Biomass

Anaerobic Digestion | Briquetting & Pelletising | Direct Combustion & Cogeneration | Pyrolysis | Gasification | Charcoal Production | Co-firing | Ethanol Production | Biomass Use | Biomass in Australia | Biomass in Asia | Further Information |

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Anaerobic Digestion

Anaerobic digestion is the decomposition of wet and green biomass through bacterial action in the absence of oxygen to produce a mixed gas output of methane and carbon dioxide, known as biogas. The anaerobic digestion of municipal solid waste buried in landfill sites produces a gas known as landfill gas, which occurs naturally as bacteria decompose organic matter over time. The methane gas produced in landfill sites eventually escapes into the atmosphere. However, the landfill gas can be extracted from existing landfill sites by inserting perforated pipes into the landfill (see Figure 11). In this way, the gas will travel through the pipes under natural pressure to be used as an energy source, rather than simply escaping into the atmosphere to contribute to greenhouse gas emissions.

Biogas is most commonly produced using animal manure, mixed with water, which is stirred and warmed inside an air-tight container, known as a digester. Digesters range in size from around 1m3 for a small household unit, to as large as 2000m3 for a large commercial installation (Ramage & Scurlock 1996). The biogas produced can be burnt directly for cooking and space heating, or used as fuel in internal combustion engines to generate electricity.

Alternatively, new landfill sites can be specially developed in a configuration that encourages anaerobic digestion. In these new sites, the pipe system for gas collection is laid down before the waste is deposited, thus optimising the gas output, which can be as high as 1000m3 per hour and last up to 20 years. The landfill gas is generally used for electricity generation, using large internal combustion engines to drive generators of around 500 –1000 kW, to match the normal gas supply rates of around 10 GJ per hour. The largest landfill gas plant in the world is the Arcadis designed and constructed plant, extracting from the 345 acre Bandeirantes landfill site, near Sao Paolo in Brazil. The gas feeds a 23 MW generation capacity and produces 170 million kWh of electricity annually, sufficient to power more than 58,000 households (Lamers, 2004). In Western Australia the total maximum generating capacity from all 9 landfill gas plants are around 18 MW. With an increasing emphasis being placed on recycling, future amount of landfill generated per person may decrease, due to the smaller size of the landfill gas resource.

 

Briquetting & Pelletising

Briquetting and pelletising involves the compaction of biomass at very high pressures. The biomass particles are compressed in a die to produce briquettes or pellets (see Figure 2). These products have significantly smaller volume than the original biomass and therefore have a higher volumetric energy density (VED), making them a more compact source of energy, which is easier to transport and store than natural biomass. The briquettes and pellets can be used directly on a large scale as direct combustion feed, or on a small scale in domestic stoves or wood heaters. They can also be used in charcoal production.

 

Direct Combustion & Cogeneration

Direct combustion is the main process adopted for utilising biomass energy. The energy produced can be used to provide heat and/or steam for cooking, space heating, industrial processes, or for electricity generation.

Small scale applications, such as domestic cooking and space heating, can be very inefficient, with heat transfer losses of 30 - 90%. This problem can be addressed through the use of more efficient stove technology. A major effort to improve the efficiencies of cooking stoves is occurring in many developing nations that use predominantly wood-based fuel to cook in their homes (see Figure 3). This effort is designed to curb domestic woodfuel shortages, environmental degradation and alleviate health problems associated with low efficiency systems and poor ventilation.

On a larger scale, biomass such as fuelwood, forestry residues, bagasse and municipal solid waste, can be combusted in furnaces and boilers to produce process heat (see Figure 4), or steam to feed steam turbine generators (see Figure 5). Power plant size is constrained by the local feedstock availability and is generally less than 25 MW. However, by using dedicated feedstock supplies, such as short-rotation plantations or herbaceous energy crops, the size can be increased to 50 -75 MW, gaining significant economies of scale (Overend 1998). In developing countries, power generation is usually required in smaller increments and feedstock requirements can easily be met by agricultural residues, such as rice husks and nut shells.

The basic technology is proven for selected feedstocks in the 10 – 50 MW size range. For example, the McNeil Generation Station in Vermont, USA, is a typical wood-fired power plant with a capacity of 50 MW (see Figure 6). There is currently (2002) over 7,000MW of biomass power generation in the United States based on steam turbine technology (U.S. Department of Energy, 2006).

Chicken litter, a mixture of straw, wood chips and poultry droppings, is another source of biomass and the Fibrowatt, Thetford project in England, is the largest biomass power station in Europe. The plant, which is based on the design of two existing 12.7 MWe and 13.5 MWe plants, has a capacity of 38.5 MW and is fueled by poultry litter. Larger biomass power stations are currently being constucted in the UK to be operational at the end of 2007.

Large biomass power generation systems can have comparable efficiencies to fossil fuel systems, but this comes at a higher cost due to the design of the burner to handle the higher moisture content of biomass. However, by using the biomass in a combined heat and electricity production system (or cogeneration system), the economics are significantly improved. Cogeneration is viable at present where there is a local demand for the heat as well as the electricity, though this can be more easily exported off the site to a distant user.

 

Pyrolysis

Pyrolysis is the basic thermochemical process for converting solid biomass to a more useful liquid fuel (see Figure 8). Biomass is heated in the absence of oxygen, or partially combusted in a limited oxygen supply, to produce a hydrocarbon rich gas mixture, an oil-like liquid and a carbon rich solid residue. Traditionally in developing countries, the solid residue produced is charcoal, which has a higher energy density than the original fuel, and is smokeless when burnt, which makes it ideal for domestic use. The traditional charcoal kilns are simply mounds of wood covered with earth, or pits in the ground. However, the process of carbonisation is very slow and inefficient in these kilns and more sophisticated kilns are replacing the traditional designs. The pyrolitic or "bio-oil" produced can be easily transported and refined into a series of products similar to refining crude oil.

Gasification

Gasification is a form of pyrolysis, carried out with more air, and at high temperatures, in order to optimise the gas production. The resulting gas, known as producer gas, is a mixture of carbon monoxide, carbon dioxide, hydrogen, methane and nitrogen gasses. The gas is more versatile than the original solid biomass, which is usually wood or charcoal. It can be burnt to produce process heat and steam, or used in internal combustion engines or gas turbines to produce electricity, or even as a vehicle fuel, as was common in Australia and Germany in World War II.

Biomass gasification is the latest generation of biomass energy conversion processes, and is generally being constructed at scales of around 50 MW to obtain good efficiencies, and to reduce the investment costs of biomass electricity generation through the use of gas turbine technology. High efficiencies (up to about 50%) are achievable using combined-cycle gas turbine systems, where waste heat from the gas turbine is recovered to produce steam for use in a steam turbine. Economic studies show that biomass gasification plants can be as economical as conventional coal-fired plants (Badin & Kirschner 1998). However, gas cleanup (removing corrosive products) to an acceptable standard for use in turbines remains the major challenge yet to be overcome.

Commercial gasifiers are available in a range of sizes and types and can be run on a variety of fuels, including wood, charcoal, coconut shells and rice husks. Power output is determined by the economic supply of biomass, which is often limited to a maximum of 80 MWe in most regions (Overend 1998). The first gasification combined-cycle power plant in the world was the 6 MW facility at Varnamo, Sweden, which is fueled by wood residues.

The proposed wood-fired gasifier to replace natural gas and reduce air pollutants from the 79 million litre annual production of ethanol in central Minnesota, will become the largest biomass gasifier in North America. The world’s largest biomass-fired power plant is the Alholmens Kraft Power Plant in Finland, with a steam capacity of 55 MW and an electrical capacity of 240 MW. The fuel for the plant includes bark, sawdust, woodchips and peat, and has the capacity to be fully operational using coal as a backup fuel (see Figure 9).

 

Charcoal Production

Charcoal production is a form of pyrolysis with very limited available oxygen, where the vapours and gases are driven off. Modern charcoal furnaces operate at about 600°C and produce 25 - 35% of the dry biomass feed as charcoal, and the waste gases can be used for kiln drying. Traditional earthen kilns (as used in many developing countries) have yields closer to 10%, as there is little control over how the process occurs (Twidell, 1998). Quality charcoal is 75 - 85% carbon, and is useful as a compact, controllable fuel. It can be burnt to provide heat on a large or small scale. High-grade charcoal is also used in laboratory and industrial chemical applications (see Figure 10).

 

Co-firing

Co-firing involves the use of biomass material in conjunction with fossil fuels, which is usually coal. The biomass is commonly woodchips, which is added to the feed coal (wood is 5 - 15% of the total) and combusted to produce steam in a coal power plant. Co-firing is currently well developed in the USA, but is also in a stage of research and development, as electricity companies examine the effect of the addition of wood to the coal, in terms of specific power plant performance and potential problems. It is hoped that co-firing will become economically viable in the future, allowing coal power stations to produce a small portion of renewable energy.

 

Ethanol Production

Ethanol can be produced from certain biomass materials that contain sugars, starch or cellulose (see Table 1). The best-known feedstock for ethanol production is sugar cane, but other materials can be used, including sugar beet, Jerusalem artichokes, wood, wheat and other cereals. The choice of biomass is important, as feedstock costs typically make up 55 - 80% of the final alcohol selling price (World Energy Council, 1994). Starch-based biomass is usually cheaper than sugar-based materials, but requires additional processing. Similarly, cellulose materials, such as wood and straw, are readily available, but require expensive preparation. The lignin by-product, which is around 50% of the material, can be combusted to provide the energy to drive the process.

Ethanol is produced by a process known as fermentation. Typically, sugar is extracted from the biomass crop by crushing, mixing with water and yeast, and then kept warm in large tanks called fermenters. The yeast breaks down the sugar and converts it to ethanol. A distillation process is required to remove the water and other impurities from the dilute alcohol product, which is generally about 10%-15% ethanol. The concentrated ethanol (95% by volume with a single step distillation process) is drawn off and condensed back to a liquid form, which can be used as a supplement or substitute for petrol in spark ignition engines. Brazil has a successful, industrial-scale ethanol project, which produces ethanol from sugar cane for blending with petrol. In the USA, maize is used for ethanol production and then blended with gasoline to produce "gasohol”, which is usually E10 (10% ethanol and 90% petrol). Current internal combustion engines in cars require some engine modifications to run with a total petrol substitute.

The remaining crop residues that are not suitable for producing ethanol (such as straw or bagasse) can be used to supply the external heat required for the process. In ethanol production there is a significant energy loss in the distillation stage, particularly the complex secondary distillation process required to achieve ethanol concentrations of 99% or better. This may be acceptable, however, due to the convenience of a liquid fuel and the relatively low cost and maturity of the technology. The final cost of ethanol produced is currently being pushed below $1 per litre (some researchers are achieving small scale levels of $0.45 per litre Chandra M.J.) at present to assist ethanol competitiveness at the bowser.

 

Biomass Use

Modern biomass now represents only 3% of primary energy consumption in industrialised countries (Ramage & Scurlock 1996), and this value has remained steady over recent years. However, much of the rural population in developing countries, which represents about 50% of the world’s population are reliant on traditional biomass, mainly in the form of wood for fuel. Traditional biomass accounts for 35% of primary energy consumption in developing countries, raising the world total to 14% of primary energy consumption (Ramage & Scurlock 1996).

The earth's natural biomass replacement represents an energy supply of around 3 Zettajoules (3 x 10²¹ J) a year, of which just under 2% is currently (1998) used as fuel. It is not possible, however, to use all of the annual production of biomass in a sustainable manner. One analysis carried out by the United Nations Conference on Environment and Development (UNCED) estimates that biomass could potentially supply about half of the present world primary energy consumption by the year 2050 (Ramage & Scurlock 1996).

 

Biomass in Australia

The installed electricity generating capacity from landfill gas in Australia was about 151.5 MW in 2005 (for more details see the pdf "Waste to Energy" by the Australian Business Council for Sustainable Energy). Landfill gas projects have been increasing in number and production recently, with only 15 projects in 1998 (AGO 1998) which has grown to 42 in operation in 2005. (Australian Business Council for Sustainable Energy, 2005). With a possible expansion of the landfill gas recovery systems, there is a possibility for a total maximum installed capacity of around 300 MW.

The use of sewage gas for electricity production is increasing in Australia. In 1997, the installed sewage gas electricity generation capacity was about 7 MW, which represented a 59% recovery of methane from wastewater treatment plants (Australian Greenhouse Office 1998). The figure at present is a total of 12 projects, producing 25.59 MW, which demonstrates the recent growth of this domestic industry.

Bagasse represents about 1% of Australia’s total primary energy consumption at present. The steam produced is used to drive sugar cane mills, for process heating and for grid-connected electricity production. The sugar mills in all states have a combined capacity of 481.12 MW (Australian Business Council for Sustainable Energy, 2005). Increasingly, bagasse is being utilised in cogeneration systems and using more efficient conversion systems, and with additional fuel inputs this output could easily treble.

Wood represents 2.4% of Australia’s total primary energy consumption (Bush, Harris & Ho Trieu, 1997). About 75% of the heat energy produced from this wood is consumed as firewood in the residential sector, with about 22% of homes using fuelwood for primary heating (DPIE, 1997). The remaining heat energy is used in the wood products, paper and food industries.

 

Biomass in Asia

Biomass is an important source of energy in South-East Asia, representing about 40% of the total energy consumption (RWEDP, 1997a) (see Figure 11). In some countries, such as Nepal, the biomass share is as high as 92%, while other countries, such as Malaysia, have switched to other fuel sources and biomass only contributes 7% to the total energy consumption. This is still much higher than the industrialised countries’ average of 3% (RWEDP, 1997b). The US Energy Information Agency estimates the market potential for biomass in the Asia-Pacific region to be 6 GW (Badin, 1998).

The main source of biomass in Asia is woodfuel. In Indonesia, the share of biomass is 38%, with woodfuels accounting for 80% of this total (RWEDP, 1998). In South-East Asian countries, 10%-50% of woodfuel supplies are sourced from natural forests. Non-forest sources, such as rubber and palm plantations, have become important sources of woodfuel. In Indonesia, the share of non-forest woodfuels in household consumption is estimated to be as high as 93% (RWEDP, 1997a). Agricultural residues, such as rice husks, straw, coconut husks, shells and bagasse are the other main source of biomass fuel both in the domestic and industrial sectors. In the Philippines, 87% of biomass fuel used in the industrial sector comes from bagasse (RWEDP, 1998).

The main biomass processes are direct combustion, carbonisation and anaerobic digestion in the domestic sector and in small-scale industries, but increasingly, biomass is being used in modern cogeneration systems. The combined production of heat and power from forestry and agricultural residues is being promoted by the private sector, particularly in Indonesia, Malaysia, the Philippines and Thailand. Two successful projects of the EC-ASEAN COGEN  programme are a 2.5 MW rice-husk-fired cogeneration plant in Thailand and a 1.65 MW cogeneration plant in Malaysia, fueled by wood residues. Both projects had a payback time of 3 years (Pennington, 1998).

On a smaller scale, anaerobic digesters are commonly used in Asia. In 2003, approximately 1 million households in China had biogas digesters, and between 2003 and 2005, some 11 million additional rural families reportedly began using them. As the digesters are powered with livestock or domestic waste, generally a family with one head of cattle or three pigs is able to supply a digester. A typical digester will produce 0.1 – 0.3 m³ of biogas per day, depending on the input and temperature at the time of year. According to REN21’s Energy for Development publication, of the families surveyed, 74% find it convenient to use biogas and nearly half of the families without a digester have decided to build one (REN21 Renewable Energy Policy Network, 2005). These digesters provide sufficient biogas for a family or several families together (see Figure 12). In addition, the waste residue from the digesters is a very rich fertiliser. On a larger scale, anaerobic digesters are utilised in Korea to dispose of municipal food waste, processing up to 15 tonnes of waste per day (see Figure 13).

An important issue for biomass use in Asia, is the need for improved wood stove and kiln technologies to increase efficiencies and reduce air pollution. The latter is particularly important, in a domestic context, for minimising the negative health effects of burning wood incompletely in confined spaces.

 

The Future for Biomass Technologies

In industrialised countries, the main biomass processes utilised in the future are expected to be the direct combustion of residues and wastes for electricity generation, bio-ethanol and biodiesel as liquid fuels, and combined heat and power production from energy crops. In the short to medium term, biomass waste and residues are expected to dominate biomass supply, to be substituted by energy crops in the longer term. The future of biomass electricity generation lies in biomass integrated gasification/gas turbine technology, which offers high-energy conversion efficiencies and will be further developed to run on biomass produced fuels.

 

Further Information

RISE Resources - Information regarding available renewable energy resources.

RISE Technologies - An extensive collection of information regarding renewable energy technologies.

RISE Applications & System Design - Renewable energy application information and system designs.

RISE System Displays - Case studies and information on installed renewable energy systems & performance data.

 

RISE Information Portal Applications

IEA Bioenergy

US Bioenergy Information Network

U.S. Department of Energy’s Biomass Power Program

National BioEnergy Industries Association

CADDET Technical Brochures

Australian Biomass

Regional Wood Energy Development Programme in Asia

Biomass Energy in ASEAN Member Countries

Turning sawdust in charcoal in Malaysia

Cookstoves for the developing world

Food Waste Disposal Using Anaerobic Digestion, Korea

Biomass Energy Development in Yunnan Province, China

Biomass cogeneration in Indonesia

 

 

References

Australian Business Council for Sustainable Energy, 2005. “Waste to Energy – A Guide for Local Authorities” (Online), Accessed 20 February 2007.

Australian Greenhouse Office, 1998. "Methane capture and use: waste management workbook" (Online), Accessed 20 February 2007.

Badin, J. & Kirschner J., 1998. "Biomass greens US power production", Renewable Energy World, vol.1, no.3, pp.40-45.

Bush, S., Harris, J. & Ho Trieu, L., 1997. Energy 1997 projections- Australian energy consumption and production, ABARE Research Report 97.2, Canberra.

DPIE (Department of Primary Industries and Energy), 1997. Renewable energy industry- survey on present and future contribution to the Australian economy, Australian Government Publishing Service, Canberra.

Overend, R.P., 1998. "Biomass gasification: a growing business", Renewable Energy World, vol.1, no.3, pp.27-31.

Pennington, M., 1998. "Biowaste fuels- South-East Asian cogen schemes", Renewable Energy World, vol.1, no.2, pp.59-63.

REN21 Renewable Energy Policy Network, 2005. “Energy for Development: The Potential Role of Renewable Energy in Meeting the Millennium Development Goals.” Washington, DC:Worldwatch Institute

RWEDP (Regional Wood Energy Development Programme in Asia), 1997a. "Biomass Energy in ASEAN Member Countries" (Online), Accessed 20 February 2007.

RWEDP (Regional Wood Energy Development Programme in Asia), 1997b. "Review of wood energy data in RWEDP member countries" (Online), 20 February 2007.

U.S. Department of Energy, 2006. "Electrical Power Generation" (Online), Accessed 20 February 2007.