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Direct Biomass Combustion


Combustion is the oldest and most frequently applied process to extract the energy content from solid biomass. During combustion, most of the energy is released in form of heat. Different thermodynamic processes can be used to transform part of this heat into electric power.

Steam cycle


Source: http://www.wisions.net/files/tr_graphics_content/biomasse_direct_combustion.png?phpMyAdmin=23fc4bebcd58t19e0f0a6

The most common conversion process uses steam as the working medium and is based on the Rankine Cycle. In this cycle, heat from biomass combustion is used to heat water in a boiler and produce steam at high temperature and pressure. The steam is expanded through a turbine where part of the energy is transformed into rotary movement and then into electric current through an electrical generator. The steam is condensed after expansion and fed into the boiler, thereby starting the cycle again.

Steam turbine plants are available for applications of around 1 MW up to 50 MW. Generally speaking, the larger the plant, the higher the electric conversion efficiency.

Steam engines

The principle of expanding steam within a cylinder to move a piston was the driving force of the industrial revolution. Steam piston engines (based on the same basic principle) are also suitable for transforming biomass energy into electricity in applications below 1 MW.

Cogeneration of heat and power

A significant part of biomass energy remains in form of heat, regardless of the conversion process. The bulk of biomass combustion for electric power therefore envisages the productive utilisation of heat. In most cases, the operation of plants is controlled based on the heat demand of an industrial facility, the generation of electric power subordinated to it.

The boiler

The quality of solid biomass varies significantly, depending on factors such as biomass sources and pre-treatment. Combustion systems (boilers) can deal with various fuel qualities. The chemical composition and moisture content of the biomass influence the combustion characteristics, such as energy content, ash deposition and emissions.

Size matters

The quality and homogeneity of the resources determine the complexity of the combustion system. Generally speaking, the smaller the combustion plant, the greater the need for quality and homogeneity.


1. Potential Contribution to Sustainable Development

Global sustainable energy supply

Modern solid biomass combustion technologies allow for the provision of heat and power in decentralised applications. Small and medium-sized plants are already able to cover the energy demand of many agro and forestry industries (e.g. pulp, wood, sugar, etc). In regions or nations with a high availability of solid biomass, these technologies may contribute to the sustainable provision of heat and power for both rural and urban areas.

Climate change mitigation

Today’s technologies for biomass combustion and the cogeneration of heat and power are suitable options to reduce greenhouse gas emissions related to the provision of energy. The use of fossil fuels can be avoided and the overall emissions of greenhouse gases reduced through the efficient use of solid biomass resources for the simultaneous provision of heat and power.

Millennium development goals

The most efficient and mature technologies are medium or large in scale and better suited for industrial applications. Small applications for local, off-grid solutions are not expected to be commercially available in the near future. The contribution of these technologies to achieving the Millennium Development Goals may be marginal.


2. Environmental Issues

Greenhouse gas emissions from biomass combustion activities are climate neutral. Other types of emissions can become an issue and require appropriate control methods.

Large plants

Emissions from the combustion of solid biomass in large applications are low compared with those from the combustion of coal. Emissions of nitrogen oxides, particulates and sulphur oxides can result in discharges that are higher than the current standards. Conventional methods for flue gas cleaning can be used to meet these standards, however.

Small plants

Small-scale combustion units are cause for some concern. Emission levels tend to be higher and emission-reduction methods are usually not cost-effective. The applicability of the technologies has to be checked against the specific standards required.

Overall greenhouse gas emissions

In addition to emissions from plant operations (which are climate neutral), greenhouse gas emissions from the biomass supply chain also need to be taken into account. It may include activities such as the mechanical preparation of soil, fertilisation, harvest and transport (see Biomass – Electric Power). Generally speaking, the lowest emission levels are achieved when biomass residues, e.g. forest residues and straw, are used, and the highest when crop production involves extensive use of inputs and energy. A plant’s emission of greenhouse gases is therefore rather case specific.

A German assessment of cogeneration plants running on solid biomass estimates emission levels between 70 g and 150 g of CO2 (equivalent) emissions per kWh of electricity 1. The lowest emission level is reported when using wood residues. These figures show significant reduction potentials, if compared with the overall emissions from efficient power plants driven by fossil fuels: i.e. over 900 g of CO2 (equivalent) per kWh of electricity in large coal facilities and over 400 g of CO2 (equivalent) per kWh of electricity in large combined cycle power plants running on natural gas.


3. Social Issues

The most efficient and mature technologies are more suitable for large-scale applications (around 1 MW or higher). The direct participation of the local population in such projects is rather unlikely. However, solid biomass can be used to enhance the supply of electricity and heat in regions where agricultural or forestry industries are predominant.

Business schemes for off-grid electrification

Official agreements between local agricultural or forest industries and local administrative entities are possible options to address the electrification of remote areas. Local industry invests in additional generation capacity in order to supply the region with biomass-based electricity. An official tariffs scheme is established and the energy service becomes an additional business for the local industry.

Potential conflicts with social realities have to be considered

Some economic and social aspects can be affected when large increases in biomass production are necessary. These aspects include food security, land use and land ownership, and agricultural and forestry development. The introduction of biomass combustion technologies for the supply of electric power can therefore have a direct impact on social development at the local, regional or national level.


4. Development Status and Prospects

The direct combustion of biomass is already in widespread use. Some agricultural and forestry industries already use the biomass residues they produce to cover internal energy demand. Prominent examples are the use of bagasse (from sugar and ethanol industries) and wood chips and dust (from the timber industry).

Stirling engines

The development of externally fired Stirling engines may allow for the production of affordable and relatively efficient small-scale solutions. Stirling engines with electric power capacities below 100 kW reach electric efficiencies of around 20%. Cost reductions through process improvements and mass production are expected. Thus small solutions for the provision of heat and electric power through biomass combustion may become competitive.

Organic Rankine Cycle (ORC)

The generation of electric power in conventional steam turbines requires very high temperatures in order to produce dry steam. The temperature requirements can be reduced by replacing water with (organic) fluids, which boil at a lower temperature than water. In this way, relatively complex and expensive steam boilers can be avoided and the use of biomass may become more competitive. Most of the biomass plants applying ORC technology are still demonstration projects. It is anticipated that ORC technology will achieve commercial maturity in the next few years.

Several improvements in the technologies are expected, particularly the further development of Stirling engines while use of the organic Rankine cycle may expand the applicability of biomass combustion for electrification.


5. Economic Issues

Investment in medium and large cogeneration plants based on biomass combustion comprises three main items: 1) the buildings and equipment to store and manage the biomass, 2) the power generation equipment, and 3) the equipment to condition and distribute electric power and heat.

Some economic and social aspects can be affected when large increases in biomass production are necessary. These aspects include food security, land use and land ownership, and agricultural and forestry development. The introduction of biomass combustion technologies for the supply of electric power can therefore have a direct impact on social development at the local, regional or national level.


Source: http://www.wisions.net/files/tr_graphics_content/CCost_Biomass_Combustion.png?phpMyAdmin=23fc4bebcd58t19e0f0a6


The graphic shows the expected development of capital costs for plants of different sizes, according to estimates made by the Energy Sector Management Assistance Program (ESMAP) 3 and the NEEDS project 4. Cogeneration plants using conventional steam turbines have been considered. The development of costs has been linearly extrapolated according to the available data. Significant cost reductions are not expected. The capital costs of the technologies may remain around USD 1,500 per kilowatt of electricity (cost in 2005) over the next few years.

The technical and economic feasibility of biomass-based power generation is heavily influenced by the availability of biomass as well as its quality. Issues such as seasonality, proximity to the production sites, local biomass prices and global markets, and storage and pre-treatment requirements are crucial to the design of an application (e.g. optimal site and size of the plant) and for its economic viability (generation costs).



Biomass direct combustion power plant
Source: http://privateequityindonesia.files.wordpress.com/2012/02/biomass-direct-combustion-power-plant.jpg?w=600&h=323













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I have no doubt that we will be successful in harnessing the sun's energy.
If sunbeams were weapons of war, we would have had solar energy centuries ago.

– Sir George Porter, quoted in The Observer, 26 August 1973

The use of solar energy has not been opened up because the oil industry does not own the sun.

– Ralph Nader, quoted in Linda Botts, ed., Loose Talk, 1980

Thank God men cannot fly, and lay waste the sky as well as the earth.

– Henry David Thoreau

There's so much pollution in the air now that if it weren't for our lungs there'd be no place to put it all.

– Robert Orben

There is a sufficiency in the world for man's need but not for man's greed.

– Mohandas K. Gandhi

Modern technology
Owes ecology
An apology.

– Alan M. Eddison

Don't blow it - good planets are hard to find.

– Quoted in Time

Nature provides a free lunch, but only if we control our appetites.

– William Ruckelshaus, Business Week, 18 June 1990

I think the environment should be put in the category of our national security. Defense of our resources is just as important as defense abroad. Otherwise what is there to defend?

– Robert Redford, Yosemite National Park dedication, 1985

Your grandchildren will likely find it incredible - or even sinful - that you burned up a gallon of gasoline to fetch a pack of cigarettes!

– Paul MacCready, Jr.

We do not inherit the earth from our ancestors, we borrow it from our children.

– Native American Proverb

When we heal the earth, we heal ourselves.

– David Orr

How long can men thrive between walls of brick, walking on asphalt pavements, breathing the fumes of coal and of oil, growing, working, dying, with hardly a thought of wind, and sky, and fields of grain, seeing only machine-made beauty, the mineral-like quality of life?

– Charles A. Lindbergh, Reader's Digest, November 1939

For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled.

– Richard P. Feynman

Living in the midst of abundance we have the greatest difficulty in seeing that the supply of natural wealth is limited and that the constant increase of population is destined to reduce the American standard of living unless we deal more sanely with our resources.

– W.H. Carothers




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