The term “solid biofuel” can be a bit misleading because many people associate biofuels with advanced refining and chemical processes. In fact, a biofuels can be any renewable, biological material used as fuel. With that definition, it becomes clear that things like wood, sawdust, leaves, and even dried animal dung all constitute biofuels. In fact, solid biofuels are how humans have been heating themselves and their food since the dawn of...well...humans!
There is no production necessary for a solid biofuels in most cases because it is often in a convenient form. On the other hand, sawdust and wood chips are not so convenient and so they are often put through a process known as “densifying.” All this means is that the biomass is compressed into a form that is easier to handle or is mixed with some sort of bonding agent (tar for instance or tree sap) to hold it together for easier transport, storage, and use. Pellets and bricks are common “densified” forms of solid biomass.
Studies have shown that solid biofuels create less environmental impact than doe solid fossil fuels like coal. The United States Department of Energy has studied the impact of both biomass and fossil fuels on global warming over the lifecycle of an electrical generating facility. When all aspects are taken into account, using biofuels rather than coal, even when the carbon from burning the coal is sequestered, leads to a 148% reduction in the global warming potential for the power plant.
On the reverse side of the environmental equation, raw biomass is known to emit a number of particulates as well as polycyclic aromatic hydrocarbons (PAHs). Burning solid biomass directly contributes to reduction in air quality, often to a greater degree than oil or other hydrocarbons. Burning animal waste creates more dioxin and chlorophenol pollutants than burning wood does. This is particularly harmful when it is burned indoors without venting.
PAHs are well-known carcinogens having the potential to damage DNA and cause birth defects. Dioxins are derivatives of PAHs and are known to be highly toxic to fish and wildlife. A dioxin level as low as 0.5 micrograms/kilogram (about 0.0000005% by mass) are lethal to some species. Chlorophenol is also an aromatic compound. It is commonly used in pesticides, herbicides, and disinfectants. It is one of the primary components of mothballs. Chlorophenol is less toxic than the above compounds, with lethal doses in the range of 600 milligrams/kilogram or about 50% by mass. Long-term exposure to relatively high levels can lead to damage to red blood cells and to the immune system.
Wood constitutes the majority of biomass that is burned for fuel and comes in the forms firewood, charcoal, chips, pellets, and sawdust. The use of wood as a fuel for cooking, heating, and other applications dates back to well before humans when Neanderthals were the predominant species of hominid. In fact, the most troubling aspect of using wood as a fuel is generating the spark to start the fire. Otherwise, wood is readily available, abundant, and can even be collected from the ground if cutting tools are not available. Today, wood is even used in some electric generating applications.
Wood is reasonably energy dense. Hardwoods have an energy density of around 14-15 MJ/kg if burned with 100% efficiency. As with any fuel, however, the efficiency tends to be lower. Wood is actually more efficient than many fuels, with about 70% of the energy content (10 MJ/kg) recoverable on average.
The downside to wood is pollution. Not only does it produce more carbon dioxide than fuels like methane, it also produces other pollutants like soot, smoke, and PAHs. Research has lead to stoves that burn at extremely high temperatures (over 600 degrees Celsius). Temperatures this high actually allow the smoke itself to burn, which reduces emissions.
More than 2 billion people across the planet burn dried animal dung for energy. Its benefits are that it is cheap, can be found in areas were wood is scarce, is renewable, and contains a reasonable amount of energy. Cow dung, for instance, is about 50% methane and 30% carbon dioxide by mass when converted into biogas. Of course, burning it directly is a different matter and less energy can be extracted. Cow dung has an energy density of approximately 12 MJ/kg if burned with 100% efficiency.
Unfortunately, burning animal dung efficiently is even more difficult than burning wood efficiently. It also produces a number of pollutants and is a major health hazard in countries where it is burned indoors with limited ventilation. Animal dung tends to have much higher levels of dioxins and chlorophenols as explained above.
Otherwise known as rubbish, garbage, or refuse, this biofuel includes pretty much everything that humans discard on a daily basis. In general, this waste is not directly burned, but rather is converted through a number of different processes into useable, cleaner fuels.
Methane can be harvested from landfills through a process known as “landfill gas capture.” Waste can also be gasified directly at high temperatures and with controlled concentrations of oxygen and steam to produce syngas. Finally, waste can be put through a process known as pyrolysis. In this reaction, decomposition is enhanced though the application of higher temperatures and anaerobic conditions. This is the process used to make char, which is similar to charcoal. Finally, waste can be burned directly, which is generally not allowed in most developed nations as it produces large amounts of pollutants and toxic gases.
The last major category of solid biofuel includes crops grown explicitly for burning. Though some people include the conversion of these crops to biofuels, they are no longer solid at that point and so are not included in this discussion.
Most crops grown for direct combustion are woody. In most cases, these crops are dried and converted to pellets for easy transport. They are then burned either alone or in cogeneration plants where they are combined with other fuels. Many home heating burners use pellets. Crops that are grown for conversion to liquid biofuel generally have high oil content or produce lipid, which can be converted into various liquid fuels. Crops grown for direct combustion include switchgrass and elephant grass, though both are also converted to ethanol in some settings.