Part 2.

Air pollution control engineering

• Gaseous• Particulates

Control of Sulfur Oxides (SOx)Emission of SOx one of the major causes of acid precipitation

London fog event of December 1952

yearly moving-averagemonth average

Sulfur dioxide in Brussels-Capital Region

Evolution of SO2 emissions in Belgium since 1990

Trend differs according to the industry / activity sector

Target fixed by the Federal Government

.

Abatement of SO2

• Selection of fuel type

• Pre-treatment of the fuel (before use)

• Flue gas desulfurization (FGD)

Ethyl-mercaptan EthaneC2H5-SH C2H6

     Boiling point 35°C -88.6°C

this implies that sulfur compounds have a tendency to accumulate in heavy fractions when fuel is refined

Sulphur compounds found in fossil fuels

Hydrogen sulphide H2S

Heavy fuels used in maritime transport are now targetted (especially in SOx emission control areas)

MARPOL Annex VI

Change to Low-sulfur fuel

Several options for control:

Coal-fired power plants - Desulphurization of a solid

Sulfur in coal is either organic (usually about 60%) or inorganic (about 40%). The inorganic form is iron pyrite (FeS2) which has a higher density than coal. From grinded coal it is removed by elutriation or flotation.

The removal of organic sulfur requires a chemical reaction that is accomplished best if the coal is gasified first. Gasified coal is like natural gas.

Separation according to decreasing fall velocity

Principle of an elutriator

Desulphurization through catalytic hydrogenation – Step 1

~ 400°C, ~ 50 bars

Coal-fired power plants – Desulphurization of a solid

Sulfur in coal is either organic (usually about 60%) or inorganic (about 40%). The inorganic form is iron pyrite (FeS2) which has a higher density than coal. From grinded coal it is removed by elutriation or flotation.

The removal of organic sulfur requires a chemical reaction that is accomplished best if the coal is gasified first. Gasified coal is like natural gas.

Desulphurization through catalytic hydrogenation – Step 2

Then a Claus converter is used to recover S vapor by staged combustion

oven ~ 1000°C

300°C, catalytic reduction of SO2

Abatement of SO2

• Selection of fuel type

• Pre-treatment of the fuel (before use)

• Flue gas desulfurization (FGD)

Packed Bed Scrubber

Henry's Law states that the amount of a gas that dissolves into a liquid is proportional to the partial pressure that gas exerts on the

surface of the liquid. A H AC = K p

where, CA = concentration of A, [mol/L] KH = equilibrium constant (Henry's Law constant),

[mol/L-atm] pA = partial pressure of A, [atm]

reused in construction

Lime treatment

• A well-developed soda scrubber is the Wellman-Lord SO2 recovery process, which has found use in powerplants, refineries, sulfuric acid plants, and other industrial installations. The process utilizes a water solution of sodium sulfite (Na2SO3) for scrubbing and generates a concentrated SO2 (about 90%), in effect removing the SO2 gas from other flue gases.

• The flue gas from fossil powerplants (or nonferrous smelters) is first pretreated by cooling and removal of particulate matter, such as by electrostatic precipitators, prior to being sent to the absorber. In the absorber the water solution of sodium sulfite absorbs the SO2 in the pretreated flue gas to produce sodium bisulfite NaHSO3 according to

SO2 + Na2SO3 + H2O -- 2NaHSO3

• Actually the sodium bisulfite is sent to a forced-circulation evaporator-crystallizer (see sketch) via a surge tank. The evaporator-crystallizer is the herth of the system. The surge tank allows steady flow rates into it despite gas flow and concentration fluctuations. Through the application of low-pressure steam (such as from a turbine exhaust), the sulfite is regenerated.

• The desulfurized gas is reheated before going to the stack in order to improve atmospheric dispersion.

Wellman-Lord (regenerative) process

Na2SO3

regeneration