Biomass

Biomass, a source of renewable energy, is biological material such as wood, wood waste, straw, sugar cane, algae, and many other byproducts derived from agricultural and forestry production as well as other sources. Since biomass derives from plants generated by solar energy in the photosynthesis process, it can also be defined as the biological material on Earth that has stored solar energy in the chemical bonds of the organic material.

Uses of biomass

The chemical energy contained in biomass can be processed to create useable fuels. Several processes have been developed to convert biomass into various forms of fuel such as biodiesel, and ethanol. The resulting fuels can provide a cleaner alternative to hydrocarbon fuels because they emit lower levels of by-products such as carbon monoxide (CO) and carbon dioxide (CO2). While there are many methods for processing biomass currently in use, the most important three are gasification, syngas cleaning/processing, and Fischer-Tropsch synthesis.

Biomass gasification

Gasification is a process of burning the biomass source at a relatively high temperature to release carbon monoxide and hydrogen. This process can occur through the use of oxygen, air, steam, or mixtures similar to these. When air is used to carry out the gasification process the required amount of heat is a relatively low to medium heating value. This process requires less thermal energy to complete but creates higher levels of unwanted by-products such as methanol and less of the useable hydrogen product. The use of steam requires a higher amount of thermal energy to carry out the process but will yield higher amounts of actual hydrogen and lower amounts of by-products. Gasification involves four main steps, drying, pyrolysis/devolatilization, reduction and combustion. The drying process consists of taking the biomass source and removing all the moisture. After the moisture is removed the resulting substance enters the pyrolysis zone. This is where volatiles are removed in the form of carbon monoxide and CO2 and also where tar is produced. After this process occurs the resulting substance goes over to the reduction zone where the raw materials are completely gasified in order to create a syngas product. Finally in the combustion zone the left over char material is burned which produces more gaseous product and also produces the necessary heat for the reactions in the previously mentioned reduction zone. The end product is known as syngas. Syngas can be used to fuel internal or external combustion engines for the use of electricity generation. Syngas can also be used to create synthetic petroleum or can undergo further processing.

Syngas purification

When syngas is used for further processing it undergoes a cleaning and purification stage. The gas cleaning stage is the first stage of syngas purification. This process involves the use of mechanical filters which remove particulate matter, and sorbents that remove the alkali and sulfur compounds in the gas. The remaining tar in the gas is then broken down through the use of catalytic steam. Syngas cleaning is a crucial step in preventing the fouling of machinery or contamination of catalysts when further processing the gas.

Fischer-Tropsch synthesis

The last commonly used stage of processing biomass is known as Fischer-Tropsch synthesis. Purified syngas will be run through a series of catalysts consisting mostly of cobalt and iron which will eventually transform the gas into a liquid fuel. This fuel can be used in standard piston engines and can substitute the use of hydrocarbon fuels. This process requires substantial thermal energy to carry out and is a relatively costly process. Efforts have been made to reduce the costs of this process to make it a reasonable competitor to the processing of oil for the creation of petroleum based fuels.

Risks of biomass processing

The risk level of biomass processing depends largely on the inspection and maintenance standards of the processing facility. From 2006 to 2010 there have been approximately 100 incidents in the United States regarding the processing of biomass for biofuels. From 2006 to 2009 there have been 8 fires and 6 explosions on the bases of about 200 biodiesel facilities. Some of these explosions were responsible for the complete destruction of their respective plants. However, 50% of the incidental cases in the last 5 years have not involved the process under normal conditions and 22 % of the incidents have been related to tank storage (overfilling, leaks, etc.). Given that maintenance is properly carried out, incidents such as these will rarely occur. The unstable nature of the materials in addition to the poor plant regulation is the reason why incidents are so frequent. As a result, improvement efforts on regulation, inspection, and maintenance of the processing plants have been carried out with the goal of drastically reducing the number of incidents.

Setbacks and the future of biomass

The cost of collecting and processing biomass is the current main concern for the energy and transportation industries. Efforts are being made to expand the production and distribution of these fuels in order to lower costs to the consumer. However, costs of biofuels are still far higher than the costs of petroleum based fuels. The environmental availability of biomass is not an issue because sources such as food waste and other forms of waste are constantly being renewed. As prices for petroleum based fuels continue to rise and more efficient technologies for processing biomass are developed, demand for these alternative fuels will begin to increase considerably. As a result, biomass may very well be the alternative fuel source of choice for the immediate future.