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Steam turbine technology is the almost all of electricity generation power plants from biomass used power generation in the world at present. This technology is well established due to availability of cheap or waste biomass in the world. As an example, USA has the installed capacity of electricity generation from biomass around 7000 MW with efficiency of 20 to 25 percent.
The biomass Boiler steam turbine systems are expected to find more applications for electricity generation in the future, particularly in situations where cheap biomass, e.g. agro industrial residues, and waste wood, are available. On the technology side, efficiency of these systems is expected to improve through incorporation of biomass dryers, where applicable, and larger plant sizes as well as higher steam conditions.
The steam boiler turbine arrangement, woody biomass is combusted in a furnace of a steam boiler with fluidized bed combustion. Heat released during combustion is utilized to raise high pressure and high temperature steam. This steam is expanded through steam turbine, which in turn drives an electric alternator. Exhaust steam from the turbine is condensed and returned to the boiler.
Wood fuel is usually shredded to appropriate size and dried utilizing a part of the flue gas, before the fuel introduced into the furnace. This technology has been in existence in many parts of the world, specifically to produce electricity and motive power in the sugar industry utilising bagasse (residue produced after crushing sugar cane) as the fuel.
In this modern version of this technology, wood fuel is shredded into very small pieces and combustion is carried out in a fluidised state. Although this improvement increases the cost of fuel preparation and air supply, it improves the combustion efficiency, thus reducing the operational costs and also reducing stack emission levels. A fluidized bed boiler could accept not only chipped wood but also residues such as rice husk, sawdust etc.
This technology is widely used all over the world to generate electrical and motive power from solid fuel. The modern versions have incorporated many new features to improve operational efficiency, thus reducing cost of operation and to reduce emission levels. Some of these improvements are: Increasing the pressure of boiler, increasing the vacuum in the condenser, combustion air pre heating and steam reheating. Figure 11 schematically shows the principle of this conventional system
Figure 11: Boiler-steam turbine system
Cogeneration is the process of producing two useful forms of energy, normally electricity and heat, utilizing the same fuel source in an industrial plant where both heat/steam and electricity are needed, these requirements are normally met by using either;
1) Plant-made steam and purchased electricity, or
2) Steam and electricity produced in the plant in a cogeneration system.
The second option results in significantly less overall fuel requirement. Steam turbine based cogeneration is normally feasible if electricity requirement is above 500 kW. Biomass based cogeneration is often employed for industrial and district heating applications; however, the district heating option would not be applicable in the tropical countries. A number of studies have been carried out on cogeneration in different agro industries, particularly, sugar mills and rice mills. These show that biomass based cogeneration technology is well established in the pulp and paper industry, plywood industry as well as a number of agro-industries, for example, sugar mills and palm oil mills. Normally, there is substantial scope for efficiency improvements in such cases. For example, bagasse is burnt inefficiently in sugar mills in most developing countries because of a number of reasons, e.g., old and obsolete machinery, disposal problems created by surplus bagasse, lack of incentive for efficient operation etc. Improving the efficiency of biomass-based cogeneration can result in significant surplus power generation capacity in wood- and agro-processing industries; in turn, this can play an important role in meeting the growing electricity demand in developing countries. India has launched an ambitious biomass based cogeneration programme. A surplus power generating capacity of 222 MW was already commissioned by the end of 1999, while a number of projects of total capacity 218 MW were under construction. The total potential of surplus power generation in the 430 sugar mills of the country has been estimated to be 3500 MW.
Co-firing is set up as an auxiliary firing with biomass energy source in coal fired boilers. The co-firing has been tested in pulverized coal (PC) boilers, coal-fired cyclone boilers, fluidized-bed boilers, and spreader stokers. Due to fuel flexibility of fluidized bed combustion technology, it is currently the dominant technology for co-firing biomass with coal. Co-firing can be done either by blending biomass with coal or by feeding coal and biomass separately and is a near term low-cost option for the efficient use of biomass. Co-firing has been extensively demonstrated in several utility plants, particularly in USA and Europe. Co-firing represents a relatively easy option for introducing biomass energy in large energy systems. Besides low cost, the overall efficiency with which biomass is utilized in co-firing in large high pressure boilers is also high. Current wood production systems in most countries are dispersed and normally can only support relatively small energy plants of capacity up to 5-20 MWe, although dedicated plantations can probably support much bigger plants in the future. Thus, biomass supply constraint also favour co-firing biomass with coal (with only a part of the total energy coming from biomass) in existing co-fired plants in the short term.
Whole Tree Energy (WTE) system:
The Whole Tree Energy (WTE) system is a special type of wood fired system, in which whole tree trunks, cut to about 25 ft long pieces, are utilized in the process of power generation in an innovative steam turbine technology that uses an integral fuel drying process. Flue gas is used to dry the wood stacked for about 30 days before it is conveyed to a boiler and burnt. Allowing the waste heat to dry the wet whole tree can result in improvement in furnace efficiency with net plant efficiency reaching comparable value of modern coal fired plants.
A Stirling engine is an external combustion engine; working on the principle of the Stirling thermodynamic cycle, the engine converts external heat from any suitable source, e.g. solar energy or combustion of fuels (biomass, coal, natural gas etc.) into power. These engines may be used to produce power in the range from 100 watts to several hundred kilowatts. Stirling engines can also be used for cogeneration by utilizing the rejected heat for space or water heating, or absorption cooling. A number of research institutes and manufacturers are currently engaged in developing biomass fired Stirling engine systems. For example, the Technical University of Denmark is developing medium and large Stirling engines fuelled by biomass. For 36 kWe and 150 kWe systems, the overall efficiency is about 20 percent and 25 per cent respectively. […..]
Gasification is the process of converting a solid fuel to a combustible gas by supplying a restricted amount of oxygen, either pure or from air. The major types of biomass gasifiers are, Fixed bed gasifier, Fluidized bed gasifier, and Biomass integrated gasification combined cycles (BIGCC)
Fixed Bed Gasification
Fixed bed gasification technology is more than a century old and use of such gasifiers for operating engines was established by 1900. During World War II, more than one million gasifiers were in use for operating trucks, buses, taxis, boats, trains etc in different parts of the world. Currently, fixed bed gasification shows for the most part possible selection into biomass based power generation with capacity up to 500 kW. Although charcoal gasification presents no particular operational problem, the actual acceptance of the technology by potential users is rather insignificant at present, mostly because of low or no cost benefit that it offers. Also, producer gas is less convenient as an engine fuel compared with gasoline or diesel and the user has to have time and skill for maintaining the gasifiers-engine system. However in situations of chronic scarcity of liquid fuels, charcoal Gasifier-engine systems appear to be acceptable for generating power for vital applications. Thus, several gasoline-fueled passenger buses converted to operate with charcoal gasifiers were reported to be in use in at least one province of Vietnam in early 1990s. As reported by Stassen (1993), a number of commercial charcoal Gasifier-engine systems have been installed since early eighties in the South American countries. Wood gasification for industrial heat applications, although not practiced widely, is normally economically viable if cheap wood/wood waste is available. On the other hand, wood gasifiers-engine systems, if not designed properly, may face a wide range of technical problems and may not be commercially viable. Research and development efforts of recent years have been directed towards developing reliable gasifier-engine systems and the technology appears to be maturing fast. Although the demand for wood gasifiers is rather limited at present, a number of gasifier manufacturers appear to have products to offer in the international market. Gasification of rice husk, which is generated in rice mills where a demand for mechanical/electrical power also exists, has attracted a great deal of interest in recent years. The rice husk gasifier design that has found quite wide acceptance is the so-called Open Core design that originated in China; this is basically a constant diameter, (i.e. throttles) downdraft design with air entering from the top. The main components of the gasifier are an inner chamber over a rotating grate, a water-jacketed outer chamber and a water seal-cum ash-settling tank. Gasification takes place inside the inner chamber. The char removed by the grate from inside the gasifier settles at the bottom of the water tank. At present, 120 to 150 rice husk gasifiers appear to be in operation in China. A third of the gasifiers are in Jiangsu Province; these include about thirty 160 kW systems and about ten 200 kW systems. A number of rice husk gasifier systems have been shipped to other countries namely, Mali, Suriname, and Myanmar. A husk gasifier system of capacity 60 kW was developed in 1980s to use in smaller mills in the developing countries. This prototype was successfully used in a mill in China, although no other such unit appears to have been built or used. Beside rice husk gasifiers, several other gasifier models have also been developed in China. Presently, more than 700 gasification plants are operating in China (Qingyu and Yuan Bin, 1997). As a result of several promotional incentives and R&D support provided by the government, gasification technology has made significant progress in India in the recent years. Up to 1995-96 about 1750 gasifier systems (Khandelwal, 1996) of various models were installed in the different parts of India. The total installed capacity of biomass gasifier system in India by 1999 is estimated to be 34 MW. Besides generating electricity for the local community, it is estimated that the project has also benefited about 11,000 people directly or indirectly.
Fluidized Bed Gasification
Fluidized bed gasifiers are flexible in terms of fuel requirements, i.e. these can operate on a wide range of fuels so long as these are sized suitably. However, because of complexity in terms of manufacturing, controls, fuel preparation and operation, these gasifiers can only be used for applications of larger capacities compared with fixed bed gasifiers, typically above 2.5 MW.
Biomass integrated gasification combined cycle (BIGCC) technology
In the gasification – gas turbine technology described above, an overall maximum efficiency attainable is 20%. This could be substantially improved, by raising steam utilizing the gas turbine exhaust and driving a steam turbine. A number of BIGCC power plants are in operation in countries such as Sweden and Finland.
Gasifier-internal combustion (IC) engine technology
In this arrangement, solid wood is first dried and shredded into appropriate size and then converted into a combustible gas in a gasifier. Gasifier is a cylindrical reactor with a throat section, which is narrower than the rest of the reactor. In this throat section, air is introduced through a set of tubes. Wood dried to a maximum of 20% moisture level and shredded into appropriate sizes is introduced at the top of the reactor through an air lock. Up draught gasifiers are widely used for heat applications as they are easier to construct and are more energetically efficient. Such gasifiers are rarely used for motive power or electricity generation purposes due high tar levels in the gas stream.
Chart 01: Gasifier-Gas Cleaning-Engine System
As the material slowly passes through the reactor, it undergoes physical and chemical changes in the many overlapping zones. First the material is dried in the drying zone, losing all the remaining water. Then the material is pyrolysed into solid char and volatiles. In the next zone – the combustion or oxidation zone at the throat of the gasifier, all the volatiles get combusted into carbon dioxide and water. This section liberates all the heat required for the gasification process. In the expanding section below the throat section known as the reduction zone carbon dioxide and steam produced in the upper sections are made to react with carbon, which has reached red-hot stage. In this reduction zone, carbon dioxide and water reacts with carbon to form carbon monoxide, hydrogen, methane and other hydrocarbon mixtures.
The oxidation is essentially an exothermic process liberating heat in the action, whereas the reduction zone is an endothermic process making use of heat. The gas mixture so produced is called producer gas.
Un-burnt materials in the wood end up as ash and are collected and periodically removed from the bottom. Hot producer gas leaves the gasifier at the bottom of the gasifier under the action of an induced draft fan. Air for combustion in the combustion zone is drawn into the section due to low pressure created under the action of the induced draft fan.
Producer gas leaving the gasifier, if mixed with air can form a combustible mixture. It can be used as a fuel in internal combustion (IC) engines or in furnaces or boilers. To be used in IC engines, the gas needs to be treated further. First it must be cooled to improve the volumetric efficiency (to facilitate the introduction of maximum quantity of fuel into the cylinders of the engine). This is done by a jet of water. The water jet also washes away a part of the tar and particulates in the gas. Then the gas needs to be thoroughly cleaned of all traces of tar and particulate matter. This is achieved by passing the gas through a series of filters.
If the gas is to be used as fuel in a furnace or a boiler, the cooling and filtering operations may be omitted.
If the gas is to be used as fuel for IC engine, then the gas mixed in the correct proportion of air is admitted to inlet manifold. In respect of spark ignition type of IC engines (petrol or natural gas engines), producer gas alone can operate such engines. For compression- ignition type of engines (diesel engines), it is necessary to utilise a minimum quantity (less than 5%) diesel fuel as the ignition source in a well optimised engine.
When standard IC engines are fuelled with producer gas, the maximum output of the engine gets de-rated. In respect of spark ignition engines, this de-rating is about 50% (i.e. the new output is 50% of the name plate output). In respect of compression ignition engines, it is insignificant if 30% diesel fuel is used as pilot fuel.
This technology to use producer gas from biomass fuel was popularised during the Second World War in the 1940s. During this war, distribution of petroleum fuel was disrupted and was in short supply. Many countries, particularly, USA and Sweden utilised this technology for transport vehicles. With the end of the war, the supply of petroleum was restored and this technology was discontinued.
With the increase in cost of petroleum in the 1970s with the formation of OPEC, this technology has once again gained popularity, particularly for off-grid application for decentralised electricity production. In many Asian countries such as India, Cambodia and Sri Lanka this technology is becoming very popular for off-grid applications.
In Sri Lanka, this technology was used prior to the introduction of Grid Electricity. In the earlier version, coconut shell charcoal was used as the fuel for the gasifiers. Producer gas from these gasifiers was used to drive slow-speed IC engines. Motive power of the engine was used to drive a single over-head shaft with multiple pulleys driving individual drives. Later, the IC engines were fuelled with furnace oil with injectors and hot bulb. When grid electricity was popularised, these devices were discontinued. At the Government Factory at Kollonnawa, near Colombo, remnants of this system are still available to see.
With the increase in oil prices in the 1970s, interests in new and renewable energy resources surfaced again. A few gasifiers with IC engines were introduced through donor-funded projects. Attempts were made by many research institutions to develop this technology locally. These attempts were successful in varying degrees. With the declining oil prices in the late 1980s, the enthusiasm shown in renewable energy declined. Almost all the gasifiers system in the country became inoperative.
Three years ago, a team of officials visited India to identify gasifier-IC engine systems for local adaptation. Later a 35kWe system was introduced from India by the Ministry of Science and Technology. For the past two years, this has been operating as a demonstrating unit for off-grid electricity generation. This system will be relocated to a rural area shortly to serve an isolated village community.
The 35kWe system consumes 1.6 to 1.8 kg wood per kWh of net electricity generated. Figure 12 below shows a photograph of this system in operation.
Figure 12: 35 kW gasifier-IC engine generator
Gasifier-gas Turbine Technology
The gasifier-IC engine system described in the previous section is more suitable for outputs in the kW to say 1 MW range. To use gasifier system for larger applications in the multiple MW range, gas turbine technology is generally more suitable. A schematic diagram of this technology is shown in 13.
Figure 13: Gasifier – gas turbine technology
Biomass integrated gasification steam injected gas turbine (BIG/STIG) technology
A method of improving the efficiency and output of the above-described BIGCC technology is to inject steam into the gas turbine combustor. This increases the output of the gas turbine without consuming power at the compressor. This technology requires very stringent water purification system and other control measures. At this early stage of biomass technology for power generation in Sri Lanka, such complicated technologies are not considered. Figure 19 illustrates this principle.
Figure 14: Biomass integrated gasification steam injected gas turbine (BIG/STIG) technology
Table03: Typical capacity/efficiency/resource data for biomass power systems
dm tonnes/yr **
Small down draft gasifier/IC engine
High operation & maintenance,
and/or low availability, low cost
Large down draft gasifier/IC engine
High operation & maintenance,
and/or low availability, low cost
Potential good availability, under development, high cost
Good reliability, high cost
Indirect-fired gas turbine
Not available commercially
Rankine Organic Cycle
Updraft gasifier/IC engine
Fixed grate or fluid bed boiler/steam turbine
Fluid bed (BIG/CC) ââ‚¬” dedicated biomass
Fluid bed gasifier –
* Indicative of range for application
** Assumes: availability at 70%, fuel net calorific value 20 MJ/kg
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