The Potential for Small-Scale Power Plants Using a Mixture of Coal and Bio-fuels
Dr. Michael C. Clarke
Managing Director, M.E.T.T.S.
Presented at the: The Third Intra-Asean Coal Workshop and Conference, Cebu City, Philippines October 1992.
Indigenous bio-fuels can be a useful and significant source of energy in the developing countries of S. E. Asia. In some situations these fuels will need to be supplemented with local or imported fossil fuels (especially local coal) to maintain an even energy output. Atmospheric fluidised bed combustors are suitable for burning the bio-fuels, allow fuel supplementation, are environmentally friendly and are a technology that is not excessively complex. Conventional grate systems offer advantages in terms of established technology and in many cases increased efficiency. Both these types of combustion should be considered in the light of fuel properties, when choosing a combustion system.
Many areas of South East Asia are energy resource deficient, in terms of major supplies of recoverable fossil fuels. There are however numerous examples of small to medium fossil fuel resources that could not support major plant [100MW(e) and above] but could provide significant electrical power close to those fuel resources - and often at the peripheries of national power grids. The fossil fuel resources can often be extended by readily available bio-fuels. The fuel resource inventories of these areas can thus be extended beyond the limits set by the economic recovery of fossil fuels alone.
The fossil fuels will include poor and low rank indigenous coals, coal washery rejects, low energy natural gas and recovered and low quality fuel oil. In this paper the emphasis is given to minor coal resources that can be matched with bio-fuels found in the South East Asian region. Examples of resources of bio-fuels are discussed, along with techniques for supplementation. These resources include domestic waste, rice hulls, sugar based stillages, palm oil effluents and bagasse and field waste from sugar farming.
The expertise required in providing the hardware and management for utilising these fuels is described, in the light of some projects that are presently being developed. The role of local people in making such systems work is emphasised, as will economic opportunities arising from such projects.
2.1 Classes of Bio-fuels
Bio-fuels fall into three classes. These are, material produced in dedicated fuel production plantations, bi-products of other agricultural activities and materials that could be classified as wastes. The economies of using such fuels will vary on which class the fuels belong to. The first class will require that it is economic to use the fuels as a primary energy source. Dendro-thermal projects are examples of the first class - dedicated plantations.
The Philippine experience with dendro-thermal plants has not been good. The attempt to use very poor quality land to grow the lipil-lipil tree was unadvisable. The cost of maintaining equipment and providing forest management during the long period of forest establishment could not be met in the financial framework that was provided for the projects. The cost of fuel harvesting was also found to be greater than the initial optimistic forecasts made out. Other schemes of growing fuel oils (eg. jojoba - Australia) have been equally unproductive.
The second class, fuels that are produced as a processing by-product, offer better economic opportunities. Bagasse, saw-mill refuse, rice hulls and palm oil pressings are all used as fuels. The manufacturing process leading to their creation produces a concentration of material (that although of relatively low specific energy when compared to oil or coal) is still valuable because of its immediate availability. The draw back with some of these fuels is that they are produced on a seasonal basis.
To run a power plant using such fuels on a continuous basis, fuel storage would be required. Given their relatively low specific energy (eg. Bagasse - 6.5 MJ/kg at 50% moisture, Rice Hulls - 9MJ/kg as milled) the storage of significant fuel stockpiles to allow for continuous year-round power production would require very large areas. Such fuels when stored can degrade from oxidation originating from bacterial action, can absorb additional moisture and may be liable to spontaneously combust. These fuels are therefore best to be considered as seasonal energy sources.
True wastes, the third class, include materials that have only marginal fuel value, and may have a negative net usable energy potential. These materials often require a considerable energy input into their collection and preprocessing. Their use as fuels is dependant on other benefits such as environmental hazard reduction being the primary reason for collection and combustion.
An example of such a waste is sewage sludge. Sludge cake is generally autothermal at greater than 29% solids. If cake can be produced at over 33% solids, the combustion of the waste can produce heat that is worthwhile recovering. The cost of producing a cake of 33% solids can however be considerable. Both physical (centrifuging and belt presses) and chemical (flocculants and filter aids) processes are needed to produce cake of high solids concentration.
Where anaerobic sludge digestion is used to reduce sludge quantities, gas produced in the process could be available for use in later combustion processes. The gas which would contain between 60 - 65% methane (with the remainder being mostly carbon dioxide and water vapour) can be considered a poor quality but useful fuel.
Municipal wastes (garbage) can have significant fuel values. Western garbage has been measured as having specific energy values of between 9 an 14MJ/kg, whilst Philippine garbage has been measured at 6.5MJ/kg. Very high energy costs can however be incurred in collecting the garbage, however since it is an environmental and social responsibility that the garbage be collected and disposed of, the cost of collection should not be assigned against the energy available from the material. The cost of specialised sorting and other handling procedures required for garbage combustion (as against sanitary landfill) however should be charged against the energy output.
2.2 Environmental, Social and Economic Considerations in Combusting Bio-fuels
Environmental considerations in combusting bio-fuels vary with the fuels. Bagasse has a low sulphur content, but with it high moisture load the combustion efficiency will be low and much water vapour will be released per unit weight of fuel. Rice hulls contain around 20% silica. If the combustion is not well controlled, the carbon content of the rice hull ash will be high making the ash useless for processing industries that could otherwise use it as a valued silica source. Other uses, such as soil conditioning, for partially burnt rice hulls must be found if a secondary environmental hazard is not going to be created by the primary combustion process.
Municipal garbage varies with the lifestyle of the population that produces it. Western and developed Asian populations produce high specific energy garbage. If not sorted either at source or on delivery to the garbage processing points adjacent to the combustion units, such garbage will contain high energy plastics (eg. PET bottles, plastic wrappings, blown plastic food containers). Some of these plastics will also have the potential to release pollutants such as chlorine, furans and dioxins. Glass contained in such wastes has the potential to fuse whilst metals may melt, volatilise or simply form incombustible entities that need to be removed from the combustion system.
The cost of sorting a "developed" populations' garbage would need to be addressed in apportioning costs on any combustion process. In a developing nation, the cost may be totally off-set by the return from recycled materials. The social benefits from the added employment prospects for garbage sorters would also have a major beneficial impact.
Of the true wastes, municipal garbage perhaps has the greatest potential to be used as a fuel. Production is normally reasonably constant (in both volume and content) throughout the year. Long term variations can be planned for by careful demographic studies. In a developing nation, variations in specific energy may occur with variations in moisture content. New handling techniques may need to be installed to lessen the potential for severe wetting of the garbage.
Good technology can reduce the environmental impact to acceptable levels. Gas emissions can be reduced even in older technology by carefully monitoring the combustion conditions.
2.3 Fuel Supplementation
Bio-fuels of the second and third classes are likely to need supplementation to maintain energy outputs where electrical energy is the desired product. The supplementation may need to be consistently necessary in terms of both availability and level of supply, always available but in varying quantities or may only be required on an intermittent basis.
Sewage cake will often require a constant supplementary energy input. Domestic garbage is likely to require some supplementary fuel, with the level of addition being primarily dependant on moisture content. Bagasse and other field wastes may be able to supply a consistent power output for a large part of the year, but a complete substitution may be required for the rest of the year.
Fuel supplementation is easily achieved by the use of fluid fuels. Natural Gas with a specific energy of 50MJ/kg and Diesel with a S. E. of 45MJ/kg are two obvious choices in terms of ease of storage and use, low pollution potential (low sulphur) and high energy content, however both fuels are relatively expensive, and in the case of the Philippines would most probably need to be imported. Cheaper bunker fuels are available, that although having excellent specific energy values, also have significant sulphur contents.
Local coals with specific energies of between 18 and 26MJ/kg are available. Unfortunately these coals often have significant sulphur contents (1 - 4%), and high ash levels (to 30%). These coals are found in small but significant deposits in many locations that are in reasonable proximity to major bio-fuel resources. Some examples would be Cagayan and Isabela (Lignite) regions of Luzon, Southern Samar, Negros, and Malangas and Bislig regions of Mindanao, of the Philippines, and the Chaing Mai region of Thailand.
Coal requires more infrastructure to use as a supplementary fuel than do fluids. Sampling for moisture, specific energy and sulphur would need to be regularly carried out, in order to regulate energy release and implement environmental controls on gaseous emissions. In some instances the nature of available local coals will be the determining factor in the choice of combustion system.
The ash fusion temperature, as influenced by the alkali metal content, will have a major influence on technology selection. The heat release rate and the working temperature of the system will determine the efficiency of energy conversion. In considering mixed fuels and fuel supplementation, a system compatible with each fuel type and mixtures of those fuels will need to be chosen.
3. TECHNOLOGY AND COMBUSTION
3.1 Fluidised Bed Combustion
Combustion using fluidised bed systems is not new. Three types of fluidised bed combustors (FBCs) are commonly found. One is the circulating bed where material fed to the combustor is picked up be a very strong blast of hot air. The fuel is burnt as it is blown up through the furnace. The unburnt particles are trapped in a high temperature cyclone, and returned to the bottom of the bed for re-cycling. This type of unit could be called a 'blow through combustor' since residence time in the unit is very small, with all particles being picked in whirlwind of hot gas.
The circulating bed combustor is efficient in burning a regular supply of quality fuel, but can be unforgiving where fuel supply is variable with respect to both the rate of addition and quality. It has a distinct advantage in encouraging the burn-out of combustible residuals in fuel that contains considerable inert matter. This type of unit requires a powerful and very reliable fan system and the provision for preheating the incoming air to the inflagration temperature of the fuel.
The second type of fluidised bed combustion system that is commonly found, is the one that uses a bubbling bed as the reaction zone. This is a true fluidised bed, in that a stream of air keeps a mixture of bed material and fuel in a 'fluidised' state. Here each bed or fuel particle is surrounded by hot gas, with the only particle-particle contact being instantaneous collisions. These types of combustors are not new, with one Australian engineer quoting a Chinese colleague, who stated that there are over two thousand such units operating in China.
These types of beds could be likened to quicksand, with particles of solid fuel being suspended in the mass. The bed material (often a course sand) is heated by the burning fuel, and which in turn acts as a source of heat to heat up new fuel particles as they enter the mass. If liquid or gaseous fuels are used in such a system they become part of the fluidising gases.
A high-bred of the bubbling bed system is the pressurised bed reactor, where the entire system is enclosed in a pressure vessel. These last units offer the possibility of combusting toxic wastes at lower temperatures than would be required at atmospheric pressure. Both circulating bed and pressurised bed reactors are considerably more expensive than the atmospheric bubbling bed type. Both systems would also require greater maintenance and have a high internal energy usage.
All fluidised bed combustion systems offer the prospect of reducing sulphur and chlorine emissions resulting from the combustion of 'dirty' fuels. By using limestone as a component of the bed material (in excess of two moles of calcium to every mole of sulphur or chlorine), the neutralisation of a large portion of the sulphur and chlorine will be achieved.
The air draught is provided by blowers, and will be injected through small stand-pipes. The efficiency of the reactor may be increased by preheating the air draught with waste heat from the system. If wet or sticky waste material is fed to the combustor, the violent movement of the hot sand particles, combined with the high draught, will quickly break up any lumps and ensure complete and rapid combustion.
Light particles such as rice hulls, should be fed to the combustors combined with other heavier fuels. A rice hull/coal slurry delivered to a fluidised combustor beneath the expanded bed surface would produce a well burnt out rice hull particle, with little erosion potential for waste heat boiler tubing.
Using this type of combustion it is very important that the temperature is not allowed to rise above the fusion point of the bed material and ash produced during combustion. As a rough rule the temperature should be kept below 950°C. Some mixtures of ash and bed material may fuse below this temperature, whilst some are safe at over 1000°C. Test work may be needed to ascertain the ideal temperature, and prevent tears after the combustor has been commissioned.
Where coals of high sodium content are used for fuel supplementation, great care will need to be taken to run the combustors at temperatures where general or spot ash fusion does not occur. Likewise bio-fuels will need to be monitored for sodium content. An example is an Australian incinerator (non FBC) where sodium, derived from domestic garbage, combined with 'hot spots' has caused slagging in the system.
In the described in the diagram at the end of this paper the hot gas passes from the bed region through the boiler, cyclones, bag filters and finally up the stack. The cyclones are designed to remove the coarse products of combustion whilst the fine ash (-20mm) will be removed by the filters. The gas temperature on leaving the stack should be below 200°C. An alternative arrangement is to have a primary waste heat boiler almost directly above the combustion chamber or as part of the chamber, with a secondary waste heat boiler (economiser) in the position of the one shown in the diagram. This system is usually more thermally efficient, but can also give increased maintenance problems.
Gaseous fuel, possibly produced from anaerobic sludge digestion, can be injected into the bed (especially useful for initial ignition) or could be injected in the position shown - item 9. By using such an over-bed burner, coupled with heat sensors in the furnace area, a higher temperature can be achieved in the boiler (with increased efficiency) and a more stable temperature regime can be achieved. This system can take advantage of a cheap, poor and variable quality fuel being fed to the main combustion area, with the performance of the overall system being ensured by the use of a more expensive gaseous fuel on a demand basis.
Diagram of a Fluidised Bed Combustion System.
[ diagram to be inserted here ]
3.2 Stoker and Grate Firing Systems
This technology is well known. Singapore has three stoker combustion units. The combustors were designed to burn a 'rich mans' garbage where little recycling of glass, metal and plastic was practised. These units burn garbage with a fuel value of 6MJ/kg.
Stoker systems can handle feeds with widely varying particle sizes and particles of varying specific energies. 'Lumps' are deposited onto the grate at the feed end, are transported through the system where they combust or not combust and are then deposited into the ash receiving system. Screening and other sorting can then be used separate the 'burnt' material.
3.3 Fluidised Bed Combustion vs Grate Firing
Fuel properties should be the primary consideration in choosing the system of combustion.
Fluidised bed combustion will be favoured where:
1. 'Lumps' that cause loss of fluidisation can be kept out of the feed. Fuel sorting, the screening and pulverisation of fuel and removal of low temperature fluxing agents (eg soda glass) may need to be practised,
2. Uses are identified for well combusted and evenly sized ash product,
3. It is necessary to use sulphur and chlorine neutralisation during combustion and
4. Fuel particle abrasion is required for complete combustion.
Grate Systems should be considered where;
1. It is desired not to preprocess incoming fuel,
2. Temperatures over 950°C are needed for successful combustion,
3. NOx emissions will not cause significant hazard and
4. No problems are foreseen in sorting the burnt product or in disposing of that material in bulk.
Supplemented and mixed fuel systems situated at the periphery of grids that can produce between 10 and 50 MW(e) would contribute greatly to the electrical capacities of the Philippines and other developing countries. These units would be designed to use local fuel resources, that would often be a mix of local bio-fuels and fossil fuels. An example of a fluidised bed system suitable for burning such fuels is given below.
A proposal for two 18MW(e) units for Cebu is currently being put together. The units will be garbage fired, with local coal supplementation. Atmospheric fluidised bubbling bed technology will be used for combustion. The units will consist of materials handling and sorting, combustors, waste heat boilers, gas cleaning systems, steam turbines and 60Hz generators. They will supply around 14% of the present Cebu grid.
Other opportunities exist in Thailand, Malaysia, Vietnam, Indonesia and Laos. Detailed fuel resource are needed to identify specific locations where there is a power need, a source of bio-fuel and a small but significant fossil fuel resource.
By using non-propriety technology that is of relatively simple design and construction, South East Asian countries will be able to participate in the supply of technology to their own utilities and to the utilities of their neighbours. Sometimes a small loss of efficiency may be experienced by the adoption of this technology, but capital savings will be made during construction and operating savings will be made from the simplified maintenance procedures required for such equipment.
Fabrication workshops, as found in many parts of S. E. Asia are capable of producing such items as stacks, hot gas ducting, conveyor structures, boiler parts (with fabrication), wet scrubbers, combustor structures, fans and plant superstructure. Some items such as turbine/generators and major drives may need to be purchased from suppliers in developed nations. Skills such as refractory construction, sensor placement and wiring, and automation installation and control can be passed onto the local workforce if required.
Recycling and the use of wastes is a worldwide movement. Any waste management schemes as described above will involve the participation of the local population. In a S. E. Asian context materials recycling coupled the maximum use of indigenous resources make very good sense. Waste management including collection, sorting, storage and use is labour intensive. Waste management can provide much useful employment with sound environmental responsibility.
The use of indigenous bio-fuels to produce significant electrical power is possible. Supplementation with fossil fuels may be required to ensure even power production. Fluidised bed combustion systems offer a means of burning bio-fuels, with or without fossil fuel supplementation, with minimal environmental damage.
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3. Barford J. P. and Clarke M. C. High Rate Anaerobic Digestion of Wastes for Energy ......, Clean Energy and the Environment, University of the Philippines, February 1992.
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8. Resource Recover at Palm Beach, Keith T. R. et al. Babcock and Wilcox. International Conference on Municipal Waste Combustion, Hollywood, Florida, USA, April 1989.
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SOURCE: M.E.T.T.S. Pty. Ltd. Website http://www.metts.com.au
Author: Michael C Clarke
(Michael C Clarke on Google+)