iomass because this is usually a disperse resource and transport can increase its cost. They are composed of a
gasifier, a quite simple gas cleaning system and an internal combustion engine [1,2]. Another interesting option that
can achieve higher efficiency is the use of biomass-fired air blown gasification combined cycle (ABGCC) power
plants like the Va¨rnamo, Arable Biomass Renewable Energy (ARBRE) and Thermie Energy Farm demonstration
projects [1,3–5]. The first plant provided 6 and 9 MW to a district heating system, while the others generate 8 and
14 MW, respectively.
Due to the complexity and cost of an air separation unit, oxy-gasification is used in large coal-fired IGCC power
plants, and for the production of H2 and chemicals [6–8]. The composition of the cleaned gas (mainly CO and H2)
opens the way to CO2 capture by using several techniques (although this possibility is not yet used). For example, a
Nomenclature
ac O2 coefficient in combustion equation
bc CO coefficient in combustion equation
cc CO2 coefficient in combustion equation
CGE cold gas efficiency
d distance
daf dry and ash free
d.b. dry basis
dc H2O coefficient in combustion equation
ec H2S coefficient in combustion equation
fc N2 coefficient in combustion equation
h hydrogen subscript in char formula
hf hydrogen subscript in fuel formula
HRSG heat recovery steam generator
i real operation point
j point in an iso-line
k point in a four-point group
LHV low heating value
max maximum
min minimum
n nitrogen subscript in char formula
nf nitrogen subscript in fuel formula
o oxygen subscript in char formula
of oxygen subscript in fuel formula
p parameter
s sulphur subscript in char formula
sf sulphur subscript in fuel formula
w moisture subscript in fuel formula
w.b. wet basis
wt weight
x independent variable
y independent variable
z dependent variable
0 coordinate of point in a iso-line
d increment
1644 A. Valero, S. Uso´n / Energy 31 (2006) 1643–1655
shift reactor can displace the equilibrium to produce CO2 and H2, and then chemical absorption or membranes [9] can
be used to separate the CO2. Another option consists on burning separately H2 and CO by using oxygen [10]. Finally,
the gas could be burnt with O2 plus CO2 recycled from the same combustion process in the gas turbine.
Oxy-co-gasification is similar to oxy-gasification but replacing part of the coal by biomass, which implies an
additional CO2 emissions reduction. Oxy-co-gasification of coal with straw or sewage sludge in an IGCC has
been studied by British Coal. The University of Essen researched the use of coal/biomass combinations for
IGCC applications, concluding that up to 10% biomass in an oxygen-blown entrained blown gasifier is
technically feasible although net electrical efficiencies would be slightly lower due to energy needed for biomass
pre-treatment [11]. Oxy-co-gasification of coal and biomass in Buggenum IGCC Power Plant has also been
proposed in a study that consists of two parts: preliminary desk study [12] and exploratory experimental
work [13].
It should be noted that the combined use of biomass and coal in the same power plant allows to use biomass
without the main problems of small biomass-fired power plants (high specific cost, low efficiency and shut-off risk if
there is a biomass shortage). One way to do this is by burning coal and biomass (co-firing) [14]. Another option
consists on gasifying biomass and burning the gaseous fuel in a coal boiler [2]. Finally, co-gasification, mainly oxyco-gasification,
allows increased efficiency and reduced environmental impact.