What is biomass?

Biomass is the name given to any recent organic matter that has been derived from plants. The energy stored in plants (and in animals that eat plants) originates from the sun. Plants use sunlight for growth in a process called photosynthesis. In this process, the plant combines carbon dioxide from the air and water from the ground to produce energy. Energy is stored in the plant in the form of carbohydrates.

The use of biomass for energy is essentially the reversal of photosynthesis. This energy may be released when the plant or waste is burned or converted to fuels.

During the energy conversion process energy is usually released in the form of heat which may be used to perform useful work such as generating electricity. Of all renewable resources, biomass is unique in that it is effectively 'stored' solar energy.

Using biomass

The solar energy stored in biomass can be used in various ways. For example:

• Combustion - For thousands of years people have been burning woods and grass to obtain heat. About half of the world's population rely on wood as their primary source of fuel. In most cases, the fuel is used inefficiently and forest resources are rapidly being consumed.
• Fuel Production - Fuels such as bio-ethanol, bio-methanol and bio-diesel can be produced from biomass. Sources such as forests, grasslands or special fuel crops can be cultivated for the purpose.
• Biomass which contains considerable amounts of cellulose, such as wood, can be distilled to produce methanol.
• Oils extracted from the seeds of plants such as sunflowers, as well as coconut oil and palm oil, can be used as substitutes or extenders for diesel fuel in compression ignition engines.
• Waste Management - Energy can be salvaged from waste products from an industrial process eg wood or paper mill waste. The biomass can be combusted for power generation, or converted, by bacterial digesters to methane. Methane gas released over time from rubbish dumps can also be collected.

Combustion and chemical conversion

The main ways that biomass can be converted to energy are combustion and chemical conversion.

Combustion

The simplest use of biomass is burning the material in a furnace. Burning biomass or bio-fuels produces combustion products such as carbon dioxide, nitrogen oxides, and particulate matter (smoke) - the same emissions as any combustion process.

Many types of furnace are available for the combustion of biomass. Typically they produce steam which can then be used for process heating, drying or electricity generation. Some of the main types used are:

• Refractory-lined furnaces - These operate at about 1000??C and need excess air (more than is needed just for combustion) to keep the wall temperatures down.
• Water-wall furnaces - These have banks of tubes positioned around the furnace wall. The burning fuel heats water circulating in the tubes to produce steam.
• Fluidised-bed furnace - In this type of reactor, air is blown through an orifice plate beneath the burning material, keeping it in a dispersed, fluid-like state that ensures effective combustion.
• Boilers - Boilers are usually used for larger applications such as power generation.
• Cyclone Furnaces - These are more economical for smaller sized feeds.

Chemical Conversion

Processes which convert raw biomass resources to a secondary fuel source (such as bio-fuels) can be grouped as being either thermochemical or biochemical. Thermochemical processes include gasification and pyrolysis; biochemical processes include anaerobic digestion and fermentation.

Gasification - is a thermochemical process which dates back to first production of gas from coal. In most modern industrial biomass processes, the gasifiers operate by heating the biomass in an environment with little oxygen until the biomass breaks into its constituent chemical components. The process requires a large amount of heat for the reactions to work.

Existing gas or oil-fired boilers can be adapted to burn the gas. The gas may also be suitable for use in high efficiency gas turbines. Wood gasification can also be more energy efficient than burning wood directly in a boiler to produce steam for electricity generation.

Pyrolysis - is a roasting process, also known as retorting, destructive distillation, or carbonisation. In the pyrolysis process, organic materials such as wood or other biomass is heated in an oxygen-free reactor. This causes the material to breakdown into new compounds, usually a hydrocarbon mixture. The result of the reaction usually appears to contain oil like liquid, a carbon rich solid residue and some gaseous fumes.

Fermentation - is a biochemical process where living organisms, such as yeasts and bacteria, change the composition of organic compounds (biomass such as grapes). Other food crops such as corn, potatoes, and barley can also be fermented to produce ethanol.

The alcohol produced is often termed 'bio-ethanol' or a 'bio-fuel'. Bio-ethanol can be combined with petrol and burned as a fuel in spark-ignition engines.

Anaerobic digestion - Wet biomass is decomposed by bacteria to produce a mixed gas of methane and carbon dioxide. These reactions only occur when there is no oxygen present - hence the term 'anaerobic'. When oxygen is present, it will either inhibit or kill the bacteria which produce the gas. The gas produced is called 'biogas'.

Bio-Diesel or Trans-esterification - The process in which 'organically derived' oils are combined with alcohol (methanol or ethanol) in the presence of a catalyst. The result of the process is a mixture of glycerine and 'bio-diesel' (methyl or ethyl esters). The mixture is separated to recover glycerol, a useful by-product, and the bio-diesel fuel.