WO2010070328A1

WO2010070328A1 - Fuel product and process

Info

Publication number: WO2010070328A1
Application number: PCT/GB2009/051707
Authority: WO
Inventors: Frederick Mcgoldrick
Original assignee: Solsys Limited
Priority date: 2008-12-20
Filing date: 2009-12-14
Publication date: 2010-06-24
Also published as: GB0823261D0

Abstract

A process for producing rigid fuel pellets from a biomass material and a binder, comprising the following steps: a) admixing the biomass material and binder, and b) agglomerating the so-formed mixture by tumbling at ambient temperature to form the rigid pellets; and c) drying the rigid pellets using a flow of air, wherein the binder is silicate-based and includes one or more surfactants, and the process is carried out at ambient temperature.

Description

FUEL PRODUCT AND PROCESS

The present invention relates to a fuel product and a process for making same.

Biomass refers to living and recently dead biological material that can be used as fuel or for industrial production. Most commonly, biomass refers to plant matter grown for use as biofuel, but also includes plant or animal matter used for production of fibres, chemicals or heat. Biomass may also include biodegradable wastes that can be burnt as fuel. It excludes organic material which has been transformed by geological processes into substances such as coal or petroleum.

Biomass falls into two main categories; 1 ) woody biomass includes forest products, untreated wood products, energy crops and short rotation coppice (SRC), which are quick-growing trees like willow; 2) non-woody biomass includes animal waste, industrial and biodegradable municipal products from food processing and high energy crops. Examples include miscanthus, switchgrass, rape seed, hemp, corn, poplar, willow, sugarcane residue and oil palm (palm oil). The particular plant used is usually not very important to the end products, but it does affect the processing of the raw material.

Production of biomass is a growing industry as interest in sustainable fuel sources is growing. Although fossil fuels have their origin in ancient biomass, they are not considered biomass by the generally accepted definition because they contain carbon that has been "out" of the carbon cycle for a very long time. Their combustion therefore disturbs the carbon dioxide content in the atmosphere.

Biomass is part of the carbon cycle and carbon from the atmosphere is converted into biological matter by photosynthesis. On death or combustion the carbon goes back into the atmosphere as carbon dioxide. This happens over a relatively short timescale and plant matter used as a fuel can be constantly replaced by planting for new growth. Therefore a reasonably stable level of atmospheric carbon results from its use as a fuel. It is accepted that the amount of carbon stored in dry wood is approximately 50% by weight.

Energy produced from biomass residues displaces the production of an equivalent amount of energy from fossil fuels, leaving the fossil carbon in storage. It also shifts the composition of the recycled carbon emissions associated with the disposal of the biomass residues from a mixture of CO₂ and CH₄, to almost exclusively CO₂. In the absence of energy production applications, biomass residue carbon would be recycled to the atmosphere through some combination of rotting (biodegradation) and opening burning. Rotting produces a mixture of up to fifty percent CH₄, while open burning produces five to ten percent CH₄. Controlled combustion in a power plant converts virtually all of the carbon in the biomass to CO₂. Because CH₄ is a much stronger greenhouse gas than CO₂, shifting CH₄ emissions to CO₂ by converting biomass residues to energy significantly reduces the greenhouse warming potential of the recycled carbon associated with other fates or disposal of the biomass residues.

The existing commercial biomass power generating industry in the United States, which consists of approximately 1 ,700 MW (megawatts) of operating capacity actively supplying power to the grid, produces about 0.5 percent of the U.S. electricity supply. This level of biomass power generation avoids approximately 11 million tons per year of CO₂ emissions from fossil fuel combustion. It also avoids approximately two million tons per year of CH₄ emissions from the biomass residues that, in the absence of energy production, would otherwise be disposed of by burial (in landfills, in disposal piles, or by the ploughing under of agricultural residues), by spreading, and by open burning. The avoided CH₄ emissions associated with biomass energy production have a greenhouse warming potential that is more than 20 times greater than that of the avoided fossil-fuel CO₂ emissions. Biomass power production is at least five times more effective in reducing greenhouse gas emissions than any other greenhouse-gas- neutral power-production technology, such as other renewables and nuclear.

One specific example of a biomass material is 'sludge', more particularly two generally used expressions 'wastewater sludge' and 'sewage sludge'.

One technology to try and 'use' sludge is incineration, and for this to be effective, the sludge needs to be dewatered to a solid content of about

25%. Many methods are known for dewatering sludge including the use of centrifuges, filter presses, generally termed in the art 'mechanical dewatering'.

For incineration, the dewatered sludge then needs to be further dewatered to have a dry solid contents of 90-95%. This is carried out by thermal drying using either direct dryers, such as hot gases from the combustion of oil, natural gas, etc, or indirect dryers where the heat transfer occurs through the barrier of a dryer, or combined dryers. All such dryers require significant use of energy before the dewatered sludge is useable in for example fluidised bed incineration. As an example, the input of 200,000 tons of dewatered sludge into a medium size power station or incinerator can provide 1 MW of electricity based on a 40% conversion of the incineration of the sludge, i.e. 2.5MW heat energy from the burning of the sludge. However, drying of this volume or mass of sludge, to allow it to be incinerated, requires 12.5MW: that is, 5 times the amount of calorific heat value of the sludge itself, and 12.5 times the amount of electricity recoverable. Whilst this may reduce the land impact of dealing with sludge, it is clearly an energy inefficient process.

Other biomass waste technologies are based around heat and pressure pelletisation processes similar to briqueting, which also requires a high energy input, with the calorific value therefrom being lower than the energy input.

Although the biomass market is still relatively small, this in some respects is "cause and effect" as, to date, there has not been a simple cost effective system which can pelletise the plethora of potential sources of biomass materials. The present demand for useable biomass solid fuels in the UK and Europe currently exceeds supply. Despite shortages, some companies are still disposing of biomass waste in landfill because the capital and running costs of presently available pelletising equipment is too high.

On a political level renewable energy production targets in the EU are not currently being met, and there is considerable impetus to increase their usage with a 12% target for 2010, although currently less than 2% is provided from renewable sources. It is an object of this invention to provide a more energy efficient process for dealing with biomass materials.

It is another object of the present invention to provide a more efficient fuel product and process by lowering costs of operations and capital requirements.

Thus, according to one aspect of the present invention, there is provided a process for producing rigid fuel pellets from a biomass material and a binder, comprising the following steps:

a) admixing the biomass material and binder, and b) agglomerating the so-formed mixture by tumbling at ambient temperature to form the rigid pellets; and c) drying the rigid pellets using a flow of air;

wherein the binder is silicate-based and includes one or more surfactants, and the process is carried out at ambient temperature.

The use of a silicate-based binder which preferably includes one or more surfactants allows the process of the present invention to create rigid fuel pellets at ambient temperature. Forming rigid fuel pellets at ambient temperature has not been achievable by any prior art process.

The fuel pellets are 'rigid' in the sense that they are handleable, and are able to be stored, stacked, and/or transported immediately, without requiring any separate active curing step or steps. That is, the pellets cure without any assistance or further treatment, especially heat and/or pressure treatment. The prior art processes require any pellets formed by tumbling and agglomeration to be actively cured with heat and/or (forced air) pressure before such pellets are rigid and handleable. Thus, the fuel pellets of the present invention could be packaged and/or transported immediately after forming.

The tumbling action, such as in a rotary drum, serves to agglomerate the particles and bind the mixture into the pellets, usually with a variable size distribution.

In a further embodiment, a rotary drum may comprise at least one or more apertures through which the ambient air may pass into said rotating drum to dry the pellets. The apertures may be covered by a protective mesh of a suitable material or a perforated covering to prevent pellets from being displaced or lost from the drum whilst allowing the ambient air to pass through the drum. The flow of ambient air may be produced by a blower or any other type of ambient air drying apparatus.

Preferably, the drying step is performed for a short period, more preferably less than 1 hour, even more preferably less than 30 minutes.

Such drying occurs at ambient temperature, and can also occur without any heat- treated curing step, as used in the prior art. After drying the rigid fuel pellets may further cure over time without any external influence. They could be allowed to stand, for example, for some time, such as 1-10 days, at a suitable position or location. Like concrete, curing may continue for some time, for example over several days. However, the present invention allows for the acceleration of the overall curing process providing rigid pellets with sufficient solidity after drying such that they are ready to be stored, stacked, transported, etc. immediately. This provides for a more efficient and time saving process which is capable of very high tonnage throughputs. No mechanical compression force is required, (with its attendant low production rate and high cost), and the process of the present invention is carried out at ambient temperature. By being able to carry out the process at ambient temperature, no additional equipment is required for any active second stage treatment, or to provide an elevated temperature. This naturally eliminates the need for a power source, e.g. fuel to be burnt, to create the elevated temperature, which action is usually a significant economic requirement of an industrial process.

Thus, in another way, the binder of the present invention allows the fuel pellets of the present invention to be formed and to cure in a 'cold fusion' process. That is, the pellets can be formed and can cure without the need for any external heat input.

In addition, the present invention is particularly advantageous by being able to be a 'single stage' process, avoiding the need for any pre-mixing or treatment of the constituents involved, and the requirement for any post- forming treatment. From a capital and economic perspective, a single stage process reduces the requirements needed to set up a plant adapted to provide the process of the present invention, and lowers the costs of operation by having a single stage process which is run at ambient temperature.

The present invention is also advantageous in using one or more inorganic silicate-based binders. The use of such binders reduces the complexity of the process, and again reduces the need for any pre-treatment or mixing of binder materials. The use of inorganic silicate-based binders has two further advantages. Firstly, such binders do not impact on the burn quality of any carbonaceous material (as they do not burn), in contrast with any organic binders used in other processes, such as starches, (which do burn, and which therefore effect the burn quality and thus heat content value of the formed material). Secondly, such binders are also clear of any environmental implications (as they do not burn), again in contrast with organic binders.

Once the rigid fuel pellets are formed, they cure naturally and at ambient temperature to provide the final form of the fuel pellets. This occurs at ambient temperature, and can also occur without any active and/or separate curing step, especially a heat treatment step, as used in the prior art. The rigid fuel pellets will cure over time without any external influence. Thus, they could be allowed to stand, for example, for some time, such as 1 -10 days, optionally at least 3, 4, 5 or 6 days, at a suitable position or location, whilst curing occurs after the tumbling. Like concrete, curing may continue for some time, for example over several days, but the invention provides rigid pellets with sufficient solidity after tumbling, that they are ready to be incinerated, stored, stacked, transported etc as they cure after forming.

The drying of the rigid pellets from the agglomeration accelerates the curing of the pellets to a final form. The curing can also include any post- step (c) drying required of the formed pellets in addition to the chemical process occurring at at least the surface of the pellets as they are being formed, preferably to provide a harder or hardened shell. Preferably, the process provides pellets having a hardened outer portion, skin, casing or shell. More preferably, the interior of the pellets is dry, and wholly or substantially has a small, preferably micro, aerated or porous form. That is, the action of the surfactant to draw the silicate binder to the surface of the pellets as they are being formed creates air pockets and bubbles in the interior, the benefit of which is discussed hereinafter. Biomass materials usable with the present invention include any material generally of a biological origin, generally being carbon based and usually based on recently living biological material, as opposed to coal and other fossil fuels. Such materials include sludges such as waste water sludge and sewerage sludge, chicken litter, bone-meal, spent mushroom composts, woods, organic plant material or residues or products, usually by-products, etc.

One legal definition of biomass is from The Biomass Research and

Development Act of 2000 (P.L. 106-224; Title III), which defines biomass as "any organic matter that is available on a renewable or recurring basis, including agricultural crops and trees, wood and wood wastes and residues, plants (including aquatic plants), grasses, residues, fibers, and animal wastes, municipal wastes, and other waste materials."

The Directive 2001 /77/EC of the European Parliament and of the Council of 27 September 2001 defines biomass as the biodegradable fraction of products, waste and residues from agriculture (including vegetal and animal substances), forestry and related industries, as well as the biodegradable fraction of industrial and municipal waste.

The Renewables Obligation Order (Northern Ireland) 2007 (no 104) entitled Electricity, defines 'biomass' as fuel used in a generating station of which at least 90 per cent of the energy content (measured over such a period and with such frequency as the Authority deems appropriate) is derived from plant or animal matter or substances derived directly or indirectly therefrom (whether or not such matter or substances are waste) and includes agricultural, forestry or wood wastes or residues, sewage and energy crops. A sludge material biomass is preferably dewatered sludge i.e. reduced water content. This allows for easier combustion of the sludge once pelletised and less energy requirements for the incinerator to deal with any excess water. Untreated sludge usually contains around 6% dry solids

(d.s) content and 94% water content. For incineration purposes the sludge is preferably dewatered to produce a cake of around 25-28% d.s. As a combustion plant performs best in a continuous mode of operation, this will favour the selection of centrifuges or filter belt presses for dewatering.

In particular, the present invention is able to use sludge material which can be derived from any definition of sludge including wastewater sludge, sewage sludge or 'biowaste', which, once treated, preferably has a moisture level of 20-40%, frequently 25-30%, which is a commonly available form of dewatered sludge. Further dewatering or drying of the sludge is unnecessary, reducing significantly the energy input required to provide a useable fuel product. If the process of the present invention uses for example, and not being limited thereto, 1 MW of energy for the process to provide the pellets based on such sludges, then this can be compared with the requirement of 12.5MW mentioned above for the need to further dry dewatered sludge for its use in prior art incineration processes. The present invention is therefore at least three or four times more efficient in terms of electricity generated by the burning of the pellets compared with prior art incineration of processed sludge.

Woods can include any waste wood material, generally being in a dust or 'fine' form, or able to be provided in such form. Processes such as torridfication are known for making wood fines, and wood fines and dust are also natural by-products in many wood shaping, forming or manufacturing processes or industries. Chicken litter, bone meal and spent mushroom compost are biomass materials, much of which can already be in a fine or particulate mode.

Chicken litter or poultry manure is by-product of poultry farming and can be used as an effective biomass energy source. In agriculture, poultry litter or broiler litter is a material used as bedding in poultry operations to render the floor more manageable. Common litter materials are wood shavings, sawdust, peanut hulls, shredded sugar cane, straw, and other dry, absorbant, low-cost organic materials. Sand is also occasionally used as bedding. After use, the litter consists primarily of poultry manure, but also contains the original litter material, feathers, and spilled feed.

Poultry and turkey litter is currently in use as a primary fuel source in several electrical generating plants in the UK, and the United States. Most of these plants were developed by Energy Power Resources (in the UK), or by their US subsidiary, Fibrowatt USA. Operating plants include Thetford, Eye, Westfield and Benson. On a smaller scale, poultry litter is used in Ireland as a biomass energy source. This system uses the poultry litter as a fuel to heat the broiler houses for the next batch of poultry being grown thus removing the need for Liquefied Petroleum Gas (LPG) gas or other fossil fuels.

In the use of organic plants such as rapeseed or hemp seed, corn, sugar cane, etc, from which products such as oils such as linseed oil are produced, again there is commonly a significant proportion of waste material in a fine or particulate form. Rapeseed dust and hemp seed dust are known by-products of processes using rapeseed or hemp seed. Similar by-products are formed in the use of other organic plant materials. In the present invention, the biomass material may be one or more biomass materials as hereindescribed. Where two or more biomass materials are used, they may be pre-mixed prior to admixing with the binder, or each biomass material is brought together either simultaneously or separately with the binder.

The admixing of the biomass material and binder may occur prior to any agglomeration by tumbling, for example as a separate step, such that the so-formed mixture is then agglomerated by tumbling to form the rigid pellets.

Alternatively, the biomass material and binder are admixed either by the same tumbling action which subsequently allows the mixture to agglomerate or on route thereto. For example, where the biomass material and binder are to be agglomerated in a rotary drum, the biomass material and binder could be provided to the rotary drum either simultaneously, or one after the other, such that their admixing to form a mixture occurs by the same action of the rotary drum as their subsequent agglomeration into rigid pellets.

Where the biomass material is provided from two or more biomass materials as hereindescribed, each biomass material could be added either separately or simultaneously with one or more other biomass materials, and/or the binder. The present invention allows for the mixing of the biomass material and the binder to be any combination of such combinations.

In one embodiment of the present invention, the biomass material has a moisture content of up to 20-25wt%. Because of the range of biomass materials that can be used by the present invention, the moisture content may be as low as between 0-5wt%, or may be as high as 20-25wt%.

In another embodiment of the present invention, the majority of the dry solids content of the biomass material is 'fines', that is having a size of 1 mm or less. Preferably, at least 50, 55, 60, 65, 70, 75, or 80wt% of the biomass material is of a size of 1 mm or less.

In another embodiment of the present invention, percentage of fines of the biomass material which is submicron in size is at least 30wt%, preferably at least 40, 50, 60 or 70wt%.

The rigid pellets formed by the present invention can be of any suitable size or dimension, including below 1 mm and above 10cm. Preferably, they are generally in the range of greater than 5mm, optionally up to 2cm. In one embodiment of the present invention, binder is added is added in the range 10-50%wt of biomass material, preferably 15-30%wt. The amount of binder required will to some extent depend on the moisture content of the biomass material.

According to another embodiment of the present invention, a particulate carbon-based material is also added to the mixture to form the pellets. Preferably the particulate carbon-based material is admixed with the biomass material and binder, and agglomerated therewith.

Carbon-based particulate material suitable for the present invention can be accepted wet or dry, and could be provided by any type of maceral fuel, including peat and lignite through to sub-bituminous coals, anthracite fines, petroleum coke fines and the like, as well as other hydrocarbon materials that could be considered a fuel source. The particulate material may also be a combination of two or more such materials, not necessarily premixed, and such as those hereinbefore mentioned, so as to provide 'hybrid' fuel pellets.

The present invention is not affected by high ash content or sulphur content in certain carbon-based materials.

In most power stations using coal, the coal is generally ground into a fine powder to be sprayed into the combustion furnaces. However, the power for crushing coal having a moisture content of, for example, 25% is relatively high. Thus, at some power stations, there is currently V≥ million tonnes a year of 'unusable coal' product in stockpiles, as it is too wet, i.e. its moisture content is too high, for efficient burning. Freshly mined bituminous coal can have a moisture content of up to 20%, lower ranking coal can have a moisture content of up to 30%, with lignite going up to 45%. To drive off this level of moisture (by turning it into steam) prior to any combustion of the actual coal requires so much energy to start with, that this coal is simply not used, as it is not efficient. Grinding such coal to be more 'burnable' is also inefficient as the moisture-rich coal generally clogs up the grinder.

It is a further advantage that the present invention can also use any type of 'wet' or 'dry' particulate carbon-based material, although any wet material preferably has a maximum water content of 10-15%. Such a moisture level can be achieved by grinding, which has a drying effect, (although the power required therefor is a lot lower than the power required for grinding coal to a powderous form ready for immediate burning as described above). Such material is generally still regarded in the art as being 'wet', especially in relation to e.g. the briquetting process, which requires its material to be absolutely dry. In some circumstances, it is preferred to have a dry particulate carbon- based material. In other circumstances, the material may be derived from a wet fuel source, such as peat and coal tailings dams, and any reduction in the amount of drying needed (compared with for example the briquetting process) reduces the overall energy input required to form the fuel product.

In another embodiment of the present invention, water is part of the material and binder mixture, either by being part of the material, part of the binder, added separately, or a combination of any of these. The amount of water needed or desired for the process of the present invention may depend upon the nature of the biomass material and the binder.

The moisture content of the biomass material may be altered by the use of two or more biomass materials having different moisture contents. For example, use of wood dust or fines can increase the dry solid contents of the overall biomass material, and therefore reduce the moisture content of the overall biomass material.

The use of two or more different biomass materials having different calorific values can also be used to provide fuel pellets having a predetermined rate and/or heat of burning. For example, it is known that the rate of burning of wood is higher than that of coal, such that the design of a wood-burning incinerator is different to that of a coal-burning incinerator. However, the present invention allows for the combination of biomass materials having different calorific values, which can be combined to provide fuel pellets which can be used in prior art incinerators or other situations. For example, the combination of spent mushroom compost, which has a low calorific value, and of wood fines, which have a high calorific value, can provide a combined biomass material for use with the present invention, whose fuel pellets have a slower rate of burning than fuel pellets formed purely from wood dust. The man skilled in the art will be aware of the different calorific values of biomass materials, and their combined calorific value based on different ratios.

The process of the present invention is directly usable with moisture-rich biomass material, as any water content of the biomass material and/or binder can be reduced in line with the level of moisture in the biomass material without affecting the process. Once the pellets have been formed, their hardened shell wholly or substantially stops or significantly reduces water ingress, especially if waterproofing additives are used. Once fully cured, the pellets can have a moisture content of at least half that of the particulate starting material, and possibly less than 5%, and thus be sufficiently dry for immediate and easy grinding to form a suitable fuel product for a power station or incinerator.

A reduction in moisture also provides a direct increase in the heat content value of the product which it is burned, hence increasing its efficiency and economic value. This economic benefit extends to transportation of such a product, in comparison with cost of transporting 'wet' or moisture-rich material as described hereinabove. Indeed, the present invention provides a process whereby with consideration of the type and amount of binder(s) used, and the process parameters, a fuel material can be provided which has a desired or pre-determined bum value or the like, which, in particular, could suit the local economic conditions for the fuel source. Different locations and countries make different waste sludge materials, and they therefore use such materials in different ways in order to try and maximise their economic value. The present invention provides a particular advantageous process to benefit what is currently regarded as waste materials from current industrial processes.

Thus, the present invention also provides significant moisture reduction in a fuel product, converting an inefficient fuel product into an efficient fuel product.

In a preferred embodiment of the present invention, the amount of water for the process is adjusted in the binder component prior to its admixing with the biomass material. The calculation of this binder to water adjustment is dependent on the moisture content of the biomass material.

According to another embodiment of the present invention, any particulate carbon-based material(s) used are generally of a maximum size or grade of 3mm or less. Coal 'dust' or 'fines' can often be of a sub-micron size. Peat is a fuel material which is generally dried/shredded/dried/crushed prior to briquetting. Some shredding of a peat material may still be required to provide a particulate carbon-based material suitable for the present invention, but to a much lesser extent than that required for briquetting.

More preferably, the particulate carbon-based material has a range of sizes or grades; preferably biased towards fine or finer particle sizes, e.g. approximately 40-60%, such as 50%, below 1 mm.

Any suitable silicate-based binder can be used for the present invention, which binder may be a homogeneous or heterogeneous material, such as cements and raw silicates like calcium, sodium or potassium. The process may include the addition of one or more further ingredients into the mixture, either separately or integrally with the binder. Such further ingredients include lime, inorganic binders, cements, and waterproofing additives. A cementitious material can assist in the green- strength of the pellets, and possibly in forming the hardened outer surface or shell for the pellets as described hereinafter.

Lime or cement helps to inhibit sulphur emission upon burning of the so- formed pellets. It is a particular advantage of the present invention that the use of lime or other types of calcium hydroxide (which are known to be sulphur-absorbing agents) are admixed with the particulate carbon-based material. The increased mixing of such sulphur-absorbing agents with sulphur-containing carbon-based materials reduces the need for current sulphur-absorbing apparatus such as scrubbers and the like at the end of fuel-burning process. Indeed, it is considered that the present invention can achieve a reduction of sulphur emission (usually in the form of sulphur dioxide) by 70-90%, or possibly more. Again, this is a significant reduction in current power station requirements, and therefore costs.

The process of the present invention can further include the step of grinding, crushing or otherwise particularising the pellets, preferably in a form ready to use in a fuel-burning power plant.

There is also a direct reduction in CO₂ emissions when biomass is substituted for coal as long as it is carbon neutral. That is, the carbon released during fuel conversion is equal to that taken up during the growing period of an energy crop or forest.

Furthermore, co-firing of biomass material, optionally with a carbon-based particulate material, does not involve the high capital costs of building a new biomass plant, but involves significantly lower retrofitting costs of an existing plant. Retrofitted boilers can fire biomass when biomass supplies are plentiful but switch back to coal when biomass supplies are low. Co- firing also increases the efficiency of the energy conversion by firing the fuel in a larger plant firing biomass alone. Biomass conversion efficiencies when co-fired range from 30% to 38%, which is very much higher than in a dedicated biomass plant. The other advantages of the use of biomass include the fact that it diversifies the power plant's fuel portfolio. In addition to reducing net CO₂ emissions, co-firing enables the coal-fired plant to reduce O₂ emissions as biofuels generally contain less sulphur than coal. Biofuels also tend to contain less nitrogen which leads to lower NOx emissions. In some cases, the operating costs of co-firing can be higher due to the higher costs of biomass compared with coal, but co-firing is often still the cheapest form of renewable energy production.

One or more other mineral additives such as zeolites or vermiculite could also be used as a further ingredient to help bind any metallic contaminants in the ash of the pellets, and so prevent any soluble metals being released from the ash.

The particulate material and binder, and any other separate reagents or ingredients to be added, can be admixed using any known process or arrangement, including simple mixing. Because the next part of the process is a tumbling action, absolute homogenous mixing of the reagents or ingredients prior to the tumbling is not essential, as the tumbling action will generally further the mixing action if necessary or desired. In some circumstances, the admixing may at least partly occur during the tumbling action, such that the actions or steps of the invention may not be distinct, either in part or completely. The waste sludge material and binder could be at least partly mixed with agitation.

In one embodiment of the present invention, the binder is coated on to the material(s). One method of coating is to spray the binder on to the material(s).

In another embodiment of the present invention, the biomass material and any carbon-based material are moving prior to and/or during mixing with the binder, and/or the material(s) are in a dispersed arrangement. One particular suitable form of this is a falling curtain of material(s), such as at conveyor transfers, inside pelletising drums or pans, and from stockpile load outs, etc.

In another embodiment of the present invention, the material(s) and binder are directly and/or immediately undergo tumbling after their contact with each other.

The tumbling action serves to agglomerate the material(s) and binder mixture to form particles of greater and greater size, generally having a spherical or ovoid shape. The size of the so-formed pellets can be adjusted based on the process conditions for tumbling, such as rotation speed, moisture content, impact force and residence time. The pellets could also be screened and/or recycled during or after pelletising to produce a desired, e.g. narrower, size distribution.

One suitable apparatus for providing tumbling action is a rotary drum. Rotary drums are well known in the art. Their output can be dependent upon the length, diameter, speed of rotation and angle of mounting of the drum, and the output can vary from single figure tonnes per hour, to hundreds of tonnes per hour per drum.

The general sizes and dimensions of agglomerator drums, such as pan, rotary and conical drums, are known in the art, as are their process variations to provide variation in the products formed. See for example UK Patent No 787993.

Rotary drums have low capital and low operating costs, especially in comparison with briquetting plants. They can even be provided in mobile form, such that the process of the present invention can be provided where desired or necessary, e.g. moved and located to where the material(s) are currently stored or 'dumped', rather than requiring significant movement (and therefore cost) for transporting the material(s) to a fixed processing site.

The agglomeration action may be carried out in one or more stages, which stages could be connected, such as the tumbling conditions changing in the same drum, or the material(s) being fed directly into another agglomerator. Or, such actions could be separate. In one arrangement for multi-stage agglomeration, the tumbling conditions are variable or varied for each stage. The conditions may be altered either in a continuous manner or action, or discretely.

Where the process of the present invention involves tumbling the mixture in a rotary drum, one or more rotary drums may be used for the agglomeration, and/or drying steps preferably in series.

The surfactant(s) serve to draw the silicate-based binder towards the surface of the forming pellets, such that as they are created and start to dry, the pellets will form and then continue to have a harder outer portion, skin, shell or surface, compared to their interior. Thus the pellets have a variable density towards the core; the density being greater at the surface. Indeed, the 'shell' layer or portion will generally have a high density in comparison with the lower density of the 'interior'.

More preferably, the pellets have sufficient hardness once formed to allow handling, stacking and/or transportation without any significant breakage.

The drying or curing of the pellets may start during or be part of the agglomeration action.

The method of the present invention may include one or more sizing steps. That is, to grade the size of the so-formed pellets to that desired or necessary. This could include extracting those pellets which are damaged or undersized, which pellet material could be recycled back into the process of the present invention.

Following any initial drying step, the formed pellets may be optionally rested for a further period, possibly a number of days such as 3-7 days, to provide or allow for at least the majority of the curing to finish. Like other curing products, the pellets continue to cure to gain strength over time, such as a further number of days or weeks.

According to another aspect of the present invention, there is provided a rigid fuel pellet product formable at ambient temperature by agglomeration of a biomass material, optionally also a particulate carbon-based material, and a silicate-based binder including one or more surfactants and being formed or formable by the present invention. Such pellets could be formed by a process as herein described.

The fuel pellet product of the present invention is a material which is easily storable. It is also easily transportable due to its variable diameter distribution. This enhances stacking concentration, which also reduces abrasion and consequential breakage of the pellets.

The product preferably allows a very high percentage of combustion (possibly 100% combustion), so as to leave little or no combustible fuel in the ash.

Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying drawings in which:

Figure 1 is a flow diagram of a process according to one embodiment of the present invention in the context of the material handling and production stages in an industrial plant;

Figure 2 is a front view of tumbling action of agglomerating pellets according to the present invention;

Figure 3 is a view of a number of pellets according to another embodiment of the present invention;

Figure 4 is a graph of moisture and strength over time for pellets of Figure 3; and

Figure 5 is a perspective view of a mobile agglomeration unit useable with the present invention. Preparation

The biomass material is prepared for agglomeration. Depending on its raw state, it may be beneficial to carry out some grinding, screening or drying. The finer the raw feed is, the more effective the process. Preferably, (but not limiting), the moisture content of the feed is a maximum of 20-25%, preferably 10-15% (by weight).

Depending on the moisture content and chemical characteristics of the biomass material, the liquid feed is adjusted to suit. This will involve balancing the quantity of water relative to the binder and surfactants used.

The above parameters can be established during pre-testing of the process and apparatus. For biomass fines agglomeration, it has been found that between 20-25% of liquid binder (to weight of raw feed) is generally desired for efficient agglomeration. Generally, the wetter the biomass feed, the less water is required to be added at this stage.

Agglomeration

The fuel feed is carried along and any dry reagents are added to the feed.

It then falls from the end of a conveyor belt. The liquid binder is sprayed onto the falling curtain of fines, which together fall into a rotating drum, generally 1 -5m (such as 3m) in diameter. As the mixture tumbles while being sprayed with the binder and water mixture, it forms small pellets which agglomerate and grow, forming rigid pellets of desired shape and size as shown in Figure 2.

The drum can be lined with loosely fitting heavy duty rubber sheet to avoid material sticking to the sides of the drum. The drum is set at an incline (e.g. 1 -3%) to aid progression of the pellets therealong, and to control the residence time in the drum. The completed pellets exit at the opposite end of the drum onto another conveyor.

Pellets can be varied in size with only operational drum adjustments (speed of rotation, moisture content and longitudinal drum angle which directly affects residence time in the drum). Expensive mould changes, such as in present briquetting operations, are not required to vary the product dimensions.

Drying

Ambient air from a blower or other air driving apparatus is allowed to pass through a conjoined or separate, preferably separate, rotary drum, in order to accelerate the curing process of the pellets. The drum is preferably perforated to allow for ambient air to be blown through it whilst the drum is rotating. The perforations or any such other apertures may be covered by a protective mesh of a suitable material or a perforated covering, to avoid loss or displacement of the pellets from the drum whilst allowing ambient air to pass through the drum. The drying step may take place directly in line with or after the agglomerating process, and the completed pellets exit at the opposite end of the drum onto another conveyor.

Alternatively, the agglomerated pellets are moved via a further conveyor belt into a rotary drum where the second step of drying takes place separately.

The second rotary drum is preferably similar to but has a larger diameter than the agglomerating drum. It may also be of greater diameter and longer than the agglomeration drum. Here the pellets progress slowly through the drum, allowing sufficient time for the pellets to dry and preferably initially cure, and optionally receive any surface treatment, and to assist immediate handling and stacking. The residence time within this drum is dependent on the fuel characteristics, and its use can be determined in pre-production tests.

Selected surface treatment additives can be added at this stage to increase the surface area of the pellet skin, to prevent sticking, and/or to prevent leaking fluid into bags, etc.

Should the green strength of the pellets be poor, certain additional binders or cementitious chemicals can be added to rapidly speed-up the overall curing process, and thereby give quicker and stronger initial green strength to aid handling, or handleability, etc. Broken and undersized pellets can be removed using for instance a slotted section of drum or a vibrating screen at the drum exit. The damaged and undersized pellets can then be returned to the agglomerating drum for reprocessing.

At this stage the pellets can be further graded to the desired cross section if necessary. Any damaged and undersized pellets can then be returned to the agglomerating drum for reprocessing.

Any pellet sizing could be designed to be made dependent upon proposed use. The pellet size can be adjusted by means of changes to process conditions, equipment configuration, and even reagent dosage.

The pellets can then be immediately used, such as in a gasifier, or stockpiled or transported for subsequent use. During this time, generally between 3-7 days for coal fine pellets, and depending on ambient temperature, the pellets may reach greater strengths as to allow more aggressive or abrasive handling. An example of formed pellets is shown in Figure 3.

The spherical shape of the pellets will also allow air to move freely through a stockpile of such pellets to assist any further curing and prevent heat build up and the risk of spontaneous combustion. At this stage, the pellet surface is also tightly sealed, preventing air ingress into the pellets, and so also slowing the effect or chance of any spontaneous combustion. If spontaneous combustion is still a problem, preventative reagents can be added during agglomeration.

Transportation and Packing

Tumble and growth agglomeration can result in a wide variation in the final pellet. This has the advantage of lowering the bulking factor of the pelletised product, resulting in lower transportation costs.

The formed product could then be incinerated directly, or allowed to continue to cure at ambient temperatures, curing time being dependent upon local humidity. Generally, the higher the moisture content of the feed, the longer the pellets will require to be cured at ambient temperatures and humidity.

Figure 4 shows the moisture reduction and strength increase for a wastewater sludge-based biomass pellet over time. The moisture content usually continues to reduce to 3wt% or less over further time. Such pellets are clearly by then very strong, but still easily burnable.

Process rates can be selected, but production rates of between 10-100 tonnes per hour of biomass material per drum would be a general rate. The production rate can be scaled up using multiple process units, or scaled down with smaller equipment, optionally mobile units.

Figure 5 shows a commercially sized agglomerator unit useable with the present invention. It is clearly transportable.

Production costs are dependent upon the production rate, particle size distribution of the feed, and characteristics of the particulate materials. However, energy input per tonne of product has been measured at approximately 0.5 to 2kWh, at least a hundred times less than the energy input needed for briquetting.

In particular, the process of the present invention can be modified to treat very high ash and/or very high sulphur biomass materials, as the pellets remain stable throughout the combustion process, allowing even for low calorific materials to burn efficiently.

The process of pelletising also simultaneously reduces fly ash by the inherent cementation, silicification and stabilisation of the residual ash instigated by the reagents used. Additionally, higher product combustion temperatures are easier to generate due to high gas transfer rates, not only between the pellets, but also between particles within the pellets, providing more rapid and/or more controllable combustion than normal fuels.

A further advantage of the present invention is the very complete combustion of the contained fuel in the pellets due to the high gas transfer rates and the maintenance of the integral structure of the pellets until combustion is complete. The retaining hardened shell, skin, etc, allows for significant heat increase or build-up inside the pellet, causing very high levels of combustion, resulting in the completion of any pre-designed chemical reactions in the interior content of the pellet. As the content is dry and porous form, generally of a 'fine' nature still, and is now preheated, rapid and so complete combustion of the content occurs. The pellets maintain their form even at white heat, and show very stable combustion characteristics.

Where water is used in the process of the present invention, the surfactant causes the binder-containing moisture to rapidly migrate to the surface of the pellet by capillary action, giving the 'egg shell' effect of a hardened surface and softer interior, due to the final heavy surface concentration of the binder. This results in a significantly enhanced skin strength, giving a very robust and low moisture content pellet (approximately 5%), which also resists moisture absorption from the air.

As can be recognised from the above, the process of the present invention overcomes or solves a number of financial and operational problems.

Once the 'egg shell' effect has been fully developed after curing, the pellet will retain its strength even during white heat combustion. This allows high temperature reactions to take place inside the pellet resulting in much higher levels of combustion of the fuel, giving effective oxidation and sequestration of any contained sulphur, and negligible unburnt carbon levels in the residue ash. The shell effect allows the pellet to retain its structure during combustion, resulting in less particulate emissions in the flue gas.

The present invention provides significant benefits compared with present technologies, including: • <3mm biomass fines can be pelletised dry or direct from a filtration plant.

• Tonnage throughput can be from 10 tones per hour (community size) up to 100 tonnes per hour per pelletising line. • High level of automation can be used during pelletising for accurate control and reagent usage.

• Accelerated curing by ambient air drying

• Pellets can be handled by bulk handling equipment when cured or alternatively bagged when 'green'.

• Pellet size can be customised from 5mm to 150mm if required depending upon coal characteristics and process parameters.

• Special heavy duty reagents can be added for high strength, for rapid cure, for high temperature strength, and for enhanced water resistance.

• Pyrite removal can be reduced or eliminated due to various binder combinations to eliminate SO2 due to gas transfer to form CaSO₄ inside the pellet.

• Due to excellent combustion characteristics, high ash fines will ignite and burn with high efficiency.

• Long lasting combustion, with high percentage carbon combustion.

• <20mm biomass dry solids can be crushed and pelletised with fines for high value pellets.

• Contaminated coal or waste products such as sawdust, rice husks, sewage, animal wastes, petroleum coke or waste oil can be included into the pellets.

• Residual ash has negligible un-bumt fuel residue and is excellent for other industrial uses.

• Residual ash can also be pelletised with similar binder reagents for concrete feedstock, aggregate blending and high porosity landfill. The present invention is usable with all types of biomass fines, which will have a varying amount of moisture and sulphur content. Generally, pellets ranging from 5-50 mm diameter are formed, which sized pellets are easily handable, storable, transportable and then burnable, and, if required, in an optimal form and size for grinding prior to burning.

The present invention provides a simple but efficient process for using waste organic carbon-based materials, and forming a useable fuel product, which is easily transportable and efficiently combustible. Rotating drum or pan agglomerators are relatively low cost to build, and are capable of very high tonnage throughputs. Customised products can be produced and the present invention enhances the economics of ash and sulphur removal in coal upgrade plants.

Low technology applications in countries where there is little investment for efficient biomass process plants can also easily utilise the present invention, therefore allowing the provision of high efficiency, environmentally friendly and cost effective process plants to be manufactured and operated. In such places, any materials not immediately useable are currently treated as waste and simply stockpiled in bigger and bigger piles, increasing the environmental hazard thereof.

Example 1

Using a biomass waste water sludge material, this had an initial analysis of:

Results of pellets tested in Figure 4 and formed by the process of the present invention were:

The pellets clearly have a reduced moisture content, and an increased BTU/lb, without any significant energy input.

Claims

1. A process for producing rigid fuel pellets from a biomass material and a binder, comprising the following steps:

a) admixing the biomass material and binder, and b) agglomerating the so-formed mixture by tumbling at ambient temperature to form the rigid pellets; and c) drying the rigid pellets using a flow of air,

2. A process as claimed in Claim 1 wherein the biomass material is one or more of the group comprising: wastewater sludge, sewerage sludge, chicken litter, bonemeal, spent mushroom compost, wood, plant residues including rape seed, hemp seed, corn and sugar cane residues.

3. A process as claimed in Claim 1 or Claim 2 wherein steps (a), (b) and (c) are carried out contemporaneously.

4. A process as claimed in any one of the preceding claims wherein the drying step is performed for less than 1 hour, preferably less than 30 minutes.

5. A process as claimed in any one of preceding claims wherein the drying step is performed using a rotary drum.

6. A process as claimed in Claim 5 wherein the rotary drum is perforated for the flow of air.

7. A process as claimed in Claim 6, wherein the perforations are covered by a protective mesh to prevent pellets from being displaced or lost from the drum whilst allowing the ambient air to pass through the drum.

8. A process as claimed in any one of Claims 1 to 7 carried out as a single stage process.

9. A process as claimed in any one of the preceding claims wherein the process is carried out without requiring a separate active curing step or steps.

10. A process as claimed in any one of the preceding claims wherein the binder is partly, wholly or substantially sodium silicate or potassium silicate.

11. A process as claimed in any one of the preceding claims wherein the process includes the addition of a particulate carbon-based material, preferably coal fines or dust.

12. A process as claimed in any one of the preceding claims wherein the biomass material and/or binder mixture includes water.

13. A process as claimed in any one of the preceding claims wherein the moisture content of the biomass material is up to 20 or 25wt%.

14. A process as claimed in any one of the preceding claims wherein a majority of the dry solids content of the biomass material is of a size of 1 mm or less.

15. A process as claimed in Claim 14 wherein the dry solids content of the biomass material being 1 mm or less in size includes at least 40% of such material being of a sub-micron size.

16. A process as claimed in any one of the preceding claims wherein the binder includes one or more surfactants.

17. A process as claimed in any one of the preceding claims wherein the biomass material and binder are at least partly mixed with agitation.

18. A process as claimed in any one of the preceding claims wherein the binder is sprayed onto the biomass material.

19. A process as claimed in any one of the preceding claims wherein the biomass material is moving prior to and/or during mixture with the binder.

20. A process as claimed in any one of the preceding claims wherein the tumbling is carried in a rotary drum.

21. A process as claimed in any one of the preceding claims wherein the mixing of the biomass material and binder occurs by the tumbling.

22. A rigid fuel pellet formable at ambient temperature by agglomeration of a biomass material and a silicate-based binder including one or more surfactants, and drying with a flow of air.

23. A fuel pellet whenever formed by a process as defined in any one of Claims 1 to 22.

24. A fuel pellet as claimed in Claim 22 or Claim 23 ready for incineration.

25. A fuel pellet as claimed in any one of Claims 22 to 24 having a hardened shell.

26. A fuel pellet as claimed in any one of Claims 22 to 25 having a variable density towards its core.

27. A fuel pellet as claimed in any one of Claims 22 to 26 having sufficient rigidity after tumbling to allow handling, stacking and/or transportation without any significant breakage.