done with ambient conditions of 10 C and a relative humidity of
90%, common winter conditions in the north of Europe. The air is
heated to the same level (123 C) with the steam. The power of the
air heater is compared to the total amount of recoverable heat,
including the heat on the water gas shift reactors and syngas
coolers. The H2:CO ratio after the last shift section is always 2.1:1.
The net thermal production is the heat produced after the gasifier,
cooling and reaction energy of the shift reactor section reduced
with the heat required for the wood drying and to raise steam for
the final water gas shift section. The synthesis gas is cooled and
delivered at a temperature of 220 C and an H2:CO ratio of 2.1:1.
To examine the importance of the dryer, the base case is run
with three different biomass water contents, 10, 15 (reference case)
and 20%. This evaluation only affects the gasification section.
Increasing amounts of water in the biomass introduces an additional
heat load on the gasifier as this water needs to be evaporated.
Table 1 presents the results of this base case for three different
water contents of the biomass.
The results of the base case show that the gasification efficiency
reduces with increasing water content of the biomass in the
gasifier. To best evaluate this efficiency, the sum of hydrogen and
carbon monoxide should be evaluated as this sum remains constant
during the water gas shift reaction.
The base case is now compared to the improved new process
scheme. The results of the simulations are presented in Table 2.
The air intake is significantly reduced by dehydrating the drying
air. This considerably reduces the steam consumption of the air
heater. The heat recovered downstream from the gasifier is 7 MW
less than in the base case, due to the vaporisation heat in the
adsorbent regeneration. The economised 31.4 Mg h1 of LP steam
combined with the economised steam on the water gas shift section
represents 19.5 MW in heat that can be used to produce
electricity. For this base case this represents around 4% of the total
heat balance of the plant that can be saved. When assuming an
electrical yield of 20e25% on a steam turbine, this heat can be
transformed in an additional 4e5 MW of electricity.
The bulk density of activated alumina is around
790e840 kg m3 [28], for the purpose of the calculations we assume
800 kg m3
. With a difference between the loading and
unloading cycles of around 10 g of water for each 100 g of alumina,
we would require 89 Mg h1 of alumina (or 112 m3 h1
) to capture
8.9 Mg h1 of water. If the loading and regeneration cycle takes
10 min, a total of 19 m3 of alumina is in adsorption and an equivalent
amount in regeneration. The heat required for heating and
cooling of the alumina represents 3.6 MW that is transferred from
the heat recovery to the air drying section and should not affect the
overall heat balance.
4