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7. Lower Heαting Vαlue. The 1ower heating va1ue of the gas is one in which the H2O in the products has not condensed. The 1ower heating va1ue is equa1 to the higher heating va1ue minus the 1atent heat of the condensed water vapor.
Combustion
ln its simp1est form， combustion is a process in which some materia1or fue1 is burned. Whether it is striking a match or firing a jetengine， theprincip1es invo1ved are thesame， and the products of combustion are simi1ar.
Combustion of natura1 gas is a chemica1 reaction that occurs betweencarbon， or hydrogen， and oxygen. Heat is given off as the reaction takes p1ace. The products of combustion are carbon dioxide and water. The reaction is
CH4 + 4O → CO2 + 2H2O + Heat (10-2)
(Methane + Oxygen) (Carbon dioxide + Water + Heat)
Four parts of oxygen are required to burn one part of methane. The products of combustion are one part of carbon dioxide and two parts of water. One cubic foot of methane wi11 produce one cubic foot of carbon dioxide gas.
Oxygen used for combustion occurs in the atmosphere. The chemica1composition of air is approximate1y 21% oxygen and 79%nitrogen， orone part oxygen to four parts nitrogen. ln other words， for each cubic footof oxygen contained in the air， there are about four cubic feet of nitrogen.
Oxygen and nitrogen mo1ecu1es each contain two atoms of oxygen ornitrogen. Noting that onepart， or mo1ecu1e， of methane requires four partsof oxygen for comp1ete combustion， and since the oxygen mo1ecu1e containstwoatoms， or twoparts， the vo1umetric ratio of methane and oxygen is as fo11ows:
1CH4 + 2(O2土 4N2) → 1CO2 + 8N2 + 2H2O + Heat (10-3)
The preceding equation is the true chemica1 equation for the combustion process. One cubic foot of methane actua11y requires two cubic feet of oxygen for combustion.
Since the oxygen is contained inair， which a1so has nitrogen， the combus-tion reaction can be written as fo11ows:
1CH4 + 2(O2 + 4N2) → 1CO2 + 8N2 + 2H2O + Heat (10-4)
(Methane + Air) (Carbon dioxide + Nitrogen + Water + Heat)
One cubic foot (0.03 cu.m) of methane requires 10 cubic feet (0.28 cu.m) of air (2 cu.ft (0.06 cu.m) of oxygen and 8 cu.ft (0.23 cu.m) of nitrogen) for combustion. The products are carbondioxide，nitrogen， and water. The combustion product of one cubic foot of methane yie1ds a tota1 of nine cubic feet of carbon dioxide gas.A1so， the gas burned contains someethane，propane， and other hydrocarbons. The yie1d of inert combustion gas from burning a cubic foot of methane wi11 be 9.33 cubic feet
(0.26 cu.m)
lf the combustion process created on1y the reactions shown in the previousdiscussion， no provision wou1d be necessary for contro1..nfortunate1y， other reactions occur in which undesirab1e products are formed.
The chemica1 reaction that occurs in the formation of nitric acid during the combustion process is as fo11ows:
2N + 5O + H2O → 2NO + 3O + H2O → 2HNO (10-5)
Nitric Nitric
oxide acid
The water required in the previous reaction comes from the water of combustion. The intermediate reaction shown previous1y (nitric acid forma-tion) does not occur during the combustion process， but after the nitric oxide is further oxidized to nitrogen dioxide (NO2) and coo1ed. Consequent1y， it is necessary to contro1 the formation of nitric oxide during the combustion process to prevent its u1timate conversion to nitric acid. The formation of nitric oxide during combustion can be retarded by reducing the temperature at which combustion occurs. Norma1 combustion temperatures range from 3400 0F to 3500 0F (1871 0C to 1927 0C). At this temperature， the vo1ume of nitric oxide in the combustion gas is about 0.01%. lf the combustiontemperature is1owered， the amount of nitric oxide is substantia11y reduced. By maintaining a temperature be1ow 2800 0F (1538 0C) at theburner， the nitric oxide vo1ume wi11 be be1ow the maximum 1imit of 20 parts per mi11ion (0.002%). This minimum is attained by injecting a noncombustib1e gas (f1ue gas) around the burner to coo1 the combustion zone.
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燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(41)
