Pollution
In class we explored the significance of the emission of several pollutants that are formed within and emitted from the major fossil-fuel-fired heat engines. The table below summarizes the problem. The rating scale varies from 0 (little significance) to 10 (great significance).
|
Coal-fired steam-electric generating station |
Natural gas fired combined cycle |
Gasoline automotive engine |
Diesel engine |
|
|
CO |
2 |
1 |
8 |
2 |
|
UHC |
1 |
~0 |
9 |
2 |
|
NOx |
10 |
4 |
10 |
10 |
|
SOx |
10 |
~0 |
1 |
3 |
|
Particulate |
10 |
~0 |
2 |
10 |
|
Toxics (HAPS) |
7 |
1 |
4 |
4 |
|
CO2 (GHG) |
10 |
5 |
8 |
7 |
|
CH4 (GHG) |
3 (coal mine CH4) |
3 (extraction & transport) |
~0 |
~0 |
|
N2O (GHG & strato ozone depletion) |
2 (fluidized bed combustors) |
~0 |
~0 |
~0 |
|
CFCs (GHG & strato ozone depletion) |
0 |
0 |
0 |
0 |
|
Summary |
NOx, SOx, and particulate are removed from the stack gases with specialized equipment. Toxics (eg, Hg) are starting to be seriously examined for reduction. |
The gas-fired combined cycle power plant is relatively clean, though NOx is highly regulated. Emissions increase if the system is switched to oil-firing during the winter. |
CO, UHC, and NOx emissions from the engine are significant. The exhaust emissions control catalyst removes about 90% of the pollutants. |
Diesel emissions are relatively low, except for NOx and black smoke (carbon particulate matter) |
|
Aircraft Gas Turbines (kerosene): UHC and CO from ground operation (taxing); NOx from take-off and landing, and cruising (especially for stratospheric flight with supersonic transports). |
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|
Gas-fired (and propane-fired) residential furnaces and hot water heaters: CO and NOx in moderate range. Cooking: the issue is indoor air pollution. |
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Combustion and Pollutant Formation
Consider the chemically correct, complete combustion of methane with air:
CH4 + 2O2 + 2(3.773)N2 ® CO2 + 2H2O + 2(3.773)N2
In this chemical equation, the carbon is completely oxidized to carbon dioxide and the hydrogen is completely oxidized to water vapor. There is no left over
oxygen or fuel. Every mole of O2 carries along with it 3.773 moles of N2. (Actually, every mole of O2 carries along 3.727 moles of N2, 0.044 moles of argon, and almost 0.002 moles of CO2. Water vapor is also present in the air, though it is not considered in the chemical equations listed here.)
Many practical combustion systems run with excess air. (An exception is your modern automotive engine, which is controlled to spend most its time running exactly at the chemically correct ratio of fuel to air, so that your 3-way exhaust emission control catalyst does its job and removes ~90% of all 3 pollutants – CO, UHC, and NOx – from the exhaust stream.)
Natural gas flames run with about 5% excess oxygen. (An exception is the modern gas turbine engine used in the combined cycle power plant. This runs with large amounts of excess air. In the flame zone, the excess air percentage is ~100%, in order to maintain a relatively cool flame and keep NOx from forming. Overall, the excess air percentage is ~200% in order to keep the turbine blade materials within the temperature range that can be withstood by the blades.)
Oil flames use about 5-10% excess air.
Coal flames use about 10-25% excess air.
Industrial wood flames use about 20-50% excess air.
We rewrite the chemical equation with excess air:
CH4 + 2(1+EA%/100)O2 + 2(3.773)(1+EA%/100)N2 ® CO2 + 2H2O + 2(3.773)(1+EA%/100)N2 + (EA%/100)O2
Note we now have 4 products: CO2, H2O, N2, and unused O2.
Generally, we wish for the excess air percentage to be the minimum amount necessary to reach as close to complete combustion as possible in a practical combustion system. If too much excess air is used, too much exhaust gas goes up the stack, carrying with it too much waste heat.
Since we are considering many types of fuels, we need a general complete combustion equation.
Let the general chemical formula for the fuel be:
CxHyOzSuNv
For natural gas and light oil the C and H are predominant.
For alcohol fuels [such as methanol (CH4O) and ethanol (C2H6O)], for wood, and for coal (especially the low rank coals, such as lignite and sub-bituminous), we need the O.
For the heavier oils and for coal we need to consider the impurities sulfur and organically bound nitrogen (a consequence of the nitrogen in the original vegetation) in the fuel.
For complete combustion of a general fuel, we write:
CxHyOzSuNv + (x+y/4-z/2+u)(1+EA%/100)O2 +
3.773(x+y/4-z/2+u)(1+EA%/100)N2 ®
xCO2 + (y/2)H2O + uSO2 + (x+y/4-z/2+u)(EA%/100)O2 +
[3.773(x+y/4-z/2+u)(1+EA%/100)+v/2]N2
Analysis of the fuel, or other knowledge of its composition gives us x, y, z, u, and v.
So far we have but two pollutants in our products of combustion: sulfur dioxide (SO2) from the sulfur impurity in the fuel, and the GHG gas carbon dioxide.
Where are the other pollutants such as CO, UHC, NOx, and particulate?
This is answered in the next lecture notes.