Solid fuel

thumb|A fire made of charcoal briquettes

Solid fuel refers to various forms of solid material that can be burnt to release energy, providing heat and light through the process of combustion. Solid fuels can be contrasted with liquid fuels and gaseous fuels. Common examples of solid fuels include wood, charcoal, peat, coal, hexamine fuel tablets, dry dung, wood pellets, corn, wheat, rice, rye, and other grains. Solid fuels are extensively used in rocketry as solid propellants. Solid fuels have been used throughout human history to create fire and solid fuel is still in widespread use throughout the world in the present day.

Solid fuel from biomass is regarded as a renewable energy source which can contribute to climate change mitigation efforts. Solid fuel from fossil fuels (i.e. coal) is not a renewable energy.

Types

Biomass

Biomass that is used for energy production can be processed into solid fuels but also into liquid or gaseous fuels. In comparison, the term biofuel is nowadays mainly (but not exclusively) used for liquid or gaseous fuels, used for transportation.

Pellet fuels are made from compressed organic matter or biomass. Pellets can be made from any one of five general categories of biomass: industrial waste and co-products, food waste, agricultural residues, energy crops, and untreated lumber. Wood pellets are the most common type of pellet fuel.

Wood

Wood fuel can refer to several fuels such as firewood, charcoal, wood chips sheets, pellets, and sawdust. The particular form used depends upon factors such as source, quantity, quality and application. In many areas, wood is the most easily available form of fuel, requiring no tools in the case of picking up dead wood, or few tools. Today, burning of wood is the largest use of energy derived from a solid fuel biomass. Wood fuel can be used for cooking and heating, and occasionally for fueling steam engines and steam turbines that generate electricity. Wood may be used indoors in a furnace, stove, or fireplace, or outdoors in a furnace, campfire, or bonfire. As with any fire, burning wood fuel creates numerous by-products, some of which may be useful (heat and steam), and others that are undesirable, irritating or dangerous.

When harvested in a sustainable fashion wood is usually considered to be a renewable solid fuel (renewable energy).

There is debate as to whether burning wood can be considered carbon neutral, as technically the wood cannot release more carbon than was sequestered during its growth, although this does not take account of other impacts such as deforestation and rotting has on the carbon footprint.

Peat

Peat fuel is an accumulation of partially decayed vegetation or organic matter that can be burnt once sufficiently dried. It is used widely in the country districts of Ireland and Scotland where alternatives are absent or expensive. It has a relatively low calorific value, even after essential drying.

Fossil fuels

Coal

Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. Throughout history, coal has been used as an energy resource, primarily burned for the production of electricity and heat, and is also used for industrial purposes, such as refining metals. Coal is the largest source of energy for the generation of electricity worldwide, as well as one of the largest worldwide The extraction of coal, its use in energy production and its byproducts are all associated with environmental and health effects. Variations such as smokeless coal can be formed naturally in the form of anthracite, a metamorphosed type of coal with a very high carbon content that gives off a smokeless flame when set alight. It is an important type of smokeless fuel.

Coke

Coke is a fuel with few impurities and a high carbon content, usually made from coal. It is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes made from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is man-made. The form known as petroleum coke, or pet coke, is derived from oil refinery coker units or other cracking processes.

Initially, the heat source – a match or any external source of heat – evaporates moisture trapped in the solid (fuel). This heat and drying stage consumes energy instead of releasing it. Then, the temperature of the fuel rises and the chemical bonds are broken down. This thermal decomposition turns the solid into flammable gases and a carbon-rich solid called 'char'. The volatile gases from the fuel mix with the oxygen in the air and ignite to create a visible flame. The remaining char reacts directly with the oxygen once the gases are spent. This is called glowing combustion, and it is slower and flameless.

Cost and transport

Solid fuels, compared to liquid fuels or gaseous fuels, are often cheaper, easier to extract, more stable to transport and in many places are more readily available.

Coal, in particular, is utilized in the generation of 38% of the world's electricity because it is less expensive than its liquid and gas counterparts. than liquefied petroleum gas and releases many volatile organic compounds, which can contribute to poor air quality through the formation of secondary pollutants such as ground level ozone and secondary organic aerosol. The emissions from solid fuels are major drivers of poor air quality in regions where solid fuels are a dominant fuel source.