Biomass Fuel Quality Enhancement and Respiratory Quotient (RQ) for Ranking Fossil and Biomass Fuels Based on CO2 Emissions - Novel Combustion Concepts for Sustainable Energy Development - page 43

1 Introduction

Coal accounts for 45 % of the total generated electricity in the United States (EIA
2013 ). Along with CO 2 , harmful pollutants such as nitrogen oxides (NO X ), sulfur
oxides (SO X ), and mercury are also released during the combustion of coal in power
generating facilities. Usage of alternate renewable energy sources is investigated to
reduce the impact of fossil fuels on the environment. Biofuels are one of the
renewable energy sources which contribute about 10 % of the total energy in the
world (EIA 2013 ). Desired characteristics of biomass crops which can be used for
power generation were listed by Mckendry ( 2002 ). They include high yield, low
energy input for production, low cost, low contaminants, and low nutrient
requirements. Though wood was the dominant source of energy in the United States
till 1850, its usage slowly decreased with the utilization of coal (Tillman et al.
2012 ). With current regulations on the emissions, the focus has again shifted toward
the use of wood and other biomass fuels along with coal. The carbon dioxide
emitted during the combustion of biomass fuels is supposed to be absorbed by the
growing plants in the carbon cycle, and hence, biomass is considered to be carbon
neutral. The term biomass includes wood, forest wood residues, agricultural wastes,
energy crops, animal wastes, and municipal solid wastes.
Although biomass has been extensively studied as a renewable energy source,
most research conducted used raw untreated biomass. While the use of raw biomass
as a renewable energy source has potential, pretreatment of raw biomass could
further enhance its capabilities as a fuel. Raw biomasses have relatively low energy
contents and generally contain high amounts of moisture resulting in reduced con-
version ef

ciency. The higher amount of oxygen contained within biomass also
results in a lower calori c value. The torrefaction process directly affects the oxygen

content via the liberation of volatiles that contain oxygen. This causes a decrease in
the O/C ratio and results in a biomass with a larger higher heating value (HHV).
Torrefaction is a pretreatment process that aims to improve the quality of bio-
mass by reducing the moisture content and increasing its calorific value via
decreasing the oxygen percentage within the biomass. This is done by heating
biomass at a constant temperature between 200 and 300

C for a set period of time
in an inert environment. Different gases which are used to maintain an inert
environment includes nitrogen (Bridgeman et al. 2008 ; Arias et al. 2008 ; Almeida
et al. 2010 ), argon (Prins et al. 2006b ) and recently wet torrefaction using hot
compressed water were studied to improve the energy density of biomass (Yan
et al. 2010 ). Effect of using a small amount of oxygen on torrefaction was studied
by Rousset et al. ( 2012 ) and Wang et al. ( 2012a ). Effect of using CO 2 as the
pretreatment medium was studied by Eseltine et al. ( 2013 ). Under certain torre-
faction conditions, the resulting biomass can have properties comparable to low-
rank coals (Chen and Kuo 2010a ). Increasing the calori

°

c content of biomass by

lowering the oxygen carbon ratio makes it more suitable for use as a fuel for

-

gasi cation (Prins et al. 2006a ). This coupled with the increased grindability makes

torre ed biomass an attractive fuel for co- ring applications as well.