BIOMASS – THE ONLY RENEWABLE FUEL
The term “Biomass” was created about 1975 to describe natural materials used as energy sources. The Office of Technology Assessment (OTA) estimated in 1980 that biomass could potentially supply more than 20% of US energy requirements – if we were serious about energy independence. In 2006, biomass as fuel, supplies 3% of US energy (see EIA and their table of biomass consumption).
While biomass is one of the best forms of renewable energy, it is not a great fuel. It occurs in a wide variety of forms (wood, paper, trash, agricultural waste, etc.). This can be reduced by densification (or pelletization) to a uniform fuel that can be easily shipped, stored and used.
Biomass varies widely in both mass and volume fuel density. It varies in chemical composition and the proximate/ultimate analysis gives records this data. It often has high water content, and the different methods of recording and measuring MC can be confusing.
While biomass can be used directly (mostly in wood fires), it can be converted to higher forms of fuels. Biomass is converted to various fuel forms in two primary processes:
• thermal (combustion, pyrolysis and gasification) and
• biological (fermentation and digestion) processes.
The chemistry of biomass and other conversion processes is simple, primarily carbon, C, hydrogen, H and oxygen, O. A brief explanation is given here in terms of a “Ternary diagram” of fuels.
Air/Fuel Ratios for biomass pyrolysis, gasification and combustion:
• Air is the primary requirement for thermal reactions of biomass, and adjusting to the correct air/fuel ratio has given us incredibly clean cars in the last 20 years. This diagram shows the air-fuel ratios for pyrolysis, gasification and combustion of biomass.
This website is devoted primarily to biomass gasification. However, all combustion and gasification processes must pass through pyrolysis at low temperature, so there is a lot of information about pyrolysis and combustion, as well.
One inch birch dowell pyrolysing at 600C
Biomass fuels are characterized by what is called the “Proximate and Ultimate analyses”. This fuel can be burned directly for heat or to make a gas for power.
The “proximate” analysis gives moisture content, volatile content (when heated to 950°C), the free carbon which remains (i.e. bio-char), the ash (mineral) and the High Heat Value (HHV) based on the complete combustion of the sample to carbon dioxide and liquid water. (The low heating value, LHV, gives the heat released when the hydrogen is burned to gaseous water, corresponding to most heating applications.)
The “ultimate analysis” gives the composition of the biomass in wt% of carbon, hydrogen and oxygen (the major components) as well as sulfur and nitrogen (if any).
The table of Proximate and Ultimate analyses is found in Appendix A of An Atlas of Thermal Data for Natural and Synthetic Fuels, by S. Gaur and T. Reed, Marcel Dekker, is available in our web store. It gives analyses of over 140 fuels, including biomass components, natural biomass (woods, agricultural products), processed biomass, other solid and liquid fuels.