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Appendix E
Basis for Calculations
All calculations were based on a typical internationally traded coal of which the
properties are given in Table E-1. All flow schemes are based on 100 kg maf coal,
and the relevant mass and energy values are all based on this quantity. Taking the
mass values per second we get the energy flows in MW.
For drying the coal from 12.5 to 2% moisture 31 MJth are required. As this heat is
supposed to be supplied by burning part of the clean fuel gas, this corresponds to an
energy penalty of about 19 MJe.
For heating the water for the optimal coal-water slurry feed gasifier to 325°C
121 MJth/100 kg maf coal are required.
For a classical Texaco gasifier, the carbon conversion has been set at 95%. In all
other cases this has been set at 99%.
The energy required for the production of oxygen was taken as 46 MJe/kmole. If
not indicated otherwise, the purity of the oxygen is 95% mole.
For making 300°C process steam for dry-coal feed gasifiers, 3 MJth/kg are
required.
The heat loss from the gasifiers has been taken as 0.5% of the coal LHV for the
coal-water slurry feed gasifiers and as 2% of the coal LHV for the dry coal feed
gasifiers. In the latter case it has been assumed that this heat becomes available for
making steam.
For all compressors and turbines, an isentropic efficiency of 90% has been
assumed. For the adiabatic compression of air to 32 bar, this corresponds to
15.6 MJe/kmole air. For wet air compression the energy data were 10.7, and
13.1 MJe/kmole wet air for compression to 32 and 64 bar, respectively. For nitrogen
compression the same figures have been taken as for air compression.
For gas quenches a recycle gas compressor is required. The energy consumption
is 11 MJe for 100 kg maf coal intake, except for the two-stage dry-feed gasifier
where it is only 7 MJe for 100 kg maf coal intake. The reason for this lower figure is
that the gas has to be quenched from 1100 to 900°C instead of from 1500 to 900°C.
In case of the Tophat cycle, corrections have been applied for the approach
temperatures in the recuperator. As a standard 25°C was taken for the temperature
difference between the turbine outlet and the humidified air leaving the recuperator.
For higher temperature differences the efficiency bonus was one percentage point
per 25°C.