Air Pollution - Cleaner Production; Pollution Control And Economic Development

Block 8: Environmental Issues and Public Health - Air Pollution Chapter 9: Cleaner Production; Pollution Control And Economic Development

Pollution Control Methods: Stationary source (stack) emission controls:

The gaseous effluent from a plant is usually discharged to the atmosphere via a stack or chimney. Stack heights may be between 6m (small plant, low pollutant concentrations) and 300 m (large coal fired power station or smelting plant, high pollutant concentrations) in height. The profile of pollutants from a stack depends on the process upstream (prior to) of the stack. For example, coal and heavy fuel oil combustion produce mainly the acidic gases SO₂ and NOx, particulate matter, CO and volatile organic compounds. In contrast, fuel gas combustion releases NOx but negligible amounts of SO₂ and PM. Specific processes such as glass making or oil refining have pollutant profiles that are characteristic of that industry.

Several air pollution control devices have been developed that are capable of reducing pollutant concentrations prior to discharge of the effluent gas via the stack. Stack gas scrubbers are mainly designed to "scrub out" SO₂, NO₂ and PM, through contact between the gas and an alkaline (usually caustic soda, NaOH or soda ash, Na₂CO₃) aqueous solution, which is in turn becomes a liquid effluent that requires disposal or further treatment. ‘Dry scrubbers’ are designed to operate to produce a sludge i.e. a solid waste) and zero liquid effluent. Scrubbers may reduce PM concentrations simultaneously with the reduction in acid gas concentrations, but have relatively lower PM removal efficiency, particularly for fine ( < PM2.5) and ultrafine (< PM1.0) particles.

A fabric filter or baghouse is a high efficiency device for filtering PM from the gas stream.

In some applications, an electrostatic precipitator (ESP) may be used to remove PM.

For high efficiency removal of acidic gases and PM from the stack gas, a scrubber system followed by either a bag filtration system or an ESP have to be used in series.

Removal efficiencies of air pollution control devices:

SO₂ removal efficiencies of 90 to 95+%, PM removal efficiencies of 99+% and NOx removal efficiencies of up to 60% are achievable. The actual efficiency attained depends on the specific application and the design of the air pollution control system.

Fuel switching:

An alternative and/ or complementary approach to the use of stack gas cleaning equipment for emission reduction is to switch to the use of cleaner fuels.

The emission factors for coal, fuel oil and gas, the three common industrial fuels, are given in Table 9.1.

**Table 9.1: AP-42 Emission factors for coal, fuel oil and gas combustion plants without emission controls¹**
Fuel	Pollutant
	SO_x^a		SO_x^b		PM10^c
		g/MJ	g/kg	g/MJ		g/MJ
Coal (x%S, m/m)	(17.5-19)S g/kg	Divide g/kg by 28.0	2.5-15.5	Divide g/kg by 28.0	(1-5A^*, 4.5 - 33) g/kg	Divide g/kg by 28.0
Fuel oil (x%S, m/m)	18.8 S g SO₂/l	0.441 S g SO₂/MJ	8.0 g NOx/l	0.19 g NOx/MJ	(1.10 S + 0.385) mg/l	0.026 S + 0.0091 mg/MJ
Gas (Natural or LPG)	~ 0			0.13g NOx/MJ	~ 0
¹ Adapted from AP-42 Section 1.2 to 1.4 ^a: SOx: SO₂, SO₃ and gaseous sulphate compounds; SO₂ constitutes over 95% ^b: NOx: NO, NO₂; about 95% of NOx emitted as NO ^* where A is the ash content of the coal, % m/m.

Emissions from a particular installation depend on the fuel composition and the combustion technology in use. Emissions due to the use of the different fuels should be compared on the basis of a unit of energy (MJ) produced rather than per unit mass or volume of fuel, as illustrated in Table 9.2.

**Table 9.2: Estimates of emission for coal, fuel oil and gas combustion plants without emission controls, per unit energy**
Fuel	Pollutant emissions
	SO_x ^a [g /MJ]	NOx ^b [g /MJ]	PM10 [mg /MJ]
Coal (1.0% S, m/m)	0.68	0.32	0.67
Fuel oil (3.5 %S, m/m)	1.54	0.19	0.10
Gas (Natural or LPG)	negligible	0.13	negligible

The sulphur content values for coal and fuel oil used in Table 9.2 are typical South African values. Gas is the cleanest energy source since SO₂ and PM emissions are negligible, and NOx emissions are significantly lower than for coal or fuel oil. The sulphur content of fuel oil tends to be considerable higher than that of coal, hence the higher SO₂ emissions, but coal plants have much higher PM emissions per unit of energy generated. Switching energy generation plants to a cleaner fuel such as gas would therefore immediately reduce SOx, NOx and PM emissions. Note, though, that the price of gas, per unit of energy, is significantly higher than that of coal or fuel oil.

The transport and storage of gas is expensive - gas is therefore not always available as an alternative fuel.