The technology can be applied in households, institutions (such as schools) and industries where it is used for boiler heating. Biomass gasification household stoves work by a high temperature conversion of biomass in a restricted oxygen environment to a mixture of nitrogen, carbon monoxide, hydrogen, and methane. The hydrogen and methane are then burned without emitting pollutants. For larger applications pure oxygen may be used which gives a higher calorific value gas without the nitrogen.
Gasification is the process of converting a solid fuel to a combustible gas. To this process usually a restricted amount of oxygen is added, either pure or from air. A carbonaceous solid material can also be gasified to produce a hydrogen-rich gas by bringing it in contact with steam at a high temperature. Air gasification of biomass produces a low calorific value gas, the producer gas containing about 50% nitrogen, and can fuel engines and furnaces. Gasification of biomass with pure oxygen results in a medium calorific value gas free of nitrogen.
According to Practical Action (2007), 2.4 billion people use traditional biomass for cooking, either wood, crop residues, charcoal, or animal waste. IEA (2006) states that an extra 200 million people worldwide will rely on biomass for their cooking and heating needs by 2030. Switching to cleaner fuels and having access to those fuels is one strategy for dealing with the problems of the health effects caused by the smoke and other pollutants released in enclosed cooking areas. The UN millennium project aims to half the number of households using traditional biomass.
Improved biomass cookstoves can aim for 30% efficiency and reduces the amount of wood fuel used and thus decreases pollutant emissions.In small-scale gasifiers, solid fuels are gasified in a fixed bed; these can be of three types: updraft, downdraft, and cross-draft reactor. With the escalating costs of fossil fuels and gas as a preferred cooking fuel (than fuelwood, residues, kerosene and others), biomass gasifiers are attracting renewed interest. The possibilities for biomass gasification technology for cooking applications are leading to a number of initiatives to demonstrate the potential benefits of introducing them in developing countries. Gasification based cooking systems can be classified in to two broad types: gasifier stoves and central gas production with pipe network for producer gas supply for cooking. Gasifier stoves, which are basically compact gasifier-gas burner devices, have been tried since mid-nineties for cooking applications. Several hundred biomass gasifier cookstoves are already in operation in countries such as China and India.
Biomass gasification household stoves work by a high temperature conversion of biomass in a restricted oxygen environment to a mixture of nitrogen, carbon monoxide, hydrogen, and methane. The hydrogen and methane are then burned without emitting pollutants. For larger applications pure oxygen may be used which gives a higher calorific value gas without the nitrogen. The technology can be used at household, community and industry level.
As a result of an EU technology transfer process the Biomass Fuel gas cooker Model JXQ-10A has been commercialised and is marketed by Wattpower.The Wattpower JXQ-10A stove has fast gas production within 1-2 minutes and can be used in continuous operation with easy recharge of fuel or can be shut down for up to 20 hours and then started again by opening the air entry. It is compact (1m x 0.5m x 1m) for household use but incorporates a blower system which requires 80 watts AC electricity.
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In most Asian countries, governments are actively promoting renewable energy projects, including biomass gasification, by providing incentives, although lack of investment capacity remains an important barrier. Firstly, biomass gasification projects are perceived to be expensive as their initial investment costs tend to be much higher than the conventional energy production means. Secondly, it is still difficult to obtain financing for such projects due to financial institutions’ non-familiarity with financing renewable energy projects. Hence, the barriers to a successful application of the biomass gasification technologies in developing countries can be summarised as follows (Asian Regional Research Program in Energy, Environment and Climate Change, 2005):
- Failure of past projects which has negatively affected people’s perception of biomass energy,
- Risk of being the first to fail, and
- Finite size of biomass energy technologies.
Improved funding opportunities encourage the development and demonstration of biomass-based technologies. At the same time, in order to avoid the risk of being the first to fail, technology suppliers need to guarantee the quality of the technology and make spare parts easily available. In order to avoid reluctance to invest in gasification technologies due to non-successful projects in the past, it is important that the latter activities are clearly analysed on shortcomings.
Other barriers to the implementation of biomass gasification technologies are both of a technical and non-technical nature. Important technical barriers are: problems with the feedstocks due to their varying characteristics; gasifier reactor which is fuel flexible, scaling-up aspects and multi-reactor designs; and gas cleaning which is also application dependent (Knoef, 2005).
Though wood handling, storage, drying, comminution and screening are well-established processes without uncertainties in operation and performance, other forms of biomass are not always so well defined and understood. Many harvested grasses can be delivered in bales and these can be handled like straw, but the materials like miscanthus and arundo require specification of harvesting systems and in the case of billets and bundles, reception, storage and handling is not well established. Similarly, good feeding practice is essential for the smooth operation of gasifiers and due to the varying characteristics of feedstock, feeding is often an important technical challenge, especially the feeding of the pressurised systems is seen as problematic due to mechanical problems, gas tightness, etc.
In addition to the development of fuel-specific gasifiers, at present, there is a strong need for fuel-flexible gasifiers that are also able to convert difficult fuels like high alkali and high ash content fuels. Scale up issues should be considered carefully. The designs for more difficiult types of fuels and under very extreme conditions like supercritical water gasification are still under development. The reactor design of gasifiers has not been optimised yet, which requires design and modelling of gasifiers by co-operation between manufacturers and universities. However, there is a confidentiality problem in providing commercial data and funding is not being spent sensibly in supporting the modeling studies when real data is being provided. Hence, the available good models for gasification seem to remain unused.
The consequences of the presence of micro pollutants in biomass and waste-like metals and organo-metals, are unidentified and needs attention in the present context, particularly for synthesis gas production. Gas cleaning for chlorine, sulphur and nitrogen, however, seems to be sufficiently reliable at the moment, although it has very high cost components. Hence, niche markets have to be found for further commercial applications, which requires commercial demonstration plants for long-term testing in association with research and lab-scale experiments. In addition to this, there should be more emphasis on hot gas cleaning such as the removal of sulphur, however, solutions for hot gas cleaning must be cost effective. Similarly, tar removal without producing toxic waste water or cracking is a major point of concern in gasification plants. Another important concern is the quality of the ashes and tendencies for sintering and ash melting; biomass with a high alkali content can create erosion problems in turbines.
Non-technical obstacles to the research, development, demonstration and commercialisation of biomass gasification activities are: financial aspects such as high intial investment costs, competing technologies, long-term contracting, etc.; procedures to acquire permits for gasification installations; and possibly negative public perceptions.
The international development of biomass gasification could be promoted by European Directives, tax measures, fiscal instruments, convenants, action plan biomass, green credits. In addition, networks like the gasification network is a very effective way of supporting development, identifying and prioritising issues, and directing future directions, particularly if they can be provided with financial resources to address the most critical problem areas. Also education and training is recognised as an important tool for promotion of biomass gasification (Knoef, 2005).
There are several existing research, development and demonstration programmes on biomass gasification going on in different countries around the globe. For instance, in India, a large number of gasifier projects have been installed in the field so far, including demonstration projects with grid-connected biomass gasifiers. Among the current objectives of the biomass gasification programme of MNES is promotion of R&D and commercialisation of biomass gasifiers (MNES, 2006). Similarly, in China, the Energy Research Institute of the Shandong Academy of Sciences has developed a crop straw/stalk based gasification and centralised gas-supply system. The system essentially consists of a unit downdraft gasifier (Model XFF-2000) coupled to a gas clearing storage and supply sub-system (China New Energy, no date; Bhattacharya and Salam, 2006).
The University of the Philippines has initiated research on gasification by developing a rice husk-gasifier with two combustion zones and a rather complex gas cleaning train with funding coming from the Philippine Department of Energy (then: Bureau of Energy Development). The primary objective of the project was to demonstrate the technical viability of a small-scale pyrolytic converter using agricultural and forestry residues, especially rice husk. Test runs were conducted at the National Food Authority compound in Northern Luzon. The result showed that pyrolysis of rice husk is difficult, but manageable. The processing of the material is particularly sensitive to temperature changes in the bed and the moisture content of the fuel. Studies on the use of other fuels then charcoal were also undertaken by other agencies such as the Industrial Technology Development Institute and the Forest Products Research and Development Institute under the Philippines Department of Science and Technology. Wood and agricultural waste such as corn cobs, coconut husk and rice husk were analysed and it was concluded that these fuels could be successfully gasified and used to run internal combustion engines for a limited number of hours (Knoef, 2005; Bhattacharya and Salam, 2006).
In Sri Lanka, the National Engineering Research & Development Center is the key institute involved in R&D activities on gasifiers. Several conceptual designs have been developed and tested during the past few decades, but none of them have been commercialised. Therefore, the systems and technologies used at present are basically the designs adopted from India, particularly the systems designed by Ankur Scientific Energy Technologies Pvt. Ltd. Several other research institutes such as University of Moratuwa, University of Peradeniya, Coconut Research Institute, and Tea Research Institute are also involved in studies on gasifiers; up to now they are involved mainly in feasibility studies and field testing rather than technology development. However, at present there is an emerging interest in the subject and these institutes are likely to become involved in the design, development, transfer, and adoption of the technology in the near future (Knoef, 2005; Bhattacharya and Salam, 2006).
The capacity of most developed stoves ranges from 3kWth to20kWth, making them suitable for domestic as well as community cooking applications. Compared to the 5-15% efficiency of traditional cook stoves in the Asian region (Battacharya et al, 1999; Battacharya, 2005), theefficiency of these gasifier stoves is in the range of 25-35%. Some indicative stoves already widely used in the market are:Wood-Gas Cook Stove of Reed and Larson use small wood chips and sticks for operation (Reed and Larson, 1996), producing charcoal as a by-product, Elsen Karstad’s Charcoal Making Wood Gas Cooking Stove for the East African households (Karstad, 1997), the Holey Briquette Gasifier Stove developed by Stanley and Venter (2003) operates using a single biomass briquette with a central hole, The San San Rice husk Gasifier Stove (SSIC, 2005) developed in Myanmar offers smokeless combustion of rice husk in an efficient manner and others.
In terms of social sustainable development benefits, biomass plantation and the gasification of this biomass would help poor rural areas to acquire access to electricity produced in decentralised power plants. This would also create more employment for local people, better opportunities for basic health care, and, as a result, will increase welfare for the rural communities.
Conversion of solid biomass into combustible gas has all the advantages associated with using gaseous and liquid fuels such as clean combustion, compact burning equipment, and high thermal efficiency. Gasifier systems offer economic advantages in locations where biomass is already available at reasonably low prices (e.g. rice mills) or in industries using fuel wood. Biomass gasification technology has the potential to replace diesel and other petroleum products in several applications (see Cogeneration.net). The circulating fluidised bed gasification technology has the advantage of enabling the substitution of expensive fossil fuels with cheaper ones such as wood wastes. In addition, through gasification noxious ash and gas constituents can be separated from the fuel gas prior to combustion in the boiler. It also provides opportunities for replacement of natural gas in heat recovery steam generators.
As explained above, switching to cleaner fuels and having access to those fuels is one strategy for dealing with the problems of the health effects caused by the smoke and other pollutants released in enclosed cooking areas.
Gasification can be considered a clean energy production technology and it can be used to produce fuel gas from different feedstock types. By using gasification instead of direct biomass combustion, non-GHG emissions can be significantly reduced to almost zero and, at the same time, the fuel gas quality can be improved to meet the requirements of different machinery (internal combustion engines included) for heat and electricity production. Gasification has a unique characteristic in that can even convert waste (from municipal solid waste to agricultural or crop residues, like coconut shells, rice husks and straw, wood residues, bagasse, and others) to a high quality energy source. Gasification has the advantage that it separates the noxious substances from the fuel gas prior to combustion. Additionally, the internal combustion engines using fuel gas from gasification emit less pollutant than engines running on petroleum derivates. SO2 and NOx are, normally, absent in fuel gas from biomass gasification (Tsamba, no date).
Apart from being fuel efficient, gasifier stoves are also emission efficient in comparison totraditional cook stoves. The traditional cook stoves, because of their very low efficiency, emitmore than 10% of their carbon as products of incomplete combustion (PIC) comprisingvarying amount of tars. In addition, about 100-180 g of carbon monoxide and 7.7 g of particulate matter are also emitted per kg of wood. Gases such as methane, total non-methaneorganic compounds (TNMOC) and N2O are added to this. These PIC emissions are evenhigher in the case of loose biomass or cow dung used as fuel in these stoves (Grover, 2003).
The greenhouse gas emission reduction effect from using biomass for energy production is subject to some controversy as it is important to consider the full life cycle of the fuel and the materials used in the technology and the products of incomplete combustion. Important aspects to consider are:
- the feedstock used (by-product of sugar production, biomass distillation or from sorghum, or raw materials which could have been used for food production). If ethanol is produced from a raw material that would otherwise have been used for food production, then the greenhouse gas emissions related to producing this food in an alternative manner need to be incorporated as well.
- the supply chain: the distance to transport feedstock to ethanol production site (or whether produced domestically or imported) and the transport mode used,
- the production process itself, and
- what is replaced by the ethanol: e.g. unsustainably harvested wood stove or fossil fuelled kerosene stoves
For calculation of these GHG emission reductions, it is recommended to apply the approved methodology for thermal energy production with or without electricity project (small scale activities) which has been developed under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html.
Several economic studies have been made on the feasibility and long-term prospects of biomass gasification. The first demonstration projects implemented in Europe showed that investment cost are far too high for commercially acceptable activities; investment costs of € 5,000/kWh or higher are not exceptional. However, it is expected that due to learning effects with operational experience and value engineering, the investment costs can be reduced to approximately € 1,000/kW electricity within the coming decade. Another aspect is the operational costs, in particularly the price of the feedstock, which can both be high (e.g. short rotation coppice) and low (e.g. waste residues). Other costs are related to transportation, fuel handling and processing, as well as labour costs, which could be lowered by process control and automation, and maintenance. Remuneration of electricity and heat can also be decisive in the overall economics of biomass gasification investments. Nonetheless, as explained above, in the short to medium term, biomass gasification is unlikely to compete with fossil-fuel produced power. However, several studies have shown that biomass gasification can compete with other renewable energy systems when capital costs can be reduced and favourable conditions created (see Btgworld).
The relative cost advantage of gasifier systems over conventional oil-based systems depends on a number of factors, such as plant size, cost of biomass fuel, cost of fossil fuel substituted, gasifier system cost, operating hours per year, interest rate, etc. The cost of gasifiers depends quite heavily on the degree of automation and instrumentation. The gasifiers currently used in China and India have low levels of automation and instrumentation in order to keep the capital cost low, but their operational and maintenance costs are consequently relatively high.
The cost of biomass gasification systems for thermal applications excluding fuels and ash handling facilities has been reported to amount to about USD 55,000 for a unit that substitutes 100 litres/hour of furnace oil; the cost per liter of oil substituted per hour tends to be higher for lower capacities. The economic and financial aspects of using a gasifier to replace liquid fuels are extremely favourable across different unit capacities; the payback period for a small and medium gasifier is around 6 months. The replacement of traditional, inefficient biomass-based heat is also favourable though not as much as in the liquid fuel case. In this case, the payback period is approximately two years (Gosh et al., 2003).
In 1998, the first US commercial scale biomass gasification demonstration plant based on the SilvaGas process began at the McNeil Power Station in Burlington, Vermont. The SilvaGas process is a particular form of biomass gasification which indirectly heats the biomass using heated sand in order to produce a medium Btu gas. The McNeil power station is capable of generating 50 MW of power from local wood waste products (Knoef, 2005).
In Sri-Lanka, two companies are involved in the design and manufacture of gasifiers. The leading company is EnerFab (Pvt) Ltd., which has recently been established and produces gasifiers for both thermal and electricity generation applications. In the case of electricity generation, the engines used are gas engines imported from China. The capacities of thermal gasifiers are in the range 100 kW to 350 kW. The efficiencies of these gasifiers are claimed to be in the range of 70 to 80% and the installation costs are in the range USD 200-300/kW. There are a few requests for installation of 5 MW range thermal gasifiers based on Indian technology. The capacities of the gasifier, designed by EnerFab (Pvt) Ltd, are in the range of 1 to 250 kWe. The overall efficiency of these plants are claimed to be around 30% and the installation costs are USD 800 to 1,500/kW. The other company involved with the design and fabrication of gasifiers is Puritas Ltd. The company has developed and constructed a 120-kW gasifier for brass melting industry under a project funded by UNDP (Bhattacharya and Abdul Salam, 2006).
China appears to have suitable conditions to develop biomass based technologies because of the vast territory and abundant biomass resources in the country. As an important decentralised power technology, biomass gasification and power generation (BGPG) has a potential market in making use of biomass wastes (Wu et al, 2002). In spite of the relatively high cost for controlling secondary pollution by wastewater, BGPG is economically feasible and can give a financial return because of the low price of biomass waste and insufficient power supply at present in some regions of China. In this work, experimental data from 1 MW-scale circulating fluidised bed BGPG plants constructed recently in China have been analysed and it was found that the unit capital cost of BGPG is only 60-70% of that of a coal power station and its operation cost is much lower than that of conventional power plants (Wu et al, 2002). A next step would be the development of medium-scale BGPG plants with capacity ranging from 1000 to 5000 kW. Generally, BGPG is a suitable technology in, e.g., rice and timber mills in developing countries due to the large amounts of biomass waste available, so that biomass collection and transportation can be avoided and the operation cost can be lowered (Wu et al, 2002).
UNDP and local authorities have launched a sustainable village-level CHP biomass gasification plant in Jilin Province in China based on the modernised use of biomass (agricultural residues from corn). The project seeks to demonstrate a viable business model and commercialisation strategy to promote project replication on a wider scale in rural China. It has facilitated a co-generation facility which obtained a power purchase agreement with the local electric utility to sell surplus power to the grid, producing additional revenue and ensuring the project’s economic sustainability.
Biomass gasification for thermal applications has become commercial in some European countries, although not at a large scale. Eight commercial Bioneer (updraft fixed bed) gasifier plants of capacity 4-5 MWth have been operational in Finland and Sweden since the mid-1980s; another plant of capacity 6.4 MWth was installed in Finland in 1996. For larger applications, fluidised bed gasifiers have been used for a number of applications; co-firing with coal has emerged as the most important option so far. Seven co-firing gasifiers have been identified thus far in Finland in the capacity range 0.5-100 MWth, including two commissioned in 1998 and one not yet realised. The most well-known among these is the 60 MWth gasifier which has been operational at Lahti, Finland, since 1998; the gas in this case is burned in a boiler along with natural gas and coal (Bhattacharya and Abdul Salam, 2006).
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