Biomass is the fourth largest energy resource after coal, oil and natural gas. It has been used as a fuel for millennia and in many parts of the world, especially in rural areas in developing countries, it still is. Today biomass fuels are being revived having been displaced by fossil fuels for energy generation. It is now being reconsidered as a fuel for advanced forms of electricity generation.
This change around from fossil fuels to sources of renewable energy has been gathering momentum since the 1970's when oil prices were raised and oil availability was questioned. Additionally rapid global industrialisation and a growing hydro-carbon dependent society has raised legitimate concerns on the intensive use of finite fossil fuels and their degrading effect on the environment. Over the past 100 years the average temperature of the earth's atmosphere has risen by 0.5° C. It is predicted that by 2100 there will be a rise of between 1°C and 3.5° C. This rise is being attributed to the 'enhanced green house effect', caused by heightened carbon dioxide, methane, halogenated compounds and nitrous oxides released into the atmosphere, EC 2000.
Currently the world energy system is based largely on fossil fuels. They provide 80% of the worlds commercially available energy. Without considerable policy changes this figure will rise to 90% by 2020. Energy demand will increase in relation to the rapid economic growth in Latin America, Africa and the Asia-Pacific region. The World Bank has estimated that over the next four decades developing countries will require five million megawatts of new electrical generating capacity to meet the anticipated needs. Currently the world's total installed capacity is three million megawatts, IEA, 1999.
Renewable energy sources will capture a huge portion of the demand for power. It is also important that renewable energy technologies are transferred to the developing world to contribute to global climate change commitments.
Renewable energy means electricity generated from solar, wind, biomass, geothermal and hydropower resources. The drive for the use of renewable sources of energy to replace fossil fuels was expressed most vociferously in 1992 at the Earth Summit in Rio de Janeiro. In response, the Kyoto Protocol was adopted in 1997 and reflects the objectives of the UNFCC (United Nations Framework Convention on Climate Change) to stabilise the concentration of green house gases in the atmosphere. The Kyoto Protocol is calling for all industrialised (annex 1) countries to reduce their emissions by 2008-2012 to levels 5% lower than those emitted in 1990 in an attempt to challenge climate change.
This paper summarises the commercially proven technologies used to maximise the energy arising from agricultural residues. The technologies reviewed are direct combustion and steam technologies and biochemical processes (anaerobic digestion). Three case studies will be examined which include: anaerobic digestion of pig slurry in China; combustion and steam cycle technologies involved in converting palm oil mill residues to energy in Indonesia and the energy conversion technology used to turn rice husks to energy, whilst producing a practical by product. The projects will avoid 30,000, 600,000 and 216,000 tonnes of carbon emissions over the next thirty years respectively. The projects described are being developed with support from the UK government's Climate Change Challenge Fund.
Biomass to Energy–Project criteria for energy conversion
The physical and chemical properties of differing biomass materials determines that gauging the most suitable technology for fuel processing and conversion to heat and electricity is critical in order to optimise conversion efficiency. Biomass is also carbon neutral, the carbon released on combustion is assimilated by the biomass source by photosynthesis when it is growing. The prime, practical concerns which need to be taken into account when developing biomass projects are listed below.
- The security and long term availability of biomass fuel supply including variations in types of fuel, seasonality and availability coverage.
- Technical issues of plant design to address moisture content, bulk density, particle size and distribution, ash content, chemical properties such as alkali content for example.
- Practical location considerations and local technological capability.
- The ability to find a contracted long term user of electricity or steam generated from biomass resources.
- The ability to make use of value added by-products of the process.
- The project economics and ability to secure equity and debt financing for the project.
- The environmental and socio-economic benefits derived from carrying out the project, Stowell 1998.
Case study – Oil palm residues to energy in Indonesia
Indonesia is the second biggest palm oil producing country in the world, after Malaysia, representing 18% of the world-wide production. The Indonesian palm oil industry continues to expand rapidly using large mills which produce hundreds of tons of waste on a year round basis. Considerable opportunity exists in Indonesia and other countries to produce significant quantities of electricity from the residual biomass while mitigating environmental impacts both locally and globally, Walden, 1997.
The case study is based on utilising palm oil residues generated from the palm oil production process. The fresh fruit bunches (FFB) or the oil palm fruit produce Crude palm oil (CPO) and Kernel palm oil (KPO). The residues in terms of percentage by weight of the incoming FFB's are:
- Empty fruit bunches (EFB) 24%
- Fibre (from fruit after oil extraction) 12%
- Shells (from fruit) 7%
- Liquid waste 29%
- Unrecovered oil 2-5%
Residues will not only accumulate from the mills but from the extensive replanting programmes ongoing in plantations including palm fronds and stems.
Two hundred and twenty thousand tonnes per year of residues are collected from six mills. The residues are additional to the wastes that the six mills use for their own steam and power use. The basic concept of the project is to collect the wastes from the mills, transport the wastes to a central site for preparation, burn the waste in a specially designed but generally conventional, water tube boiler to generate electricity by a steam turbine generator. Steam will be extracted from the steam turbine for use by the mill as process steam. A substation will be constructed here to transmit the surplus electricity generated from the residues. Maximum generating capacity is estimated to be 10.5 MWe.
The principle fuel will be the EFB's with smaller quantities of shell and fibre. All fuels will be procured under long term supply contracts. Since the quantity of waste varies with the season, the blended waste fuel will also vary. In the high growth season, it is anticipated that up to 100% EFB will be burned. In the low growth season, the EFB quantity will be supplemented by shell and fibre which has been stored during the high growth season.
Traditionally the EFB's have been burnt in simple incinerators, as a means of disposal and the ash recycled into the plantation as fertiliser. Slow burning incineration gives rise to incomplete combustion resulting in excessive emissions of particulate matter and partially oxidised derivatives, some of which are toxic. Chemically bound nitrogen and sulphur present in the biomass are oxidised to nitrogen and sulphur oxides. Pollution also arises from open burning of the wastes. Incineration is progressively being phased out and has already been banned in Malaysia.
At the centralised plant, a well designed combustion and air pollution control system will be installed to limit emissions to internationally acceptable standards. By standing down diesel elsewhere, 10.5 MW of biomass fuelled generation will avoid approximately 16,800 tonnes/year of carbon emissions. An additional 3,500 tonnes/year of carbon emissions from open bonfires will also be saved. Over the thirty year life of a project there will be a total saving of approximately 600,000 tonnes of carbon.
Direct Combustion and Steam Cycles
A traditionally designed palm oil mill burns all its fibre and about 50% of its shell in order to produce the electrical and process steam needs of the plant while the mill operates. The boilers are normally low pressure, typically 20 bar (or lower) and supply back pressure steam turbines. A 60 T/hr FFB mill requires approximately 700kWe and 21,000kWth (30 T/hr process steam at 3.5kg/cm and 270°C). This represents a very low cycle efficiency for the production of electricity (approximately 3.5%) but a high overall co-generation efficiency exceeding 90%.
By raising the boiler pressure and introducing condensing steam turbines with extraction to provide process steam, the use of palm oil residues can be optimised in more energy efficient systems. Using a 45 bar medium pressure boiler, with a condensing steam turbine and the same process steam as before, the cycle efficiency for electrical production rises to about 13%. In addition, the boiler can be designed to burn all of the wastes not just the fibre and shell. At 13% efficiency, the fibre from one tonne of FFB will produce about 41kWh, shell about 47kWh and EFB about 46kWh. The total waste from 1 tonne of FFB will, therefore, produce about 134kWe. This compares to only about 12kWe per tonne of FFB for the waste from the traditionally designed plant. For example a 60 T/h FFB mill could produce 8040 kWe. With mill use remaining at 700kWe, the surplus available for export would be 7340 kWe.
A high pressure boiler system, 65 bar, 450°C, costs about 35% more than a 45 bar system for both capital and O&M costs. The increase in efficiency going from 47bar/370°C to 65 bar/ 485°C will be about 25%. Alternatively, the MW output should increase from 7.34 MWe (net) to about 10.78 MWe (net).
Boiler combustion and EFB
Utilising EFB's in the boiler can create combustion problems due to the moisture content of the fuel (60-65%), its fibrous nature and the amount of soluble alkalis contained within it. Machine wear is minimised by using a shredder to reduce material size and moisture content. The moisture content needs to be reduced to 50% or less before it can be fed to the boiler without use of supplemental fuel. This can be achieved by suitable preparation using a shredder and screw press.
Solutions concerning the combustion of palm oil wastes include:
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The fluidised bed approach. This approach however, is not useful when the soluble alkalis are as high as they are in palm oil waste, since the temperatures in the bed will exceed the eutectic temperature, indicating that slagging in the bed will result. It is also more expensive than a stoker approach.
- Combustion should be staged in order to minimise the production of nitrogen oxides and to promote better burnout of the carbon in a fuel. This approach has been refined to handle high moisture wastes, such as EFB, where temperatures in the lower furnace remain low to promote drying before combustion occurs.
Case Study – Maximising energy from rice husks by producing electricity and a practical by product
In Indonesia over 11 million hectares are under rice cultivation producing approximately 50 million tonnes of rice in 1997. This resulted in over 14 million tonnes of residual rice husk being produced. The majority of this is disposed of by open burning
This case study is based on a plant which will require 31,000 tonnes/year of rice husk to produce 3 MW. Approximately 20% of harvested rice remains as husk after milling which means that the plant will consume the husk from about 155,000 tonnes of rice paddy. The plant will therefore be located in a catchment rice producing area of 8-14,000 hectares. As with the palm oil wastes, utilising the wastes to fuel a power plant with the correct combustion system will result in rice husks being converted to electricity efficiently. The environmental benefits include providing a disposal route for a large quantity of waste and avoiding uncontrolled burning. In addition, if the combustion process is properly designed, the high silica ash can be used as a valuable by-product for the steel industry or for the manufacture of light weight, high strength blocks for building. Value is being maximised from the biomass by producing additional by products.
The project will lead directly to reductions in greenhouse gas emissions because diesel generation will be replaced with carbon neutral biomass. Assuming that the 3 MW plant results in the backing off of fossil fired power plants burning oil, the avoided carbon emissions accumulated at a rate of about 0.3kg/kWh would result in credits of 7,200 tonnes/year of carbon emissions. Over the thirty year life of the plant, 216,000 tonnes of carbon can be avoided.
The husks are combusted in boilers and electricity generated by steam. Due to the rice husks having different physical and chemical properties to the oil palm residues, the boiler/combustion process is different. Rice husks are a relatively low density, low moisture but high ash fuel with a good calorific value. In addition the ash has a high silica content. Due to its low density there is a possibility for unburned husk to be carried out of the boiler with associated loss of efficiency and increased loss on ignition (LOI) of the ash. The high silica content of the ash can result in accelerated wear in the convection and final stages of the boiler. Provided the boiler is designed to produce an ash with high amorphous silica and low granula silica, the ash is a valuable additive for production of concrete blocks and for other purposes.
Conclusion
Developing renewable energy projects, in countries which are rapidly becoming industrialised affords a challenge. These countries are seeking to grow economically whilst being confronted with the environmental issues encountered in the process of economic growth by the industrialised countries. Within developing countries much of the infrastructure that would support an industrialised economy is not yet in place. This allows greater scope for alternative energy sources and patterns of end use consumption as new capital stock is put into place. Of course, rapidly industrialising economies are not going to adopt patterns of energy production and consumption if they are perceived as policies more costly and likely to undermine impending growth objectives. However, in certain locations, agricultural residues such as palm oil residues are already competitive with existing fossil fuels, so the combined objectives of economic growth and reduced environmental impact can be achieved.
Maximising energy from biomass with specific projects requires a practical view to be taken. These include the security of long term fuel supply; awareness of the technical issues of plant design to address the wide variety of physical and chemical properties of biomass; a suitable location and knowledge of local technological capacity; a long term user of electricity or generated steam; the ability to maximise the use of value added by products and sound project economics. The case studies described are designed to be highly replicable whilst maximising the energy produced from biomass. This will lead to creating not only a clean environment but will contribute to providing employment, improving local and national economies, creating awareness of the environment and help with the challenge to prevent further adverse climate change.
