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Monitoring Carbon Monoxide (CO) accurately : Week 7/1/2012

In our last article, we discussed about Electro Static Preciptator (ESP) & why there is danger of explosion at ESP. We also discussed about the importance of CO & need to measure it accurately. In this article, we will discuss about care to be taken at ESP, ways to avoid CO generation & methods of monitoring CO accurately to avoid explosion at ESP.

Care to be taken at ESP:

Startup of an electrostatic precipitator is generally a routine operation. It involves heating a number of components such as support insulators and hoppers. If possible, the ESP should not be turned on until the process reaches steady-state conditions. This is particularly important for ESPs used on cement kilns burning coal as fuel. The internal arcing of the ESP could cause a fire or an explosion. If an ESP is used on a coal-fired boiler, the ESP should not be started until coal firing can be verified. This will help prevent combustible gases from accumulating in the unit and causing explosive conditions.

How to avoid formation of CO:

To ensure complete combustion, it is necessary to provide a certain amount of excess air.

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.

If requisite amount of excess air (i.e. excess oxygen) is not provided, there is a risk of incomplete combustion & therby generation of CO. Sometimes, this can happen due to external factors such as poor quality coal. This leads to improper and ever changing combustion conditions, and proper control of air flow rate becomes difficult, thereby causing occasional high CO concentration in the exit gas which may cause explosion in ESP.

Very often the CO concentration even exceeds the explosion limit of 0.4-0.6 per cent mainly owing to the poor and variable quality of coal; as a result, the ESP trips causing high dust emission. Feeding uniform quality coal could be an answer to this. This will be possible through stabilising coal supplies, such as coal washing and recourse to coal blending etc.

How to monitor CO accurately:

Although it is essential to provide a certain amount of excess air to ensure complete combustion, the same excess air will have heat loss. On the other hand, plant running under reducing atmosphere i.e less air, will have fuel loss & risk of CO formation. To avoid this, online monitoring of Oxygen and CO gas is must.

Oxygen and CO gas need to be monitored as near 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, flue gas temperature is around 350 Deg.C, dust content is 40-50 gm/m3 and gas velocity is at 15-20m/s. For such hot, dirty and aggressive flue gas location, it is always recommended to use In-situ probe type Oxygen analyser and In-situ probe type CO gas analyser. These analyzers work 365 days a year, are fast in response, accurate, demand negligible maintenance and are far better than any extractive gas analysers for various reasons.

In our next article, we will discuss about in-situ method of analysis & extractive method of analysis