Biomass Feasibility Project Final Report - Xcel Energy

CHAPTER V : BIO-POWER CONVERSION
TECHNOLOGIES
Human beings have used biomass fuel since cavemen learned how to set wood on fire.
Biomass eventually was supplanted with coal, gas and petroleum, which remain the
predominant fuels for generating power to the present day. Until recently, advances in energy
technologies meant extracting more energy from the combustion (or burning) of hydrocarbon
fuels, not inventing other ways of extracting it.
Most non-nuclear power plants still rely on combustion; they burn fossil fuels (coal, oil, natural
gas) in boilers to turn water into steam that drives turbines connected to electrical generators.
Back in the gaslight era, engineers began to develop a different way to produce energy.
Instead of burning fuel in oxygen-rich boilers, carbonization or gasification heated coal to high
temperatures in oxygen-lean “anaerobic” vessels. Since the vessels lacked oxygen to support
combustion, the superheated coal didn’t burn. Rather it gave off a synthetic gas, called “town
gas” or “coal gas,” that could be used for lighting, heating and cooking. The remaining
charcoal could fuel heat-intensive industrial processes like smelting. In the 1930s and 1940s,
German scientists made further innovations in order to replace petroleum supplies cut off by the
war.
Our discussion divides into those two main topics: combustion and gasification. Both conversion
technologies are in use today. But combustion systems far outnumber gasification systems, and
within combustion plants, fossil fuels (coal, natural gas, petroleum) far overshadow biomass fuels.
Recently, however, the environmental consequences of burning fossil fuels have created new
interest in humankind’s original fuel source, biomass.
In a combustion power plant, differences in processing the two classes of fuels, fossil and
biomass, appear in the front end where fuel is handled, processed and burned. Beyond the
boiler, both fuels drive power generation in exactly the same way. So we focus on the
distinctive characteristics of biomass fuels that affect their handling, processing, burning and
waste removal in a power plant.
A more recent technology than combustion, gasification, has a host of potential applications in
waste reduction, biorefining, transport fuels and so on. But we are concerned here with only its
application to the generation of electric power. As is the case of combustion, biomass in a
gasifier usually substitutes for fossil fuel. And since gasification can produce both gaseous and
liquid fuels, that means it can offer substitutes for natural gas, oil, and diesel fuel.
The diagram below illustrates our selection of topics. Issues in boldface are subjects of this
chapter.
Identifying Effective Biomass Strategies: Page 53
Quantifying Minnesota’s Resources and Evaluating Future Opportunities