Automation Tech 2009

Currently 78% electricity comes from fossil fuels, mostly coal and India is the 3^rd largest coal consumer in the world behind China and the United States. Coal is a finite resource and its burning for electricity generation is harmful to the environment. To make full use of coal and to control pollution, monitoring of gas and dust emission is vital. FORBES MARSHALL experts share from their vast experience in Thermal Power Plants.

Online Oxygen and CO Analyser for monitoring the boiler combustion efficiency

In power boiler, coal with the help of Oxygen (i.e. Air) is burnt for heating water to generate steam. To ensure proper utilization of coal i.e. to produce maximum heat, plant need to control Air + Fuel ratio so called combustion process.

Perfect Combustion process is = Coal (C)  + O2 (Air ) à Heat + CO2

Combustion is the act or process of burning. For combustion to occur, fuel, oxygen (air), and heat must be present together.

The combustion process is started by heating the fuel above its ignition temperature in the presence of oxygen. Under the influence of heat, the chemical bonds of the fuel are split. If complete combustion takes place, the elements carbon (C), hydrogen (H) and sulphur (S) react with the oxygen content of the air to form Carbon dioxide CO₂, water vapour H₂O and Sulphur dioxide SO₂ and little amount of sulphur trioxide SO₃.

If enough oxygen is not present or the fuel / air mixture is insufficient then the burning gases are partially cooled below the ignition temperature, and the combustion process remains incomplete. The flue gases then still contain unburnt components, mainly carbon monoxide CO, carbon C (soot) and various hydrocarbons C_xH_y. Since these components are, along with NO_x, pollutants which harm our environment, measures have to be taken to prevent the formation of them.

So, to ensure complete combustion, it is necessary to provide a certain amount of excess air. Combustion optimisation saves money!

Combustion optimization is determined by a maximum percentage of complete combustion, along with a minimum of excess air (commonly 5 to 20% above the necessary level for ideal combustion.

For perfect combustion, CO₂ emission should be maximum and O₂ should be close to or, zero in the flue gas. Since perfect combustion is not practically possible due to incomplete mixing of the fuel and air, most combustion equipment is set up to have a small percentage of excess oxygen present. The lower the temperature for a given O₂ or CO₂ value the higher is the Combustion efficiency. This is because less heat is carried up the stack by the combustion gases.

As shown in the graph, the optimal combustion mixture is not displayed as a single point, but as a band of possibilities. The optimal combustion depends on various factors and gives the possibility of adjusting for a reducing or oxidising gas, both of which are necessary in an industrial process. The maximum allowable CO value is also an important factor. This should not exceed at any time.

Carbon monoxide is poisonous gas. CO robs our blood of oxygen. When inhaled, it bonds with the haemoglobin in blood, displacing life-giving oxygen. This produces a toxic compound in blood called “Carboxyhaemoglobin” (COHb). Over time, exposure to CO can make sick us. Victims exposed to enough carbon monoxide can suffer brain damage, or even die.

One of the long-term exposure limits is set at 9 ppm with a maximum of 35 ppm for one hour. These low concentrations show just how dangerous carbon monoxide really is.

Hence, excess air, i.e less coal will have heat loss whereas less air i.e excess coal will have fuel lost. To avoid such losses, online monitoring of Oxygen and CO gases is required. Oxygen and CO gas need to be monitored as close as possible to the boiler but after the combustion is over. For coal fuel – combustion process is ON till the gas temperature is above 600 Deg.C. Thus, for coal fired power boilers, the ideal location for monitoring Oxygen and CO is after the Economiser i.e. Air pre-heater (APH) inlet.

At APH inlet – gas temperature is around 350Deg.C, dust content is 40-50 gm/m3 and gas velocity at 15-20m/s. For such hot, dirty and aggressive flue gas location, Insitu probe type Oxygen analysers and Insitu probe type CO analysers  work  with very negligible maintenance and are far better than any high maintenance prone, unreliable extractive  gas analysers.

Get more from online monitoring of Oxygen and CO in flue gas

Energy savings by about 3 to 4 % due to increase in combustion efficiency, which saves 3 to 4 % of fuel costs

NOx (pollutant gas) will reduce because the thermal oxidising of nitrogen is lower

CO (pollutant gas) will be kept within limits due to high accuracy measurement.

The total CO2 emission will be reduced because less fuel is burned. Another benefit of carbon trading !.

The life span of the combustion area / heat exchanger walls is increased because excessive CO values will not happen and therefore CO corrosion is at a minimum.

Prevention against ESP explosion risk – caused by more unburned inside the ESP.

Online Dust, SO2,NOx gas analysers for monitoring the stack emissions i.e pollution monitors

During the combustion process, Nitrogen in Air (air used for burning coal) oxidizes with Oxygen and forms oxides of Nitrogen i.e NOx. Sulphur in coal after oxidation forms SO2. Similarly coal after burning becomes ash i.e dust. SO2, NOx gases are highly toxic and are harmful to the environment. Hence the pollution control board governs the industrial emissions of dust, SO2 and NOx gases. The legislation has been empowered to force industrial units to install online emission monitors and adhere to emission norms. The pollution board has the power to close down industrial units, which do not conform to their stipulated levels.

To monitor the gas emission, earlier the only available technique was Extractive gas analyser. Later in the 90’s up gradation in design and construction of direct On Stack mounted analyser(i.e Insitu analyser) became popular and resulted in the demand for this technology in the market for Continuous emission monitoring.

Hence, now days all major Consultants, End users prefer In-situ Gas analysers as they avoid the need for sampling systems.