Biofuels are manufactured from various types of biomass. Most biofuels are liquids, principally ethanol and biodiesel, produced to replace or supplement fossil fuels used in transportation. Used most commonly as a supplement to gasoline to reduce carbon emissions and reliance on imported petroleum, ethanol is the most common biofuel worldwide.
Biomass comes from recently living plants and animals. It is a renewable energy source. Following are some of the sources and uses of biomass:
- Wood and the waste from wood processing is used to heat buildings, in industrial processes, and to generate electricity.
- Crops and waste are used as fuels or biofuels.
- Animal manure and human sewage are converted to biogas, a fuel.
A fuel's energy density is a measure of how much energy can be released from a given volume of the fuel. For example, the energy densities of coal and gasoline are both much higher than that of wood, because wood contains about 50% water, plus a substantial amount of other materials which cannot be burned. Thus, the energy released from burning a cubic foot of wood is much less than that derived from burning a cubic foot of coal or gasoline.
Low energy density for biomass leads to problems in gathering and transporting sufficient amounts of it to locations where the raw fuel can either be burned or processed. Biomass tends to be bulky and inconvenient to handle, contributing to the expense and difficulty of using it as a source of fuel.
The environmental impact of using biomass is mixed in its good and bad effects. Burning crops such as wood or switchgrass is simply a recycling of CO2. It has a net zero effect on the presence of most greenhouse gasses. On the other hand, burning such products can produce smoke with pollutants that can cause harm. Also, if the growth of vegetation, such as wood, cannot keep up with the use of it as fuel, then depletion of the vegetation (e.g. deforestation) can produce considerably negative impacts on the local population.
Burning animal or human waste in disposal plants can result in the release of chemicals hazardous to both local people and the environment in general. These types of problems can be solved, but only by installing and maintaining expensive removal processes for the hazardous compounds emitted in the smoke from the burning or the ash remains of the combustion process.