Environmental Engineering Reference
In-Depth Information
temperature of the dry sludge from 105
o
C to reach target reactor temperature of 500
o
C to be
about 770 kJ per kg, while the energy consumed during pyrolysis they assumed to be 300
kJ/kg. Thus, the total energy consumption for pyrolysis at 500
o
C is 3290 kJ/kg. Energy
outputs for pyrolysis include the amounts and heating values of the oil, gas and char products,
which is not easy to estimate. Using data from Kim and Parker (2008), the average yields of
oil, gas and char were used as 25%, 10% and 65%, respectively, based on dry sludge (or
2.5%, 1% and 6.5% yield of oil, gas and char, respectively, based on TWAS). Average
calorific values in kJ/kg of oil, gas and char were used and are 22,500, 1400, and 16,500
respectively. The total energy output is thus estimated to be 1649 kJ/kg. The net energy
efficiency, calculated using Eq.1 is determined to be about 0.5, suggesting that pyrolysis is
still an energy inefficient process. This conclusion may be supported by the work of Fytili and
Zabaniotou (2008), which shows that pyrolysis is a rather endothermic process, of a
magnitude of 100 kJ per kg of dry sludge.
2.3. Gasification
Gasification is a thermo-chemical process during which carbonaceous content of coal,
biomass or lignocellulosic wastes is converted to a combustible gas at a high temperature (as
high as 900-1400°C) into combustible gases (e.g. H
2
, CO, CO
4
, and CH
4
) in the presence of
air, O
2
or steam (Fytili and Zabaniotou, 2008). Gasification technology has been widely used
around the world for the production of a fuel gas (or producer gas) from coal biomass.
Typical producer gas from an air-blown gasification process has the following compositions:
9 vol% H
2
, 14 vol% CO, 20 vol% CO
2
, 7 vol% CH
4
, and 50 vol% N
2
, with a calorific value
of 5.4 MJ/Nm
3
(Furness et al., 2000). As a comparison, an O
2
-blown gasification process
produces a gas of the following compositions: 32 vol% H
2
, 48 vol% CO, 15 vol% CO
2
, 2
vol% CH
4
, and 3 vol% N
2
, with an increased calorific value of 10.4 MJ/Nm
3
(Furness et al.,
2000).
Biomass gasification technology has received increased interest in the last decade since it
offers several advantages over direct combustion, such as reduced CO
2
emission, compact
equipment with small footprint, accurate combustion control, and high thermal efficiency
(Rezaiyan and Cheremisinoff, 2005). Gasification technology if integrated with combined
cycle gas turbine system has an overall thermal efficiency of 70-80% and an electrical
efficiency as high as 50%, offering better perspectives for power generation from biomass
(IEA Bioenergy Executive Committee, 2007). It has been shown that biomass gasification
plants can be as economical as conventional coal-fired power plants (Badin and Kirschner,
1998). Gasification, in particular larger scale circulating fluidized bed (CFB) concepts, also
offers excellent possibilities for co-firing schemes. The gas product of gasification, also
known as producer gas or syngas, can be mainly used for steam or heat generation as the fuel
gas, for hydrogen and substitute natural gas production, for fuel cell feed, and for the
synthesis of liquid fuels such as methanol and F-T liquid through various gas-to-liquid
catalytic conversion processes (Dry, 1999). Gasification of biomass has been achieved with
various types of gasifiers, ranging from fixed-bed to fluidized-bed and entrained bed gasifiers,
whose advantages and disadvantages are as summarized by Rampling and Gill (1993) in
Table 3. As revealed in the Table, the major technical challenge for biomass (as well as for
sludge) gasification is associated with ash slagging and the formation and removal of tar (high
