The Value of Biomass
The term biomass is defined as “organic material of recent biological origin”. What’s important about the “recent biological origin” is that this plant material has removed carbon dioxide (a dominant “greenhouse gas”) from the atmosphere during the recent past. Biomass is a renewable energy feedstock because its abundance could persist in perpetuity on the earth with proper cultivation. In general usage, the term is used to refer to fibrous plant material such as corn stalks, wood chips, forestry residues, wheat straw, grasses, rice husks, etc.
Potential as Transportation Fuels
According to a report by the USDA & USDOE, the current availability of forestry and agricultural biomass which could be utilized for energy is approximately 473 million tons per year. Theoretically, this amount of biomass could be converted into 30 to 50 billion gallons/year of ethanol – about ¼ of the equivalent of our national gasoline consumption. With reasonable assumptions for continued yield increases, plant improvements for biomass production, increased perennial and wood fiber cropping, and improved utilization of waste streams, the study projects that such biomass availability could be tripled in the next 50 years. This biomass availability is above and beyond what’s necessary to meet food, feed, and export demands and is mostly underutilized. Many forms of renewable energy only suit electricity production and are, therefore, difficult to use as vehicle fuel. Biomass is uniquely suited among renewable energy sources for conversion to transportation fuel.
Renewable Carbon Cycle
In the process of plant growth, light from the sun provides energy for the conversion of carbon dioxide and water into the carbohydrates that make up plant matter. This process of photosynthesis produces oxygen while removing carbon dioxide from the atmosphere.
When this reaction is reversed during gasification, the carbon dioxide given off is carbon dioxide that was previously removed from the atmosphere as the plant grew. While a minor amount of fossil fuel will be needed to produce and transport biomass, its net carbon balance is close to zero.
Biomass vs. Coal
Plant matter generally has a complex structure of glucose polymers known as cellulose and hemicellulose as well as structurally-important lignin. On a moisture-free and ash-free basis (maf), most biomass contains ~80% volatile matter and 20% fixed carbon. In contrast, group-1 bituminous coal has 80% fixed carbon and just 20% volatile matter (maf basis). So biomass is generally more reactive than coal, being dominated by volatile matter with a less significant portion of fixed carbon content.
Biomass ash contains essentially benign minerals from air, water, and soil pollution perspective (in contrast to coal that has a host of bad actors). Biomass ash typically has a lower melting point than coal ash because it is less refractory in composition (less alumina and silica, more iron, potassium, phosphorous). Ash content for biomass can vary from less than 1% for wood to as much as 12% for straw and bagasse (sugar cane residue). A much higher portion of feedstocks like animal manures results in ash, often exceeding 20% of feed composition. But this is a good thing. Biomass ash is valued as a soil amendment because of mineral nutrients such as phosphate and potash, unlike coal-derived ash. Because of biomass ash properties, Frontline utilizes dry-ash gasification (non-slagging) which keeps the minerals in the ash available for plant nutrition when added to soils.
Biomass Heating Value
The raw heating value of biomass is typically 7000-9,000 Btu/lb (higher heating value) on a moisture-free, ash-free basis. By comparison, one ton of biomass in its raw state contains the equivalent energy of 150 gallons of gasoline! For more information on characteristics of specific types of biomass check the sites of DOE and ECN.