at one end and a combustion space at its opposite end; restraining means arranged at the opposite end of the chamber to be engaged by the compressed material following passage of the material along the chamber to the combustion space, the restraining means sub-dividing the combustion space into a primary combustion space which faces the inlet and downstream thereof a secondary combustion space; means for moving the compressed material through the chamber so as to press continuously against the restraining means whereby combustion can take place while the chamber is substantially filled with material;; first nozzles arranged to blow one or more jets of primary combustion air into a lower portion of an end face of the material engaging with the restraining means; and a baffle arranged across an upper part of the combustion space so as to cause partial re-circulation of burning and burnt gas to the primary combustion zone and to allow combustion products to be discharged from the combustion space.

In an alternative mode of operation, the first nozzles may be arranged to blow one or more jets of a steamloxygen mixture or a steam/air mixture into a lower portion of the end face of the material, when it is desired to generate water gas, which may be used subsequently in ammonia synthesis.

Preferably, second nozzles are arranged to introduce secondary air into the secondary combustion zone to complete combustion of gaseous material which passes thereto from the primary combustion zone.

However, in an alternative embodiment, in which the second nozzles are absent, the furnace acts as a gasifier producing a gas which can be burnt further along.

The restraining means may comprise restraining bars arranged substantially vertically on the side walls of the chamber.

Conveniently, the through-flow chamber is generally uniform in cross-section, and provides a substantially horizontal feed path for the material from the inlet to the combustion space adjacent to the restraining bars. In such event, means for moving the material is a pushing means in the form of a simple reciprocating pusher arrangement at the inlet end of the chamber. Preferably, the pusher arrangement provides a substantially sealed closure of the inlet end when it is not feeding material.

This will resist any tendency for gaseous product to migrate from the combustion space towards the inlet end of the chamber.

While it is preferred that the chamber provides a substantially horizontal feed path, this is not essential to the invention. By way of example only, the inlet end of the chamber may be arranged higher than the combustion space, whereby movement of the material fed to the inlet towards the combustion space at the outlet end is assisted by gravity.

By virtue of the arrangement whereby hot exhaust gases which emerge from the primary combustion zone via the outlet mix with the incoming primary combustion air, this provides pre-heating of the combustion air and also further promotes combustion of any products present in the exhaust gases, which are not fully burnt. The mixture of recirculating exhaust gases and hot incoming combustion air provides a powerful combustion action within the combustion space which is able to consume even densely packed agricultural waste. The extent of the mixing of the gases may be con trolled by varying the distance of the nozzles from the combustion face of the material A furnace according to the invention is particularly suitable for burning straw when the latter is supplied thereto in tightly compressed bales.When the furnace is designed for such use, preferably the through-flow chamber will have a cross-section which is only slightly larger than that of a typical straw bale, which will be fed lengthwise into the chamber. Conveniently, the length of the chamber, from the inlet to the outlet, will be more than twice a typical straw bale length, whereby two or more bales can be accommodated in the chamber at any one time. Evidently, as one bale is fed forward while combustion takes place within the combustion space, a further bale must be introduced at the inlet end which will cause the adjacent, and so far substantially unburnt bale to advance to the combustion space.

In addition to providing very efficient combustion of agricultural waste product, and especially straw, a furnace according to the invention also provides a very useful source of heat which may be used as required on a farm, or in a works situated within an economic transport distance from a number of farms. A main use of the furnace will be for water heating and steam raising all the year round with stored bales. However, bearing in mind that plentiful supplies of straw will be available at the time of cereal harvesting, the heat generated by the furnace, when burning the straw gathered at the same time can be used to supply heat to a grain drier. The heat may be extracted from the hot exhaust gases andlor the combustion space by the use of any suitable heat exchangers.

According to a further aspect of the invention there is provided a furnace for burning relatively bulky and compressed material, such as bales of agricultural waste or straw, the furnace comprising: a chamber through which the material is to pass and having an inlet at one end and a combustion space at its opposite end; restraining means at the opposite end of the chamber to be engaged by the material; means for moving the material through the chamber so as to press continuously against the restraining means so that combustion can take place while the chamber is substantially filled with material; and means for blowing one or more jets of primary combustion air, or a steam/oxygen mixture, or a steam/air mixture, into the combustion space and into contact with a lower portion of the end face of the material restrained by the restraining means, whereby a reaction takes place at the point of impact, and the gases leave through said end face along generally horseshoe shaped paths.

One embodiment of furnace according to the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a side view, in longitudinal section, of a furnace for burning straw; Figure 2 is a rear end view of the furnace shown in Figure 1; Figure 3 is a plan view of the furnace shown in Figures 1 and 2; Figure 4 is a schematic view of an alternative construction of furnace, illustrating in more detail the manner by which combustion is promoted; and Figure 5 is a schematic view of a modification of the construction shown in Figure 4.

Referring now to Figures 1 to 3 of the drawings, there is shown a small furnace for use on a farm for burning baled agricultural waste product, and especially straw. The furnace will usually be fixed to a boiler or air heater.

The furnace is designated generally by reference 10 and comprises a through-flow chamber 11 having an inlet 12 at one end for incoming material to be burnt and an outlet 13 at an opposite end for discharging burnt residue. The through-flow chamber 11 provides a substantially horizontal feed path for material from the inlet 12 towards an abutment-restraint provided adjacent the outlet end of the chamber, and taking the form of a pair of vertical restraining bars 14, on the side walls of the chamber. Means is provided for feeding the material towards the bars 14, which comprises a reciprocating pusher arrangement 15 at the inlet end of the chamber 11.

The pusher arrangement 15 is shown only schematically in Figure 1, and may be manually or motor-operated as required, or driven by a falling weight and pulleys. Support legs 16 are provided at the inlet end of the furnace.

The restraining bars 14 serve the purpose of restraining the unburnt material, and also define a primary combustion zone in the chamber 11 on the side of the bars facing the fuel inlet. The furnace 10 is primarily designed for combustion of straw bales, and therefore has a cross-section which is only slightly greater than a typical cross-section of a lengthwise-fed straw bale.

Nozzles (not shown) are arranged downstream of the bars 14 and serve to direct a supply of combustion air to the end (combustion) face of a straw bale restrained by the bars 14. The manner by which combustion is promoted in the combustion space will be described in more detail below with reference to Figure 4.

A primary combustion zone 26 is defined on one side of the bars 14 which face the inlet 12, and a secondary chamber 20 is defined on the other side of the bars 14 to the primary combustion zone 26, and forms a secondary combustion zone in which combustion is completed of gaseous product from the primary zone 26. A first nozzle arrangement 21 is provided for blowing a stream of primary combustion air into the chamber 20, through the inlet (defined between bars 14) and into the primary combustion zone 26. A baffle 22 is provided in the chamber 20 and is arranged in the path of gases emitted from the primary combustion zone between the bars 14, the baffle 22 serving to direct such hot exhaust gases so as to mix with the stream of primary combustion air supplied by nozzle arrangement 21, and to enter the primary com bustion zone with the combustion air.A second set of nozzles 28 is arranged to introduce secondary air into the chamber 20 in order to complete the combustion of only partly consumed gaseous product from the primary zone 26.

Referring now to Figure 4 of the drawings, there is shown in more detail the manner by which combustion is promoted in the furnace. The furnace shown in Figure 4 is generally similar to that shown in Figures 1 to 3, and corresponding parts are designated by the same reference numerals.

However, in regard to the material feed arrangement at the inlet end 12, it will be noted that the pusher arrangement 15 has a peripheral seal 24 which serves to prevent the escape of exhaust gases via the inlet end of the furnace. By way of example only, the furnace of Figure 4 is shown being used to burn baled agricultural waste, illustrated schematically by reference 25.

The items of agricultural waste 25 are pressed consecutively towards the combustion zone 26 adjacent to the bars 14, where unburnt or partly burnt waste is restrained while combustion takes place. The nozzle arrangement 21 blows a stream of combustion air 27 into the chamber 20, through the inlet provided by the bars 14 and into the combustion zone 26. The combustion which takes place in the primary zone 26 tends to form a cavity in the waste material 25 along the curved path 27a. As previously mentioned with reference to Figures 1 to 3, the incoming stream of combustion air also includes a portion of re-cycled exhaust gases, and this combined mixture then follows a curved path 27b, 27c as it passes through an outlet in the bars 14.The stream of gases along portion 27c will include substantial portions of hot exhaust gases, some of which will be wholly oxidised products, and some of which will be only partly consumed.

The portion 27c passes through outlet apertures in the bars 14 so as to enter the middle and upper portions of the chamber 20. These emitted exhaust gases, together with secondary air stream 28a from secondary nozzles 28 are then guided by the baffle 22 into gas stream 27b, which splits into a recirculating exhaust gas portion 27e and an exhausting portion 27f. Straw falls down from the upper part of the front face of the bale to be consumed lower down.

The portion 27e, which is hot, and may include only partly consumed combustion products, mixes with the incoming combustion air stream 27 in order to preheat the latter, before passing to the primary combustion zone 26. The exhausting gas portion 27f may be wholly consumed, or only partly consumed, when it emerges from the rear of the furnace, but any necessary further combustion can take place by blowing in air through nozzle 28.

The combustion gases may then pass through a sealed connection from the outside air (see Figure 5) to a heat absorbing region, which may be boiler tubes.

When the furnace 10 is used to burn baled straw, the bale will occupy substantially the entire crosssection of the chamber 11, but the hot gas flow burns a hollow cavity (defined by path 27a) in the front end of each bale, which causes collapse of overhanging portions of the straw bale into the burning region so as to complete the combustion of the leading end of a freshly fed bale. As this leading end is consumed, the residual part of the bale is then urged continuously into the combustion space by being pushed longitudinally along the chamber 11 by the pusher arrangement 15. A fresh bale is inserted by withdrawing the pusher when one whole bale has been consumed.

It has been found in practice that a furnace, constructed as shown in Figures 1 to 4, operates very efficiently to consume continuously, by substantially complete combustion, agricultural waste product and especially straw bales. It is believed that the operating efficiency of the furnace is derived, at least in part, by the provision of the baffle 22 which is arranged in front of the upper part of the burning face, and which deflects the emerging hot exhaust gases leaving the combustion space so that part of these very hot gases is entrained by the incoming air jets, and these are then heated up to an ignition temperature before the jets impinge on the straw surface, thus ensuring immediate combustion. Means (28) is provided, not shown in detail, to add secondary air above the combustion gases leaving the baffle, to ensure complete combustion.However, in an alternative arrangement, the secondary nozzles 28 may be omitted from the secondary combustion chamber, in which case the furnace operates as a gasifier, producing a gas which may be used elsewhere.

Evidently, alternative flue arrangements may be provided, if desired, for discharge of the completely consumed exhaust gases. Following combustion of the solid matter, ash residue is pushed over the edge 13, as fresh material is fed to the furnace, and fall under gravity into suitable collection arrangements provided adjacent the outlet end 13.

A furnace as shown in the drawings may be used for space and water heating, steam raising, for use in keeping livestock warm in winter, or for supplying drying air to a grain drier. Evidently, other uses may be found for such an economic and efficient source of heat as is provided by the furnace, when used to burn agricultural waste and straw bales.

While the furnace has been described above in relation to the burning of agricultural waste and straw, it should be understood that the furnace may also be used in the gasification of straw.

Thus, the straw may be used as a source of hydrocarbon fuel to make water gas. In such event, instead of using air only in the nozzle arrangement 21, separate jets of steam and oxygen may be used to provide the incoming stream 27. A composite set of reactions then takes place within the combustion space 26, resulting in generation of water gas.

Instead of supplying combustion air via nozzles 21, a steam/oxygen mixture or a steamiair mixture may be introduced, when it is desired to generate water gas.

Referring now to Figure 5, this shows a modifi cation to the construction of Figure 4, but parts corresponding with those already described will be designated by the same reference numerals, and therefore not described in detail again. This furnace modification has a ram-type pusher 15, operable through the inlet end 12, which has a removable closure cap. As in the previously described arrangements, bales of compressed agricultural waste are progressively advanced along the furnace until engaged by the restraining bars 14. Combustion takes place within a combustion space defined by a fire brick lining having a throat downstream of the bars 14, and a sealed connection is made to this throat along which hot combustion gases are directed so as to heat boiler tubes 30 or the like arranged in their path. Ash and clinker are pushed by the movement of the bales into the pan 29.

The constructions of furnace disclosed herein enable tightly packed bales of straw or similar dry agricultural waste material to be pushed continu ousiy along a tightly fitting duct until an end face is restrained by the bars 14, and then air or other reaction gases are blown into this end face so that a reaction takes place at the point of impact, and the gases leave through the same end face along a generally "horseshoe" shaped path.

Claims (4)

1. A furnace for burning relatively bulky and compressed material, such as bales of agricultural waste product or straw, the furnace comprising a chamber through which the material is to pass and having an inlet at one end and a combustion space at its opposite end; restraining means arranged at the opposite end of the chamber to be engaged by the compressed material following passage of the material along the chamber to the combustion space, the restraining means sub-dividing the combustion space into a primary combustion space which faces the inlet and downstream thereof a secondary combustion space; means for moving the compressed material through the chamber so as to press continuously against the restraining means whereby combustion can take place while the chamber is substantially filled with material;; first nozzles arranged to blow one or more jets of primary combustion air into a lower portion of an end face of the material engaging with the restraining means; and a baffle arranged across an upper part of the combustion space so as to cause partial re-circulation of burning and burnt gas to the primary combustion zone and to allow combustion products to be discharged from the combustion space.

2. A furnace for burning or gasifying relatively bulky and compressed material, such as bales of agricultural waste or straw, the furnace comprising: a chamber through which the material is to pass and having an inlet at one end and a combustion space at its opposite end; restraining means at the opposite end of the chamber to be engaged by the material; means for moving the material through the chamber so as to press continuously against the restraining means so that combustion can take place while the chamber is substantially filled with material; and means for blowing one or more jets of primary combustion air, or a steam/oxygen mixture, or a steam/air mixture, or separate jets of air, oxygen enriched air or oxygen and of steam into the combustion space and into contact with a lower portion of the end face of the material restrained by the restraining means, whereby a reaction takes place at the point of impact, and the gases leave through said end face along generally horseshoe shaped paths.

3. A furnace according to claim 1 and substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.

4. A furnace according to claim 2 and substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.