Page 264 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

240 8 Pre-combustion Air Emission Control

8.4 Biofuels

With the depletion of fossil fuels and the growing concern of climate change,
renewable energy emerges on to the global stage. On the scale of centuries, the
supplies of fossil fuels will be severely depleted. The only sources that could supply
energy indefinitely beyond that time horizon are likely to be nuclear fusion and
renewable energy. Heat and energy can be produced by solar, fuel cell, wind,
geothermal, etc. There is minimal fuel combustion in these energy production
processes and consequently are believed to be able to dramatically reduce the air
emissions from fuel combustion.
On the other hand, cost is the major barrier preventing these technologies to be
commercialized at large scales. Thus far, these alternative energy resources are still
at the research and development stage with limited applications. In addition, it also
takes energy to produce the devices for energy harvesting and conversion. The net
benefit of these technologies should be evaluated based on a life-cycle analysis.
Among all the relevant alternative energy technologies, biofuels are closely
related to the scope of this book. Thereby the following is focused on biofuels.
Biofuels are produced from biomass, which is biological material derived from
living organisms. They grow on the planet earth by converting carbon dioxide
(CO 2 ) with solar energy into HC organic compounds. Biomass can be simply
divided into three categories (Table 8.3), lignocellulosic, starch based, and tri-
glyceride-producing biomass.
The type of biofuels produced from biomass depends on the feedstock and the
process. Table 8.3 summarizes the platforms for biofuels from biomass. Like fossil
fuels, biofuels can also be solid, liquid, and gaseous.

8.4.1 Solid Biofuels

As seen from Table 8.3, solid, liquid and gaseous biofuels can be produced from
different biomass following different engineering processes. Without deviating too
much from the main scopes of this book, typical processes are briefly introduced as
follows.

8.4.1.1 Pulverized Biomass

After drying, biomass can be pulverized and mixed with coal for power generation,
and it is expected to reduce the net carbon emission because carbon in the biomass
is from CO 2 in the atmosphere. Biomass drying consumes energy. Due to the high
moisture content in biomass, loss in thermal efficiency is also a concern. Other
technical questions about fuel feed, boiler combustion chemistry, and ash deposi-
tion and disposal have been raised too [37].