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Industrial Emission Controls: Nitrogen Oxides

Introduction

Nitrogen Oxide (NO_X) is a collective term used to refer to species of oxides of nitrogen, such as: nitric oxide (NO) and nitrogen dioxide (NO₂). Nitrogen dioxide is a reddish-brown gas. It is a strong oxidant and soluble in water and can be oxidised within the atmosphere to form nitric acid (HNO₃), which along with sulphuric acid (H₂SO₄) falls as acid precipitation or "acid rain".

Nitrogen oxides are formed at two stages during combustion:

1. The reaction of oxygen with nitrogen compounds in the fuel - this is termed Fuel NO_X;
2. The reaction of nitrogen with oxygen in the combustion air - this is termed Thermal NO_X;

The relative contribution of fuel and thermal NO_X depends on the type of fuel being used and the operating conditions. A reduction in the atmospheric emissions of NO_X produced by fossil fuel combustion processes can be achieved at one of two stages:

1. Preventing the production and release of nitrogen oxides during combustion;
2. Removal of nitrogen oxides after combustion.

Removal of Nitrogen Oxides

Unlike sulphur, it is not possible to reduce the nitrogen content of the fuel by physical cleaning as it is combined within the organic matter of the fuel, and at present there are no commercially available methods to reduce organic nitrogen. Fuel switching is also not effective as this will only reduce the fuel NO_X contribution and not the thermal NO_X.

1. Nitrogen Oxide Removal During Combustion

Low NO_X burners ensure that initial fuel combustion occurs within fuel rich conditions, that is with low oxygen concentrations, such that any gaseous nitrogen produced is reduced to N₂. Once initial combustion has taken place, further air is added to the combustion chamber to ensure complete combustion of the fuel. This greatly reduces the opportunities for NO_X production. Advanced low NO_X burners can reduce NO_X concentration by up to 30%. Low NO_X burners can be installed on either new or existing combustion plants, and as such have been retrofitted to a number of UK power stations. They have also been incorporated into the design of many of the new gas-fired power stations that are under construction or operational.

The combustion modifications required for staged combustion involves introducing air and fuel at different levels within the furnace. In general a furnace contains a number of burners with similar fuel / air ratios. Staged combustion involves altering the fuel / air ratios of individual burners whilst maintaining an overall ratio. For example, by allowing less air into the hottest part of the flame at the bottom of the furnace, less thermal NO_X is formed. Further air admitted to the coolest part of the flame at the top of the furnace allows complete combustion of the fuel, whilst maintaining an overall ratio of fuel / air.

Fluidised bed combustion involves the combustion of coal in a bed of inert material, commonly sand, with air being blown up from beneath the bed at high velocities. As velocity increases individual particles are forced upwards and become 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. The low operational temperatures in the region of 750 - 950°C prevent the formation of thermal NO_X. Furthermore, the FBC system can achieve in the region of 80 - 90% SO₂ removal. This system can also burn coals of low or variable quality which are relatively cheap, with combustion and emission control carried out in a single unit. Consequently, the flue gases do not have to be removed from the boiler for treatment. The FBC system can not easily be fitted to existing plant and therefore will only be suitable for new generation plant.

2. Nitrogen Oxide Removal After Combustion

Emissions of NO_X generated during the combustion process can be reduced, as with SO₂, by treating the flue gases. There are a number of systems available, and a brief description of the following systems will be given.

• Selective Catalytic Reduction (SCR)
• Selective Non-Catalytic Reduction (SNCR)
• Activated Carbon Process

Within the SCR, ammonia is injected into the flue gas. The nitrogen oxides present in the flue gases react with the ammonia and are converted to nitrogen and water. This reaction takes place in the presence of the catalyst, which is usually vanadium or tungsten oxide. The catalysis allows the reaction to take place at low temperatures between 300 - 400°C. This process is suitable for fitting to existing plant and new build applications, and can achieve a NO_X reduction of up to 80 - 90%, with minimal waste production. The main disadvantages with this system are the high costs involved; the cost of the catalyst can amount to between 40 - 60% of operating costs, and it is necessary to replace the catalyst every 2 - 3 years. This system is not operational within the UK, although it is being used extensively in Japan.

Denitrification of the flue gas is achieved by a reaction with ammonia as with the SCR, however, within this system there is no catalyst. As no catalyst is used higher operational temperatures are required, between 930 - 1030°C to ensure that a reaction between the ammonia and nitrogen oxides occurs. This process is also suitable for fitting to existing plant and new build applications, and can achieve a NO_X reduction of up to 50 - 60%, again with minimal waste production. This system is far cheaper than the SCR, but maintaining the correct operational temperatures to ensure an adequate reduction of NO_X is a drawback of this system. Consequently, the NO_X reducing capabilities of the SNCR are less than the SCR. This system is also not operational within the UK but is being used extensively in Japan.

The Activated Carbon (coke or char) Process reduces NO₂ produced during combustion to NO via a reaction with carbon at about 80°C. Ammonia can then be added to reduce the NO to N₂ and water. NO_X removal can be in the region of 40 - 60%.

Summary

For small processes combustion modifications of the combustion chamber are considered to be the most appropriate technique. For larger installations combustion modifications will generally be effective but if the process generates high levels of NO_X then a combination of combustion modifications and flue gas treatments may be required. The costs of flue gas treatment are, however, significantly greater than combustion modifications which may inhibit their usage. At present there are no post combustion techniques in operation within the UK and none planned for the future.

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