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Industrial Emission Controls: Sulphur Dioxide

Introduction

Sulphur dioxide (SO₂) is a colourless gas. It reacts on the surface of a variety of airborne solid particles, it is soluble in water and can be oxidised within airborne water droplets to form sulphuric acid (H₂SO₄), which falls as acid precipitation or "acid rain". SO₂ emissions arise from the oxidation, during combustion, of the sulphur contained within fossil fuels. Fossil fuels, including coal, oil and to a lesser extent gas, contain sulphur in both organic and inorganic form.

Removal of Sulphur Dioxide

A reduction in the atmospheric emissions of SO₂ produced by fossil fuel combustion processes can be achieved at one of three stages, as follows.

1. Reducing the sulphur content of the fuel before combustion:

Emissions of SO₂ are proportional to the sulphur content of the fuel, although with regard to coal a proportion, usually less than 10%, is retained in the ash. Therefore, one of the simplest ways to reduce the amount of SO₂ released from the combustion process can be achieved by switching to a fuel that has a lower sulphur content, i.e. burning low sulphur coal or gas instead of high sulphur coal. The coal sulphur content can vary from below 0.5% to over 10% by weight; for the majority of coals currently in use within the UK this sulphur content is in the range of 1 - 3%.

Average Sulphur Content of Various Fuels

Innogy, PowerGen and other power station owners presently import a large amount of the coal burnt in power stations. The primary reason for this is low sulphur imported coal is, at present, cheap and it will allow the generators to meet the SO₂ reduction under the Large Combustion Plants Directive (LCPD) for reduced emissions (1988), without excessive cost.

Sulphur in coal is found in both inorganic and organic forms and sulphates. Inorganic sulphur, in the form of pyrite (FeS₂), can be removed from coal relatively easily simply by washing the coal. This method can result in a reduction of 10 - 50% of total sulphur content. However, again as with fuel switching the reduction is limited, plus large quantities of waste water are produced. Washing can also change the physical characteristics of coal, therefore operational problems may arise when combustion takes place.

2. Sulphur Removal During Combustion

A number of technologies to prevent the production and release of SO₂ during combustion have been developed over the past decade, but very few have achieved wide commercial application to date. The most developed are the Fluidised Bed Combustion (FBC) process and the integrated Gasification Combined Cycle (IGCC) system.

Fluidised Bed Combustion

This process involves the combustion of coal in a bed of inert material such as sand, with air being blown up from beneath the bed at high velocities. As velocity increases individual particles begin to be forced upwards until they reach a point at which they remain suspended in the air stream. The bed in this state behaves like a liquid and can be described as fluidised. Tubes containing water are immersed in the bed to absorb the generated heat (this water is converted to steam which is used to drive the steam turbine and thus produces electricity). The fluidised movement within the combustion chamber results in a greater heat transfer efficiency to the water filled tubes and therefore operating temperatures are lower than in a conventional system. SO₂ emissions can be controlled in this system by adding a sorbent (a substance used to absorb any SO₂ present, for example lime or limestone) to the bed of inert material. The limestone effectively absorbs the SO₂ as it is released from the coal and retains it within the ash, which is removed regularly. The low combustion temperatures allow efficient combustion to take place without causing the ash to soften, thereby allowing easy removal of the ash containing the absorbed SO₂.

The FBC can achieve in the region of 80 - 90% SO₂ removal. Two main disadvantages of this system are firstly the large quantities of sorbent required (approximately twice that of an FGD system (see later) to achieve the same SO₂ removal), and secondly the large quantities of strongly alkaline waste produced, which is generally disposed of in landfill.

Integrated Gasification Combined Cycle System

This process does not require the addition of a sorbent. Instead the coal is gasified under pressure with a mixture of air and steam. The resulting gas is expanded through a gas turbine to produce electricity. The waste heat from the gas turbine is then passed through a second steam turbine, the second stage of the combined cycle process, again producing electricity. As the coal is converted to gas the sulphur present is converted into hydrogen sulphide which can be easily removed and sold for use within the chemical industry. Gas cleaning can be integrated into the gas production process, and emissions can be reduced by more than 99%. Also very little waste is produced. This system has not been adopted for use in the UK because the success of the technology has not been sufficiently proven.

3. Removal of Sulphur after Combustion - FGD

Emissions of SO₂ generated during the combustion of fossil fuels can be reduced by treating the flue gases before they are emitted into the atmosphere via the stack; this is termed Flue Gas Desulphurisation (FGD). Flue gas desulphurisation systems can be classified as either Regenerable or Non-regenerable.

Limestone/Gypsum System

This process is the most globally used FGD system. This system is relatively simple: crushed limestone / lime is mixed with water to form a slurry which is then sprayed into the sulphur containing flue gases. The sorbent reacts with the SO₂ to form an aqueous slurry of calcium sulphite. Compressed air blown into the slurry oxidises the calcium sulphite to produce calcium sulphate. This product is then treated to remove excess water and either sold to the building trade or disposed of as landfill. SO₂ removal can be in the region of 90%

Spray Dry System

Within the spray dry system, a slurry of alkali sorbent, usually slaked lime, is injected into the flue gases in a fine spray. The heat from the flue gases causes the water to evaporate, cooling the gases as it does so. The SO₂ present reacts with the drying sorbent to form a solid reaction product, with no waste water.

Seawater Scrubbing Process

The seawater scrubbing process exploits the natural alkalinity of seawater to absorb acidic gases. Flue gases are contained in an absorption tower where they flow counter current to seawater. The heat of the flue gas causes the seawater to be heated and the gases cooled. During this process SO₂ is absorbed by the seawater, before passing to a water treatment plant where further seawater is added to increase the pH. Air is supplied to oxidise the absorbed SO₂ to sulphate and to saturate the seawater with oxygen. The seawater is then discharged to the sea. This system is a simple and inherently reliable one with low capital and operational costs, which can remove up to 99% of SO₂, with no disposal of waste to land. However, heavy metals and chlorides are present in the water released to the sea.

Wellman-Lord Process

A third FGD process is the Wellman-Lord process, which can be divided into two main stages. 1) Absorption: the hot flue gases are passed through a pre-scrubber where ash, hydrogen chloride, hydrogen fluoride and SO₃ are removed. The gases are then cooled and fed into the absorption tower. A saturated solution of sodium sulphite is then sprayed into the top of the absorber onto the flue gases; the sodium sulphite reacts with the SO₂ forming sodium bisulphite. The concentrated bisulphate solution is collected and passed to an evaporation system for regeneration. 2) Regeneration: the sodium bisulphite is broken down, using steam, to release the sodium sulphite, which is recycled back to the flue gases. The remaining product - the released SO₂ - is converted to elemental sulphur, sulphuric acid or liquid SO₂. This system offers a number of advantages over alternative systems, the main one being that the sorbent is regenerated during the combustion process and is continuously recycled.

Summary

There are various methods for reducing the atmospheric SO₂ emissions from power generation. Each method has both advantages and limitations related to cost, removal efficiency, operational experience and waste products produced. Therefore the choice of control technology should be based on the criteria required for each individual combustion plant.