Technology Status - NET Nowak Energie & Technologie AG

104
Conversion
Combustion is the most widely-used type of biomass-derived energy
conversion. The burning of biomass produces heat and/or steam for
immediate cooking, space heating and industrial processes, or for indirect
electricity generation via a steam driven turbine. Most of today’s biopower
plants are direct-fired systems – the higher the steam temperature and
pressure, the greater the efficiency of the overall plant. While steam
generation technology is very dependable, its efficiency is limited. Bioenergy
power boilers are typically in the 20-50 MW range, compared to coal-fired
plants in the 100-1,500 MW range. The small-capacity plants tend to have
lower efficiency because of economic trade-offs: efficiency-enhancing
equipment cannot pay for itself in small plants. Although techniques exist to
boost biomass steam generation efficiency above 40%, plant efficiencies
today are typically in the 20% range.
Co-firing means that biomass can substitute for a portion of conventional
fossil fuel in an existing power plant furnace. Often, the biomass is chipped
wood that is added to the feed coal (wood being 5-15% of the total) and
combusted to produce steam in a coal power plant. Co-firing is welldeveloped
in the US but is still undergoing research as electricity companies
examine the effect of adding biomass to coal, in terms of specific power plant
performance and potential problems. Because much of the existing power
plant equipment can be used without major modifications, co-firing is far less
expensive than building a new biopower plant. Compared to the coal it
replaces, biomass produces less sulphur dioxide (SO 2 ), nitrogen oxides
(NOx) and other air emissions. After “tuning” the boiler for peak
performance, there is little or no loss in efficiency from adding biomass. This
allows the energy in biomass to be converted to electricity with the high
efficiency (in the 33-37% range) of a modern coal-fired power plant.
Pyrolysis is the process of decomposition at elevated temperatures (300-
700°C) in the absence of oxygen. Products from pyrolysis can be solid (char,
charcoal), liquids (pyrolysis oils) or a mix of combustible gases. Pyrolysis has
been practised for centuries, e.g. the production of charcoal through
carbonisation. Like crude oil, pyrolitic, or “bio-oil”, can be easily transported
and refined into a number of distinct products. Recently, the production of
bio-oil has received increased attention because it has higher energy density
than solid biomass and is easier to handle. Bio-oil yields of up to 80% by
weight may be obtained by the process of fast or flash pyrolysis at moderate
reaction temperatures, whereas slow pyrolysis produces more charcoal
(35% to 40%) than bio-oil. A main advantage (with respect to energy density,
BIOPOWER X5