Emission control is undoubtedly one of the main requirements in boiler plant operation. Depending on fuels and boiler equipment, exhaust gases contain by-products in different quantities, some of which are air pollutants. Small and medium-sized boiler plants, which cannot economically accommodate abatement systems, must cope with this problem by a correct choice of fuel and/or of burners and auxiliary equipment.
Although every country has its own environmental protection regulations, a general classification of major pollutants can be made as follows: sulfur dioxide, nitrogen oxides, carbon monoxide, particulate matter, and hydrocarbons. In addi- tion, other pollutants are always present depending on the particular fuel and operating conditions.
Typical ranges of values of permitted pollutant concentrations in air emission from boiler plants are reported in Table6.8 for the above-listed pollutants. The permitted values vary from country to country, but boiler plants must be conducted so as to ensure values lower than those in the table.
6.11.1 Sulfur Dioxide
Sulfur dioxide SO2is formed during combustion (see Sect.6.2) by the combination of sulfur contained in the fuel with oxygen from combustion air.
Table 6.8 Typical ranges of values of permitted pollutant concentrations from combustion
Pollutants
Level of concentrationCRa
mg/Sm3 ppmb
Sulfur dioxide 1,500–2,000 525–700
Nitrogen oxidesc 50–200 102–410
Carbon monoxide 100 80
Particulate matter 100–150
aReferred to dry gas and to a concentration of O2equal to 3 % in volume. To convert from the actual pollutant concentra- tionCx(with an oxygen concentrationWx) to the concentra- tion referred to 3 % value of oxygen concentration, use the following relationship:CR¼(18/(21Wx))Cx
b10,000 ppm¼1 % in volume
cExpressed in NO2
Except for sulfur compound particles, most of the sulfur contained in the fuel is converted into SO2, but a small percentage of the total oxides is converted into sulfur trioxide SO3. Sulfur oxides combine with the moisture present in stack flue gas to form sulfuric acid, some of which can condense and corrode metallic parts of the boiler. Similarly, in the atmosphere a portion of SO2is converted into SO3and then into sulfuric acid and sulfur compounds.
The quantity of sulfur oxides formed during combustion is closely related to the sulfur content of the fuel and does not depend on boiler operating conditions. Thus, reduction of sulfur oxides can be obtained by using low-sulfur fuel, natural gas in particular, or by introducing abatement devices such as stack gas scrubbers, which remove SO2from stack flue gas.
The maximum SO2concentration in stack gas must be kept lower than 1,500–2,000 mg/Sm3or 525–700 ppm(see Table6.8).
Emulsifying fuel oil with water or other compounds and atomization at the burner are other techniques widely applied to solve problems due to the use of heavy oil.
6.11.2 Nitrogen Oxides
Nitrogen oxides NOxare formed during combustion by the combination of oxygen and nitrogen at high temperature. Both components are naturally present in com- bustion air and nitrogen in the fuel itself.
NOxemissions are mainly NO (95 % in mass) and NO2(5 % in mass). In the atmosphere NO combines with oxygen to form NO2.
NOxformed during the combustion process can be classified as Thermal, Fuel, or Prompt:
• Thermal NOx (via Zeldovich mechanism) is formed due to the oxidation of atmospheric nitrogen at temperatures higher than 1,273 K (1,000C, 1,832F).
It becomes important when the temperature reaches the range 1,600–1,800 K (1,327–1,527C, 2,421–2,781F);
• Fuel NOxis formed due to the oxidation of the nitrogen in the fuels (its value ranges between 0.05 and 1.5 % in mass depending on the kind of fuel; lower values for natural gas and light oil, higher values for heavy oil and coal). The oxidation is accelerated if excess air value is high;
• Prompt NOx derives from the reaction of atmospheric air with hydrocarburic radicals.
The concentration of NOxin stack gases is related mainly to the quantity of nitrogen in the air;typical values are 50–200 mg/Sm3or 102–410 ppm (see Table6.8).
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Although the reduction of nitrogen in the fuel is a technique that should be practiced, most technologies applied to reduce NOx are based on lowering peak flame temperature and on reducing the amount of oxygen available in the flame.
Low excess-air burners, fuel-rich or staged-firing burners with separate primary and secondary combustion zones and flue gas recirculation are the main means used to reduce the production of NOxinside the boiler. In the case of coal, fluidized-bed boilers are suitable for this purpose. Steam injection in the combustion chamber is also used to reduce NOx.
Post-combustion NOxcan be reduced by means of abatement devices generally installed downstream of the combustion zone but above air preheaters such as selective-catalytic-reduction units (SCR) and selective-non-catalytic-reduction units (SNR). The choice of the best solution, both technically and economically, must be made individually for each plant in the light of local regulations and boiler operating modes.
Table6.9lists the main techniques used and comments on how they can reduce NOxemissions and influence combustion efficiency.
6.11.3 Carbon Monoxide
Carbon monoxide CO is always produced with incomplete combustion (see Sect.6.2), and its concentration in stack flue gas is closely related to boiler operating conditions. CO concentration at the stack must be very low; concentrations higher Table 6.9 Main techniques used to reduce NOxemissions
Techniques
Effects on
NOxemissions Combustion efficiency
Excess air (+) (+) ()
Flame temperature (+) (+) (+)
Preheating of air (+) (+) (+)
Steam injection () ()
Gas recirculating (+) ()
Combustion chamber size (+) ()
Combustion chamber charge (+) ()
Heat production rate (+) (+)
Heat production exchange (+) ()
Duration of combustion (+) (+)
Burners integration (+) (+)
Nitrogen in fuel (+) (+)
Type of fuel
Coal (+)
Oil (+)
Natural gas ()
(+) means increase, () means decrease
than 0.5 % signal poor combustion conditions and generally low excess air. CO emitted from the stack is dispersed into the atmosphere where it adds to that from other sources of CO such as internal combustion engine vehicles.
The maximum CO concentration in stack gas must be kept lower than 100 mg/Sm3or 80 ppm (see Table6.8).
6.11.4 Particulate Matter
Particulate matter includes a wide variety of materials such as unburned fuel, sulfur compounds, carbon, ash, and non-combustible dust that enter the combustion chamber with combustion air.
The quality and quantity of particulate matter are influenced mainly by the type of fuel, the boiler’s operating mode and the type of burner. Natural gas and some light oil fuels produce little solid matter and ash. Most coals and heavy oils produce a great quantity both of particulates in stack flue gas and of ash, some of which remains in the stack flue gas.
In the case of coal, ash can amount to 20 % or more of the total weight of the fuel. Particular attention must be paid during both the design and the maintenance of the boiler to avoid the accumulation of ash on internal boiler surfaces.
To prevent particulate concentration from exceeding emission standards and to reduce health hazards, various abatement techniques are used, such as filtration, mechanical separation, and electrostatic precipitation. These techniques can be applied to both coal and heavy oil boiler plants.
The maximum particulate matter concentration in stack gas must be kept lower than 100–150 mg/Sm3(see Table6.8) depending on the quality of the fuel.
6.11.5 Hydrocarbons
Hydrocarbons can be grouped as either unburned fuel components or compounds from chemical reactions occurring during combustion.
Hydrocarbons can be reduced by proper combustion, but traces of their compounds will always be present in stack flue gas.
6.11.6 Other Pollutants
Other pollutants can be found in stack flue gas depending on the type of fuel.
With heavy fuel oils, in addition to the pollutants already mentioned, asphaltenes can form a finely dispersed colloid producing tar particles which deposit in storage
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tanks and travel through the burner without volatilizing. Asphaltenes can be stabilized by chemical additives; the same technique can be applied to prevent formation of vanadium pentoxide which is known to promote sulfuric acid reactions.
When refuse-derived fuels (RDF) are used, the stack flue gas contains particulates, complex hydrocarbons, trace metal emissions, and chlorides such as HCl, depending on both the nature of the refuse and the kind of treatment upstream the combustion for separating and/or recycling part of the waste.
6.11.7 Carbon Dioxide and Greenhouse Gases
Carbon dioxide, which is naturally found in the atmosphere as part of the earth’s carbon cycle, is the primary greenhouse gas emitted through human activities.
These are mainly electricity production, transportation, and industrial processes involving fossil fuel combustion and other chemical reactions.
In fuel combustion carbon dioxide emission is strictly related to combustion efficiency: the higher the efficiency, the greater the quantity of carbon dioxide emission.
Since greenhouse gases are considered one of the main causes of climate change, many countries have set regulations to limit the maximum amount of CO2that each site can produce.
Worldwide accepted indicators are:
2 kg CO2/Sm3natural gas 3.2 kg CO2/kg oil 2.4 kg CO2/kg coal 32.04 lbCO2/Sft3natural gas 3.2 lb CO2/lb oil 2.4 lb CO2/lb coal Notice that the most important GHG (greenhouse gas) emission due to human activity includes CO2, CH4 (methane), NO2 (nitrous oxide), water vapor, O3 (tropospheric ozone), and the so-called F-gases that are often used in insulations, foaming materials, fire extinguishers, solvents, pesticides, and aerosol propellants.
F-gases include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sul- fur hexafluorides (SF6). These gases are mainly used as substitutes for ozone- depleting substances such as chlorofluorocarbons (CFCs), hydroclorofluorocarbons (HCFCs), and halons.
For the application of these gases as coolants in cooling plants, see Sect.12.3.