Environmental Engineering Reference
In-Depth Information
content of cellulose and lignin such as agricultural wastes, wood materials, municipal waste,
and energy crops (see Chapter 14). A large amount of resources have being invested in trying
to transform these materials with high content of cellulose into ethanol, but results have been
elusive so far.
Biomass, in areas where it is abundant, can be used for on-site generation of heat, which can
be converted into electricity or used as process heat. Biomass direct burning is probably the
oldest method known by man to produce heat. Today in modern boilers, biomass can be trans-
formed into steam via combustion with efficiencies between 60 and 70 percent (Bessette, 2003).
Even when biomass burning is considered to have no “net emissions” of carbon dioxide, emis-
sions of soot, nitrogen oxides, and sulfur dioxide still take place during combustion. However,
nitrogen oxide and sulfur dioxide emissions happen at a lower level than for fossil fuels.
A second way of using biomass is by first heating the feedstock in a reduced oxygen
chamber at high temperatures (600 to 800°C) and converting the biomass into “syngas,”
which is a mixture of carbon monoxide, hydrogen, and carbon dioxide with heating values
of 4 to 7 MJ/m
3
(Belgiorno et al., 2003). Syngas can be used to fuel a piston engine or a gas
turbine coupled to a generator. It is claimed that gasification increases efficiency and reduces
emissions in relationship to traditional direct burning, but the real advantage is that syngas can
be can be used directly in an engine, a turbine, or in combined heat and power systems.
Use of biomass for on-site power generation has several problems that need to be addressed:
First, there is variability in heating values among different feedstocks, mainly in terms of
moisture content and bulk density. So unless a particular type of biomass, or a constant
mixture, is available on constant bases, burning different feedstocks may create variability
on the power output.
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Second, biomass contains moisture that reduces its heating value. Moreover, dried biomass
is susceptible to absorption of moisture if it is not properly stored.
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Third, because biomass has a low energy density per unit of volume (
see
Fig. 11.2), it
cannot be transported efficiently for long distances.
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The forth problem is the shape factor. Biomass from difference sources comes in different shapes,
so the technology to burn or gasify needs to be flexible to accept different shapes of material.
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Combined heat and power
Typical efficiencies of electricity generated by power utility companies in simple cycles range
between 25 and 38 percent for gas turbines, 20 and 41 for internal combustion engines, 25 and
40 percent in coal boilers coupled to steam turbines, and 15 to 25 percent for wood boilers with
steam turbines. In combined cycle systems, efficiencies reach 40 to 57 percent for a gas turbine
with a heat recovery steam generator and a steam turbine (Bessette, 2003). In the best case (the
combined cycle) for every 100 units of heat contained in the fuel, only 57 units are converted into
electricity and the rest, 43 units, are wasted as heat that is discharged to the atmosphere. This
wasted heat does not have enough energy to produce work, but it still contains sufficient energy
that can be used as a heat source for process heat or district heating. Unfortunately, power plants
are located normally in remote areas far away from industries and cities.
Consumers that need both electricity and process heat, such as in the case of food-
processing plants, can take advantage of this concept by bringing the “power plant” to their
site, so they can produce their own electricity and recover heat that can be used in the process.
This concept is called combined heat and power (CHP) or cogeneration.
The main advantage of CHP is the high-fuel utilization efficiency that can reach up to
80 percent. Because of the higher efficiency, operating costs and emissions of pollutants are
