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WEEK 03: GENERATION: ALTERNATIVE SOURCES: BIOMAS OR REFUSE-DERIVED FUEL Sections: Biomass | Waste | Conversion | Boiler | Challengers | Cogeneration Introduction On a global scale, biomass - vegetable matter used as source of energy -- meets a significant proportion or our energy needs. In Ethiopia, Tanzania, and Nepal, it accounts for over 90%. In most other developing countries, wood, crop residues and animal dung provide over 40% of the fuel burnt. Such fuel is the only source of energy for cooking and heating for some 2000 million people.1 Crop residues, such as straw, which were once left to rot or burnt on the fields, can now form the fuel for compact boilers used to heat farms or factories, to generate power, or to fuel industrial processes. Crops rich in starch, and sugar can be fermented to produce alcohol, which is added to petrol to form what is known a. gasohol. Biomass4 is clean, renewable, and plentiful fuel for power generation, but is encompasses many types of agricultural, industrial and domestic waste, and reclaimed materials -- all of which require different combustion processes. Boilers are considered to be carbon dioxide neutral: the quanity of CO2 released during combustion equals that released during natural decomposition in a field.
Fuels. A lot biological refuse are being used as fuel for electricity generating stations. Some are illustrated above with corresponding heat rates. The variability of biomass fuels, both between differing fuels and within one type it is difficult to assign absolute design parameters. Biomass fuels can be divided into four main categories: wood by-products; sugar cane by-products; agricultural by-products; and domestic/industrial refuse. The most common category would include by-products from the forestry and pulp and paper industries such as wood chips, bark, sludges, and reclaimed materials. Wood chips are considered a benchmark biomass fuels for which boiler designs are well established. Sludges and reclaimed materials present problems of fuel moisture and ash constituents. The co-combustion of high concentration reduced sulphur gases from the pulping process can significantly change total sulphur contribution and hence result in severe fouling and slagging problems. THe sugar industry has long burne. bagasse to generate steam for evaporation process. The quality of this fuel depends on harvesting techniques: field burning followed by hand harvesting provides a clean cane and high-quality fuel, but mechancially harvested cane contains more entrained earth and causes potential problems in the fuel ash constituents. Where the bagasses is processed to recover the fiber in a pupl mill, the pith returned to the boiler is typically much finer and more moist. Where the fuel is entirely pith, supplementary firing may be required to stabilize combustion. Agricultural byproducts include nuts, shells, prunings, husks, etc. Generally these fuels have moderate to high concentrations of ash constituents (alkali compounds) which can lead to severe depositis in the furnaec and convective passes. Of all the biomass fuels, the successful combustion of these by-products is the most difficult. Domestic and industrial refuse in power boilers can cause catastrophic corrosion rated and severe fire-side deposition problems. Boiler design for this fuel can be considered as the extreme for high alkali and corrosive environments. Sections: Biomass | Waste | Conversion | Boiler | Challengers | Cogeneration Introduction Domestic and commercial waste is expensive to dispose of in dumps, yet it could form a valuable fuel. A dry weight of 100 million tons of rubbish could replace about 15 million tons of coal. Where waste is dumped into the ground, it can still yield useful energy. Aa rubbish decomposes, it produces methane (the principal component of natural gas). The gas is extracted from buried refuse dumps and fed to natural gas turbines. Even sewage can be used as a source of energy. At their simplest, th. biogas plants consist of cement-lined tanks buried in the ground, which receive human waste and animal manure. Bacterial action produces methane, o. biogas. Sections: Biomass | Waste | Conversion | Boiler | Challengers | Cogeneration
The largest fluidized bed boiler has a capacity of 295 MWth and burns peat and wood wastes. The smallest are only a few MWth. Many of the biomass fuels are good for combustion and do not create any major operating problem. But with some waste materials complications can arise due to high alkali metal or chlorine content, resulting in sintering, fouling and corrosion of heat transfer surfaces. These can be solved by proper design of the process and materials.
Combustion Systems4 There are three main categories of combustion systems used for biomass: pure and partial suspension firinf, and mass burn. 1. Pure suspension firing systems include scroll-type fine burners and are limited to fuel sizes less than 1.5mm and fuel moisture below 10%. Supplementary firing is frequently required particularly at lower loads below approximately 75% of the maximum design heat input on a per burner basis. Fuel is usually taken to the burners using dense phase pneumatic transport similar to pulverized coal installations. The fuel's low moisture does not require significant air preheat and the transport air is usually preheated to less than 149°C -- normally to reduce condensation problems. The mass ratio of transport air to fuel is typically between 1 and 3. 2. Partial suspension firing systems, also known as spreader stoker systems, rely on a pneumatic or mechanical fuel distributor set some distance above a grate with the fuel burning both on the grate and in suspension. These systems allow a high degree of flexibility in firing supplemental fuels. Water-cooled vibrating grates are clearly better for high-moisture fuels, and traveling grates are superior where ash on the grate may slag. Grate heat release rates affect NOx and CO emissions. 3. Mass burn systems include positive displacement grates, roller grates, slopping or inclinded grates, and pile or hearth type burning. Positive-displacement grates are used extensively un the domestic refuse industry and occationally for high-moisture biomass. All amss-burn systems are characterized by extremely low heat-release rates which are seldom economically viable. It also require higher rates of excess air to assure fuel bed penetration. There are often problems in maintaining oxidizing atmospheres on heat transfer surfaces in the combustion zone. Pile or hearth burning requires extensive manual cleaning and has effectively been replaced by grates during the last four decades. Combustion Air Systems4 comprise of primary air, the air in first contact with the fuel either through the grate or as transport air for burners; and secondary air, the air being added to promote combustion turbulence and complete combustion. Primary air systems are provided by air fans through the grate, providing 25% to 40% of the needed air for combustion. Secondary air systems are provided by the admission of air through multiple levels of 'over fire' air nozzles. These are on both the front and rear walls of the boiler. The nozzles are selected and arranged in combination with the secondary air fan static pressure to ensure optimum jet penetration and turbulence, and that opposing jets interlace to promote large-scale turbulence between them. Sections: Biomass | Waste | Conversion | Boiler | Challenges | Cogeneration Biomass Boilers The technologies used for biomass combustion today are: a. Fluidized bed combustion is particularly suitable for high reactive fuels like biomass. CFB boilers range from 25 to 290MWth in biomass applications. Boiler Design development has focused on extending the range of biomass fuels being fired, combined with dramatic improvements in combustion efficiency and operation flexibility. The key elements in boiler design for biomass fuels are the fuel sizing, moisture, and ash constitutents. Fuel sizing dictates the type of firing or combustion system. Combustion air system design a, and firing system design are both influence by fuel moisture. The flue ash constituents have the most dramatic effect on boiler design with ramifications in firing systems, furnace sizing, sootcleaning equipment, convection pass design, and supplementary combustion systems. Fuel drying is an important part of the combustion process with high moisture content biomass fuels. Dryers are divided into two categories. flue gas dryers an. steam dryers The most common are flue gas dryers which use flue gas, taken from two or three locations in the flue gas channel. Two types of the flue gas dryers exist. flash dryer an. mill dryer. In mill drying, where the technology has been adapted from lignite combustion technology, the fuel is dried with hot flue gases prior to blowing into the furnace. Drawback, however, include the release of organic compounds from teh fuel during heat treatment resulting in odor emissions, fouling the electrostatic precipitators and risk of fire in the dryer channels. Drying technologies are now increasingly using steam to avoid these problems. Steam drying method makes it possible to recover the latent heat of evaporation back to process, again increasing the efficiency of energy production. A new steam drying technology is th. bed mixing dryer, utilized with fluid bed combustion. It extracts hot bed material from the fluidized bed and use that as an energy source in drying fuel. The drying takes place at steam atmosphere making it possible to recover energy used for drying by condensing steam in a heat exchanger. Incinerators as Boilers3 In the case of refuse-derived fuels, large scale incineration facilities are built where the combustion can be properly controlled. In many cases, these facilities use the heat to generate electricity. These plants mainly operate on a simple cycle in which steam is generated in the waste incineration furnace. The thermal efficiency is low and output is correspondingly small. In Japanese designed Super Garbage Generating System, a more complicated cycle is used in which the steam from the waste furnace is superheated by the exhaust gas from the gas turbine. Thermal efficiency of 26-30% can be achieved and the plant's output can range from 16.5 to 36 MW. The prime objective is to dispose of the rubbish as effectively as possible and the electricity production is almost coincidental. The Japan Ultra Engineering Corporation (JUEC) is spearheding cogeneration and energy from waste technology. One of the projects of JUEC involved a plant for disposing of used domestic appliances. The appliances will be taken to a disassembly area where they will be broken up manually. Metal parts and other recyclable materials will be separated and sent for reprocessing. The remaining plastic material will be used to produce electricity. The plant will use a process in which the plaastic is shredded and passed through two furnaces where it is heated to 600°C and thermally decomposed. The gaas produced in the furnaces is scrubbed and purified before being sent to a gas turbine driven generator. Some of the gas is taken off and used to heat the furnaces. The solids remaining in the furnaces are removed by a screw conveyor and can be used as road surfacing material. The plant will generate about 36MW of electricity. Although the process is rather expensive, it has the advantage that the gas can be used directly in a gas turbine to give a high thermal efficiency. Sections: Biomass | Waste | Conversion | Boiler | Challenges | Cogeneration Particle Size. Basic firing systems can be divided into three categories depending on the amount of combustion anticipated to occur in suspension. Particle sizes of less than 1.5mm and a moisture content of a maximum of 10% can be fired in suspension through a scroll-type boiler. Higher moisture fuels are occastionally fired in suspension with a grate for final burnout. Fuels of up to 32x75mm are best fired in a spreader stoker. Larger fuels may be fired with a mass-burn type grate. Moisture Content. Fuel moisture varies from 10% for rice husks to 55% for bagasses, and over 55% for pulp and paper residue streams. For a moisture content over 58%, a supplementary fuel is required for stable combustion. Primary air temperature and heat release rate in the combustion zone are the main design parameters affected by fuel moisture. Primary air preheat provide additional ignition energy and stability to the combustion process, redcing furnace pressure pulsations. Ash Constitutents The critical ash constituents for boiler design are potassium, sodium, silica, chlorine, calcium, magnesium compounds and sulfur. Sulphur, in combination with biomass fuels, can lead to increased levels of fouling and both high and low temperature corrosion. The alkali earths, MgO and CaO, are also important with their high-fusion temperatures tending to reduce the deposits arising from the potassium and sodium compounds.
Chlorine Corrosion High chlorine content may create severe hot corrosion problems. Chlorine corrosion starts in the steam tempertaure range of 480-500°C. WOod and esppecially green parts of trees have an effect on chlorine corrosion because of the deposit formation in the superheater. When wood is the main fuel and high live steam temperature (510-510°C) is desired for the power plant process, the following ways control the corrosion of the superheater:
· use coal or peat as secondary fuel with a 30% minimum share; Economic Feasibility The ecomy of a biomass-fired CHP plant and the profitability of the investment depend to a great extent on the local conditions, such as the heat consumers, the price of the available fuels and the volume and the permanence of the heat load. The solutions and practices to reduce the power plant's costs and improved profitability of operations include: centralized operation and maintenance; automation of the plants; fuel treatment methods; boiler and steam cycle systems; and standardization of these methods and systems. The key factors for economical biomass utilization are:
· Large enough plant size Thus, the economic feasibility of a biomass plant must always be studied case by case, because of the significant variation of the economic as well as local factors. Sections: Biomass | Waste | Conversion | Boiler | Challenges | Cogeneration Introduction.5 Biomass-based cogeneration is expected to spread both in residue-producing agro-industries and to purpose-built, plantation-based dendropower stations as renewable energy technoloies are refiend, and as market and environmental presures force governments to look further in cogeneration. This could happen is two international biomass programmes will succeed, namely yhe EC-Asean cogen programme and the UNFAO's Regional Wood ENergy Development Programme [RWEDP]. In Asia, the pioneering nation to launch dendropower was the Philippines a decade or so ago, but this failed because of government neglect. With the renewed interest in cogeneration coupled with its positive impact on power generation carbon emissions, and the global concerns over climate change, the programme has identified a broad array of agro-wastes producers to justify cogeneration. These are namely: rice and sugar mills, wood processing plants (plymills and sawmills) and palm oil factories. Other field crop residues or wood fuels are being studies, to include maize, coconut, groundnut, rubber, rice and wheat straw, sugar cane wastes, among others. There is also the option of purpose-grown biomass; either field crop or trees. AdvantagesThe environmental advantages of cogeneration are also impresive:
a. 30 - 40% reductions in primary fuel consumption due to cogeneration's high efficiency leading to corresponding reductions in Greenhouse gasses [GHG] emissions Institutional Barriers to Entry The reason that the Philippine experience did not succeed were more of institutional rather than technical. Potential investors are often unaware of the viability of modern biomass energy technologies or do not have access to funds. Initial market support is therefore needed. Governments should finance pre-feasibility studies and provide 'obe-window' regulatory agencies to cut through all the red tape. Firm, detailed contracts between parties are also essential. On the biomass side, "biomass feedstock prices and availability need to be guaranteed through firm, long-term pricing and quality agreements. To avoid conflicts, both production and use of biomass for energy should come under the same ownership." On the power side, "the electric utility is, in most cases, the potential purchaser for the electricity generated. Utilities should therefore come up with firm power purchase agreements, attractive to both generators and the utility. This includes long-term pricing agreements, guaranteed minimum plant load and availability factors with default clauses for non-delivery." Institutional encouragement to generate power from biomass could be strengthened by regulations against dumping or leaving the material to rot. 1. Author. TitleBook Title Publisher, Place, Year, page. 1. The Guinness Encyclopedia, 1990, The Guinness Publishing Ltd., Great Britain, p. 306 |
Grate firing was once the dominating technology but today the most common type of boiler in the modern plants is a fluidized bed boiler. Grate fired boilers, however, are still used in small scale and with special fuels. The grate construction is typically underfeed rotating grates or step grates for sizes less than 20MWth.
