Environmental Engineering Reference
In-Depth Information
counterpart, high heating value (HHV) of a fuel. LHV means the fuel and air
combustion starts at 25 C, and when complete, the water product remains as vapour
at 150 C. When the combustion proceeds in the same manner but the products
return to the initial temperature, then the additional heat recovered is greater than
the LHV value; hence, HHV and the water product is in liquid state at 20 C.
Stage I
reformer
(12.2)
Gas (12.3)
separation
NG
High pressure
storage tank
CO
H 2
Steam
Gas
shift
reactor
FC
cathode
stack
H 2
Comp
Boiler
H 2
*
Water
I fc
A ir
Comp
Anode
CO 2
exhaust
P elec
*
I fc
P e _comp
ESS
and
Control
P e _air
U dc
Figure 12.1 Illustration of essential components of a reformer as may be used in a
conventional FCHV
For example, if we take methane, the major constituent of natural gas, and
combust it in air, the reaction products are just water vapour and carbon dioxide.
When this exothermic reaction proceeds as complete and perfect combustion, then
1 mole of methane, CH 4 , reacts with 2 moles of oxygen to produce 2 moles of water
vapour and 1 mole of carbon dioxide as shown in (12.1)
exothermic
CH 4 þ 2O 2
¼)
2H 2 O þ CO 2
ð 12 : 1 Þ
Using (12.1) as a starting point, the essentials of a steam methane reformer
using NG or methanol fuel input can be described. Steam methane reformer is the
conventional means of producing hydrogen by means of an endothermic reaction
with methane and steam to produce carbon monoxide and hydrogen as the initial
reaction (12.2). The subsequent reaction is to convert the carbon monoxide, a
poison to a fuel cell stack, by reacting it in a slightly exothermic reaction (12.3)
with steam as shown in Figure 12.1. The first stage reaction (12.2) produces the
majority (3 moles) of hydrogen with the second stage consisting of gas separation
and gas shift reactor (12.3) yielding an additional 1 mole of hydrogen:
endothermic
CH 4 þ H 2 O
¼)
CO þ 3H 2
ð 12 : 2 Þ
exothermic
CO þ H 2 O
¼)
H 2 þ CO 2
ð 12 : 3 Þ
In (12.2) the input of heat energy via steam in the presence of a nickel catalyst
promotes the reaction in the direction noted. The reaction is of course reversible
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