Environmental Engineering Reference
In-Depth Information
Steam
Electricity
Turbine
Generator
Boiler
Condenser
Biomass
Ash
Stack
Figure 3.9
Direct combustion process of biomass for electricity generation through a steam-
turbine generator system.
Figure 3.9 depicts the process of electricity generation from biomass combustion
through a steam turbine-generator system. Initially, biomass is milled and burnt in a
boiler to produce high-pressure steam. The steam passes through a series of turbines
which are connected to electric generators.
The direct combustion of biomass for energy generation faces several
problems. Biomass is diverse in nature, and generally has the detrimental
combination of low physical and low energy density as a fuel. The calorific
value (heat of combustion) of biomass is typically around 50% of coal. Poorer
physical characteristics such as higher water content and poorer grind ability
generally mean that collection, transportation, and use is not only challenging in
terms of logistics and processing, but also questionable in terms of overall envi-
ronmental benefits. In addition, the direct combustion of biomass also results in
a lost opportunity with regards to the co-production of chemicals and energy.
Several options are available for the valorization of biomass by condensing the
energy within. In terms of thermally based treatments, the most common are as
follows.
1.
Gasification
: a high-temperature process (between 750 and 1800°C) of degra-
dation of biomass to produce energy-rich gases.
2.
Pyrolysis
: a medium- to high-temperature process (400-750°C) whereby the
biomass is degraded to yield mainly liquid products for energy generation.
3.
Torrefaction
: a low-temperature treatment (below 300°C) of biomass to
produce a higher-energy-density solid fuel.
Commercial uptake of these technologies faces a number of barriers. The set-up
and processing costs are typically high, the quality of products in comparison to
fossil fuels is typically poorer, and the enhancement in the energy/calorific value
is low (e.g., on average only about 10% in the case of torrefaction). Furthermore,
