Solid Biofuels

Sufficient reserves of solid biomass fuel must be available at biomass power plants to ensure continuous operation.

The most commonly used solid biomass fuel is wood, in the form of logs, pellets, briquettes or wood chips, as well as agricultural residues such as straw. If the original biomass is easy to transport, such as logs, it can be used directly as fuel in suitable boilers. Biomass that is less suitable for transport, such as forestry and woodworking residues or straw, is often processed first. A common method is size reduction followed by compression into pellets or briquettes, which are easier to transport, store and use in suitable boilers.

Solid biomass is widely used for heat production. The heating value of woody biomass depends strongly on its moisture content. Dry wood has a heating value of about 18 MJ/kg, while wood with around 50% moisture has roughly half that value.

Woody biomass generally has a relatively low ash and sulphur content compared with many fossil fuels, although the values vary with the type and origin of the fuel. The carbon dioxide released during biomass combustion was previously absorbed from the atmosphere during plant growth, but the overall greenhouse-gas balance also depends on cultivation, harvesting, processing, transport and sustainable replenishment of the biomass.

Logs

Wood chips

Pellets

Briquettes

Straw

4 pictures

4 pictures

Liquid Biofuels

Liquid biofuels are fuels derived from biomass that are stored, transported and used in liquid form. Important groups include alcohol fuels such as bioethanol, biodiesel and bio-oils produced by thermochemical conversion.

Alcohol-Based Fuels

A fuel pump dispensing biodiesel.

Bioethanol is produced by fermentation of sugars from crops such as sugar beet and sugar cane, or from starch-rich crops such as maize and wheat after the starch has first been converted into sugars. The fermented mixture is then distilled to produce concentrated ethanol with a heating value of about 27 MJ/kg. Ethanol is commonly blended with petrol in various proportions, depending on fuel standards and vehicle compatibility. The energy and greenhouse-gas balance of bioethanol depends strongly on the feedstock and production process. Cellulosic ethanol can also be produced from lignocellulosic residues and other non-food biomass, although these processes are technically more demanding than conventional starch- or sugar-based production.

Other alcohol-based biofuels include biomethanol and biobutanol. Biobutanol has a higher energy density and is less hygroscopic than ethanol, while methanol is toxic and requires careful handling.

Alcohol fuels can reduce some exhaust pollutants compared with conventional petrol, although the overall emissions depend on the fuel blend, engine design and operating conditions.

E85 is a high-ethanol motor fuel containing up to about 85% ethanol, with the exact proportion varying by season and fuel specification. It is intended for flexible-fuel vehicles.

Biodiesel, Bio-Oils

A bioethanol production plant.

Biodiesel is a renewable liquid fuel produced from vegetable oils, animal fats or used cooking oils. It can be blended with petroleum diesel and, depending on the blend and engine specification, used in conventional diesel engines. Common biodiesel feedstocks include rapeseed, sunflower and soybean oils, as well as animal fats and used cooking oils. The oil or fat is purified and then converted by transesterification into fatty acid methyl esters (biodiesel), with glycerol formed as a by-product. Biodiesel has a heating value of about 37—40 MJ/kg, somewhat lower than petroleum diesel, and is commonly used in blends with conventional diesel. Blending biodiesel with conventional diesel can reduce some exhaust emissions, although the effect varies by pollutant, engine and fuel blend.

Synthetic Liquid Fuels

Gaseous products from biomass gasification can be converted into liquid hydrocarbons by processes such as Fischer-Tropsch synthesis. Purified biomethane can also be used directly as a transport fuel, either compressed or liquefied. Fischer-Tropsch synthesis converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons in the presence of a catalyst, typically at temperatures of about 200—350 °C and elevated pressure. The process was used extensively in Germany during the Second World War when access to crude oil was limited.

10 pictures

10 pictures

Gaseous Biofuels

Gaseous biofuels are combustible gases produced from biomass by biological or thermochemical processes. Important examples include biogas, biomethane, wood gas and hydrogen produced from biomass.

Biogas

Similar to LPG, purified biogas (biomethane) can be used as a vehicle fuel at suitable filling stations.

Biogas is a mixture consisting mainly of methane, typically about 50—70%, and carbon dioxide, with smaller amounts of water vapour and trace gases. Hydrogen may also be present, typically at around 1% or less. Hydrogen sulphide and ammonia are undesirable impurities and are often removed or reduced before use to prevent corrosion and meet fuel-quality requirements.

Biogas is produced in biogas plants by anaerobic digestion in sealed digesters. Suitable feedstocks include agricultural residues, animal manure, sewage sludge and biodegradable waste from the food industry. The biogas is then cleaned and dried before being used for heat or electricity generation. Biogas is commonly used in combined heat and power plants. After upgrading to biomethane, it can also be used as a transport fuel.

Landfills containing biodegradable organic waste can also produce methane-rich landfill gas as the waste decomposes anaerobically. The gas can be collected through wells and piping and used for energy production.

During World War II, wood gas was frequently used to power automobiles as there was a shortage of petroleum-based fuels.

Wood Gas

Anaerobic digesters at a biogas plant.

Wood gas is produced by gasifying woody biomass at high temperature with a controlled supply of air, oxygen and/or steam. The resulting gas contains mainly carbon monoxide, hydrogen, methane, carbon dioxide and nitrogen, depending on the gasifying medium and process conditions. The combustible components of wood gas are carbon monoxide, hydrogen and methane. Air-blown wood gas typically has a relatively low heating value of about 4—6 MJ/Nm³. Wood gas can be used in boilers, engines or combined heat and power plants. For use in combustion engines, it usually requires cooling and cleaning to remove particulates, tar and other contaminants.

Over 100 years, methane has a global warming potential about 27—30 times greater than carbon dioxide.

Hydrogen

Biogas is a methane-rich gaseous fuel produced by anaerobic digestion.

Hydrogen has a high specific energy and can be used as a fuel or energy carrier. When used in a fuel cell to generate electricity, its direct reaction product is water; combustion in air can also produce nitrogen oxides.

Hydrogen can be produced from biomass by gasification followed by processing and reforming of the resulting gas. It can also be produced by steam reforming of biomethane or other biomass-derived gaseous fuels.

7 pictures

7 pictures