Fuel Product and Process
A process for producing fuel pellets is described. The pellets are obtainable from a particulate carbon-based material and a binder, the process comprising the following steps: admixing the material and binder, and agglomerating the so-formed mixture by tumbling. 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. No mechanical compression force is required, and with the binders used, the process can be carried out at ambient temperature. The process provides a simple but efficient process for using waste 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 process enhances the economics of ash and sulphur removal in coal upgrade plants.
The present invention relates to a fuel product and a process for making same.
A continuing problem in many solid-based fuel extraction processes is dealing with waste ‘fine’ materials. As much as 10% of run-of-mine coal can end up as fine (generally about <3 mm) or ultra fine (generally about <0.1 mm) coal dust. This fine coal is often unsuitable for the end process and, even where the size is not a problem, retains large amounts of water (10%-30%) which can make it “sticky”, difficult and inefficient to handle transport and burn.
One solution has been to form briquettes. These are formed by compressing the fines at very high pressures to physically form a secondary fuel material. However, the high capital and operating costs of briquetting plants have prevented their use beyond some high cost countries. In many places, coal fines are currently simply ‘dumped’ near the coal mine.
Another solution is to agglomerate carbonaceous fines using various processes, including pelletizing and extruding. For this, various binder materials have been suggested. In U.S. Pat. No. 4,219,519, the major material of the bonding agent is lime or an associated calcium compound. U.S. Pat. No. 3,377,146 lists various organic binders, and U.S. Pat. No. 4,357,145 suggests tall oil pitch. U.S. Pat. No. 4,025,596 describes a method for pelletizing finally divided mineral solids using a latex, optionally with bentonite or starches.
However, all of these processes involve the need for some sort of treatment of the pellets after their formation, generally drying at an elevated temperature so as to provide the final form of the pellets. Thus, all of these processes require some form of heat treatment, usually in line with the use of one or more organic binders. More importantly, all these processes are over 20 years old, and none are known to have been actually used, or used with any success.
Another problem is the weight of moisture. High moisture levels in coal make transportation and combustion inefficient. Sub-bituminous coals, which comprise a large and valuable part of the world's coal reserves, contain “chemically attached” moisture within the coal structure (up to 20%-30% moisture). This “moisture” severely limits the use and value of sub-bituminous coals. For example, for every 3 truckloads of coal that is transported, one truckload of water must also be transported. That moisture also takes (robs) energy from the flame (to turn the water into steam) as the coal is burnt. Attempts to drive the moisture out by heating have proved unsuccessful because the coal falls apart as it dries and also becomes susceptible to spontaneous combustion. As a result, very little sub-bituminous coal is traded internationally.
Another industry using the briquetting process is the peat industry. To form a suitably crushable material, the peat must be significantly dried, often two or three times, as well as shredded and crushed, adding to the overall cost of forming the briquettes.
It is an 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 particulate carbon-based material and a binder, comprising of the following steps:
admixing the material and binder, and agglomerating the so-formed mixture by tumbling to form the rigid pellets,
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 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 required fuel pellets formed by tumbling of agglomeration to be actively cured with heat and/or (forced air) pressure before such fuel pellets were 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. No mechanical compression force is required, (with its attendant low production rate and high cost), and the process of the present invention can be 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.
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 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 inorganic binders, as opposed to the generally organic materials used as binders in prior art processes. The use of inorganic binders reduces the complexity of the process, and again reduces the need for any pre-treatment or mixing of binder materials. The use of an inorganic silicate-based binder has two further advantages. Firstly, such binders do not impact on the burn quality of the carbonaceous material (as they do not burn), in contrast with organic materials such as starches, (which do burn, and which therefore effect the burn quality and thus heat content value of the formed material). 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 to provide the final form of the fuel pellets. In view of the present invention, such curing can occur 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, 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 stored, stacked, transported etc as they cure.
The concept of curing as used herein includes any 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 hardened shell. As such, it is not intended that the present invention provides any separate drying step or action, (being in relation to one or more liquid materials or substances, such as water, evaporating from the pellets as they are formed and cured). Any such post pellet-forming drying action is regarded as secondary or minor compared to the act of forming and curing the pellets.
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.
In one 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 particulate material and the binder.
For example, listed below are various types of mined coal, and their generally found moisture content (m/c) as the coal is mined, their heat content (h/c) and their carbon content.
The heat content of coal can be directly linked to the moisture content. Therefore, the heat content of high grade anthracite with a moisture content of 15% will have a heat content of 26-33 mj/kg on a moist mineral-matter free basis. At the other end of the scale, lignite, the lowest rank of coal, will have a moisture content of up to 45%, with a heat content of only 10-20 mj/kg on a moist, mineral matter free basis.
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 ½ 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. As mentioned above, 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 particular advantage that the present invention can 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 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.
The process of the present invention is directly usable with moisture-rich coal fines and similar products, as any water content of the binder can be reduced in line with the level of moisture in the coal 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.
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 burn value or the like, which, in particular, could suit the local economic conditions for the fuel source. Different locations and countries mine different types and grades of coal, and they therefore use such coals 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 a waste material 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 particulate material. The calculation of this binder to water adjustment is dependent on the moisture content of the particulate material.
According to another embodiment of the present invention, the particulate material is generally of a maximum size or grade of 3 mm or lower. 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 the peat material may still be required to provide a particulate material suitable for the present invention, but to a much lesser extent than that required for briquetting.
More preferably, the particulate material has a range of sizes or grades; preferably biased towards fine or finer particle sizes.
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 sewerage wastes, biomass, animal wastes and other hydrocarbon materials that could be considered a fuel source. The particulate material may also be a combination of two or more starting materials or ‘ingredients’, not necessarily premixed, and such as those hereinbefore mentioned, so as to provide ‘hybrid’ fuel pellets.
Suitable materials also include low grade or processed fuels, as well as hitherto ‘waste’ products, whose clean combustion would help lower overall pollution levels.
The present invention is not affected by high ash content or sulphur content in the particulate material.
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 mix, 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.
In most coal-burning power stations, the coal is generally ground into a fine material and then injected into the fuel burner to provide the power. The addition of a sulphur-absorbing agent into the pellet-forming process, along with grinding of the pellets for subsequent use in a fuel-burning plant, therefore provides two particular advantages. Firstly, the ability of the process of the present invention to provide wholly or substantially ‘dry’ pellets reduces the energy input required to effect the grinding of the pellets prior to their burning, as described above, and secondly, grinding of the pellets increases the mixing of the sulphur-absorbing agent(s) with the carbon-based material, thus increasing the efficiency of the sulphur-absorption, and so reducing the sulphur-emission.
There is increasing legislation around the world to reduce sulphur emissions, especially from coal-burning power plants. The present invention helps achieve such reduction without requiring additional or other physical and/or chemical sulphur absorption apparatus or processes such as scrubbers and the like, (which also require regular regeneration to work effectively, which is another energy intense process).
Thus, 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.
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 of the invention may not be wholly distinct.
In one embodiment of the present invention, the binder is coated on to the particulate material. One method of coating is to spray the binder on to the material.
In another embodiment of the present invention, the particulate material is moving prior to and/or during mixing with the binder, and/or the material is in a dispersed arrangement. One particular suitable form of this is a falling curtain of particulate material, such as at conveyor transfers, inside pelletising drums or pans, and from stockpile load outs, etc.
In another embodiment of the present invention, the particulate material and binder are directly and/or immediately undergo tumbling after their contact with each other.
The tumbling action serves to agglomerate the particulate material 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 a particulate material is currently stored or ‘dumped’, rather than requiring significant movement (and therefore cost) for transporting the material 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 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, 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 cure, 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 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. When coal is mined, cleaned and transported, considerable quantities of fine coal (particles less than 5 mm) are generated. The present invention can form this fine coal into approximately 50 mm lumps with very low moisture, without any change to the chemical properties of the coal. The pellets can then be handled, transported and used as normal lump coal.
Following any initial curing, the formed pellets are rested for some time, possibly a number of days such as 3-7 days, to provide or allow for 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.
In another aspect, the present invention provides a process for producing rigid fuel pellets at ambient temperature from a particulate carbon-based material and a silicate-based binder which includes one or more surfactants, the process comprising the steps of:
admixing the material and binder, and
agglomerating the so-formed mixture by tumbling to form the rigid fuel pellets.
According to another aspect of the present invention, there is provided a rigid fuel pellet product formable at ambient temperature by agglomeration of a particulate carbon-based material and a silicate-based binder including one or more surfactants.
According to another aspect of the present invention, there is provided a fuel pellet product whenever 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 of the present invention is ready for use as a fuel in many situations, e.g. domestically such as in a home fire, industrially, such as in a power plant, etc.
The product is formed from currently ‘waste’ materials, thereby increasing the efficiency of current solid-fuel extraction and production.
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:
Fine coal recovery systems are now a common part of modern coal process operations, but there has been a requirement for a cost effective high tonnage solution for utilising the wet coal fines generated by the various beneficiation (benefaction) processes.
High capital and operating costs of briquetting plants have prevented numerous operations from maximising their coal reserves. Briquetting is a process where some type of material is compressed under high pressure. Compression of the material causes the temperature to rise, which makes the raw material liberate various adhesives.
There are low-priced hydraulic briquetting presses which are designed to operate for only a number of hours a day. Bigger mechanical presses are used for large-scale installations making hundreds of kilograms per hour, but these require approximately 200 kWh energy input (for drying and processing) per tonne of briquetting material. The cost of this is prohibitive in countries where the cost of coal is already low, such that coal fines are currently simply dumped on nearby ground in many countries around the world.
Similarly, the current method of forming peat briquettes requires initial drying of the dug peat to about 55% moisture, shredding, further drying to a lower moisture content, followed by crushing, followed by high pressure briquetting. Each mechanical step requires significant energy input.
Other waste materials include petroleum coke, a by-product from cracking oil, which is sold off at a low cost.
The process of the present invention allows for use of all these materials in a cost-efficient process, to provide a beneficial fuel product.
The raw fuel feed 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 10-15% (by weight) at most.
Depending on the moisture content and chemical characteristics of the raw fuel feed, 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 coal 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 raw feed, the less water is required to be added at this stage.
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 1-5 m (such as 3 m) 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
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.
Some forming and even some curing may take place in another rotating drum, similar to but having 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 initially cure or receive surface treatment, and thereby allow 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 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.
Final Sizing (If Required)
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.
The pellet sizing could even 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 stockpiled for curing. During this time, generally between 3-7 days for coal fine pellets, and depending on ambient temperature, the pellets reach such strength as to allow full handling. No heating or force draught drying is required. An example of formed pellets is shown in
The spherical shape of the pellets will allow air to move freely through the stockpile to assist the curing process 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 size—as in natural lump coal. This has the advantage of lowering the bulking factor of the pelletised product, resulting in lower transportation costs.
The formed product could then be bagged or stacked and 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.
Process rates can be selected, but production rates of between 10-100 tonnes per hour of coal 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.
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 2 kWh, 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 coals, as the pellets remain stable throughout the combustion process, allowing even for low rank coals to burn efficiently.
The present process is also suitable for fuel products that need to lower ash and sulphur to be sellable. The present process allows fine grinding to release contaminants by gravity or flotation methods, generating a much higher quality fuel source. The process also provides the manner of re-forming the fine pure concentrate into a usable stable and valuable product form.
Sulphur emissions, even from very poor quality coal, can be wholly or substantially eliminated by simple adjustment of pelletising additives, significantly reducing or even possibly eliminating any sulphur dioxide pollution leading to acid rain. 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 pre-heated, rapid and so complete combustion of the content occurs. The pellets maintain their form even at white heat, and show very stable combustion characteristics.
In particular, the process of the present invention can involve no forced drying of the pellets because the action of any surfactant(s) used is maximised in ambient temperatures. Moreover, where water is used, 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.
One further application of the present process is lowering the feed moisture of pulverised coal fuels in power and heat stations, where the coal fines or coal tailings are pelletised and allowed to thoroughly cure and dry before being pulverised and burnt in the furnace. The general moisture content found in current coal fines dumps is usually in the range 12-35%, making them very difficult to use or blend with other feeds.
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 egg shell pelletisation could also be used on sulphide concentrates and iron ores to allow the manufacture of pre-fluxed furnace feeds which can lead to ‘sulphur emission free’ smelter technology. This could be used in existing operations cost, effectively with high industrial tonnage output.
The present invention provides significant benefits compared with present technologies, including:
- <3 mm coal/lignite 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.
- Pellets just air dry while chemically ‘curing’.
- Pellets can be handled by bulk handling equipment when cured or alternatively bagged when ‘green’.
- Pellet size can be customised from 5 mm to 150 mm 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 coal fines will ignite and burn with high efficiency.
- Long lasting combustion, with high percentage carbon combustion.
- <20 mm coal 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-burnt fuel (e.g. coal) 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.
- Lignite and peat can be treated with identical technology or can be blended with other fuel sources to create hybrid pellet fuels with pre-designed characteristics such as smokeless burning.
The present invention is usable with all types of coal 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 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 coal 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.
1. A process for producing rigid fuel pellets from a particulate carbon-based material and a binder, comprising of the following steps:
- admixing the material and binder, and agglomerating the so-formed mixture by tumbling to form the pellets,
- wherein the binder is silicate-based and includes one or more surfactants able to form rigid fuel pellets as the process is carried out at ambient temperature.
2. A process as claimed in claim 1 carried out as a single stage process.
3. A process as claimed in claim 1 wherein the process is carried out without requiring a separate active curing step or steps.
4. A process as claimed in claim 1 wherein the so-formed rigid pellets cure after tumbling at ambient temperature.
5. A process as claimed in claim 1 wherein the pellets form a hardened shell.
6. A process as claimed in claim 1 adapted to provide pellets of a variable size distribution.
7. A process as claimed in claim 1 wherein the particulate material and/or binder mixture includes water.
8. A process as claimed in claim 8 wherein the binder includes water prior to admixture with the particulate material.
9. A process as claimed in claim 1 wherein the particulate material is generally of a maximum size or grade of about 3 mm or lower.
10. A process as claimed in claim 1 wherein the particulate material is coal dust or coal fines.
11. A process as claimed in claim 1 wherein the particulate material is partly, substantially or wholly peat.
12. A process as claimed in claim 11 wherein the peat is in combination with coal fines.
13. A process as claimed in claim 1 wherein the particulate material is a combination of two or more starting materials.
14. A process as claimed in claim 1 wherein the binder is partly, wholly or substantially sodium silicate or potassium silicate.
15. A process as claimed in claim 1 wherein the process includes the addition of one or more further ingredients.
16. A process as claimed in claim 15 wherein the or each further ingredient is selected from the group comprising: lime, inorganic binders, cements and waterproofing additives.
17. A process as claimed in claim 1 wherein the particulate material and binder are at least partly mixed with agitation.
18. A process as claimed in claim 1 wherein the binder is sprayed on to the particulate material.
19. A process as claimed in claim 1 wherein the particulate material is moving prior to and/or during mixture with the binder.
20. A process as claimed in claim 1 wherein the pellets have a spherical or ovoid shape.
21. A process as claimed in claim 1 wherein the pellets are screened after tumbling.
22. A process as claimed in claim 1 wherein the tumbling is carried out in a rotary drum.
23. A process as claimed in claim 1 wherein the process does not require any pre-treatment of the particulate carbon-based material.
24. A process as claimed in claim 1 for reducing moisture in the carbon-based material, preferably to less than 5%, compared with the weight of the moisture in particulate carbon-based starting material.
25. A process as claimed in claim 1 wherein the mixing of the particulate material and binder occurs by the tumbling.
26. A process as claimed in claim 1 further comprising the step of grinding the formed pellets.
27. A rigid fuel pellet product formable at ambient temperature by agglomeration of a particulate carbon-based material and a silicate-based binder including one or more surfactants.
28. A fuel pellet product formed by a process of claim 1.
29. A fuel pellet product as claimed in claim 27 ready for combustion.
30. A fuel pellet as claimed in claim 27 wherein the pellet product includes one or more sulphur-absorbing agents.
31. A fuel pellet as claimed in claim 27 in a ground form, preferably ready for fuel-burning.
32. A fuel pellet as claimed in claim 27 having a hardened shell.
33. A fuel pellet as claimed in claim 27 having a variable density towards its core.
34. A fuel pellet as claimed in claim 27 having a dry interior.
35. A fuel pellet as claimed in claim 27 having sufficient rigidity after tumbling to allow handling, stacking and/or transportation without any significant breakage.
36. A fuel pellet as claimed in claim 27 being wholly or substantially combustible, so as to leave little or no combustible fuel in the ash.
37. A fuel pellet as claimed in claim 27 being formed from a coal dust or coal fines.
38. A fuel pellet as claimed in claim 27 having substantially no sulphur emission during combustion, preferably 70-90% or more reduction in sulphur emission.
39. A fuel pellet as claimed in claim 27 wherein the moisture in the pellet is substantially less than the moisture in the starting particulate material.
40. A fuel pellet as claimed in claim 27 wherein the pellet has a moisture content of <5%.
International Classification: C10L 5/12 (20060101); C10L 5/36 (20060101);