Concern for the environment must translate into appropriate remedial measures

Indian industry has grown rapidly in the past couple of decades with many power, cement and steel industry projects under construction or in the planning stage. With industrialisation, environmental pollution is also on the rise. Environmental issues directly impact health, which in turn has an impact on livelihoods. Existing as well as upcoming industries need to change their approach from a Continuous Emission Monitoring driven one to an Application Oriented one. Industries need to install appropriate air pollution control devices to ensure that air pollution levels are well maintained within the limits prescribed by the Ministry of Environment and Forests/Central Pollution Control Board.

Engineers at Forbes Marshall, having realised this social and ecological responsibility, are corroborating their efforts towards a clean, safe environment. The company emphasises the need of online monitoring of flue gases to improve combustion efficiency and stack emission monitoring.

Online Monitoring of Oxygen and CO Gas for Combustion Control

The perfect combustion process: Coal (C) + O2 (Air) →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 to 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 air to form carbon dioxide (CO2), water vapour (H2O) and sulphur dioxide (SO2), and a little amount of sulphur trioxide (SO3).

If enough oxygen is not present or the fuel/air mixture is not in the correct proportion, 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 (CxH). Since these components, along with NO, are harmful pollutants, appropriate measures have to be taken to prevent their formation.

To ensure complete combustion, it is necessary to provide a certain amount of excess air. Combustion optimisation 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, which should not exceed at any point in time.

Hence, excess air will have heat loss whereas the plant running under reducing atmosphere, i.e., less air will have fuel lost. To avoid losses, online monitoring of oxygen and CO gas is must. 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 crosses 600°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°C, dust content is 40-50 gm/m3 and gas velocity at 15-20m/s. For such a hot, dirty and aggressive flue gas location, in-situ probe type oxygen analyser and in-situ probe type CO gas analyser are recommended, which work 365 days a year, respond quickly, give accurate readings and demand negligible maintenance; far better than any extractive gas analysers.

Benefits of Monitoring Oxygen and CO

• Achieve proper combustion
• Energy saving due to increased combustion efficiency, which saves 3 to 4 % of fuel costs
• NOx reduction, because the thermal oxidising of nitrogen is lower
• Total CO2 emission will be reduced, because less fuel is burned
• Lifetime of the combustion area/heat exchanger
• walls is increased and therefore CO corrosion is at a minimum
• Prevention against explosion risk - caused by unburnt combustible gases inside the electrostatic precipitator, and
• Online monitoring of stack gas emission.

During the combustion process, the nitrogen in air 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. CO, SO2, NOx gases are highly toxic and are harmful to the environment. Hence the pollution control board governs industrial emissions, and online monitoring of CO, SO2, NOx and dust emission is a must. To continuously monitor the stack gas emission, earlier, the only available technique was extractive gas analysers. In the 1990s upgradation in design and construction of direct on stack mounted analysers, (i.e., in-situ analyser) became popular and resulted in the demand for this technology for continuous emission monitoring.

Now, all major consultants and end users actively state: "In-situ gas analyser which avoids the need for sampling systems, is the best available technique"

Requirements for In-situ Gas Analyser System

• n-situ probe - probe mounted transceiver unit for the ordered gases
• Only 24 V DC, 400 W power supply
• Local/remote mounted digital display and control unit, and
• Plant instrument air for zero calibration and reference air purging.

Technological Benefits of In-situ Gas Analyser

• Very rugged and reliable, works 365 days a year
• Field mounted (no AC room/shelter required)
• IR measurement technique for measuring gases like CO, SO2, NOx, CO2, H2O, HCL, CH4
• Measures CO, SO2, NOx, HCL, CH4 gases in ppm with an accuracy of 2 to 5 ppm
• Works on IR gas filter co-relation technique, no moisture interference
• Works on gas diffusion technique, hence no filter choking
• No sampling/conditioning of gases
• Built-in measurement of CO2, H2O, temperature and pressure for online normalisation. No need for a separate oxygen analyser for online normalization when CO2 measurement can do a similar job
• On line calibration using audit gases
• Bulk measurement - no straight run required
• Negligible maintenance - AMC not required
• Low power consumption (only 400 W), and
• Heated probe option available for below dew point flue gas temperature applications.

System limitation

Not suitable for flue gas temperature above 500°C application. For such applications, closed coupled hot extractive gas analyser, without sample pre-treatment technique, is the most reliable.

The in-situ gas analyser is perhaps the best system a user can have. One doesn't have to worry about the typical problems of an extractive system like, analyser room, heat tracing of sample lines, conditioning of sample, condensation of sample, clogging of sample lines and so on. In-situ analysers are practically 'fit-and-forget' type analysers. The maintenance cost works to around 3% of an extractive system"

Stack Dust Emission Monitor

The most preferred dust emission measurement technique is non-contact in-situ cross duct type opacity/dust density monitors. They are more economical, rugged, reliable and suitable for a wider range of applications as compared to other types of dust monitors. Many technological developments have taken place in this range from single beam single pass to single beam dual pass and now dual beam single pass. Dual beam single pass i.e twin transceiver opacity monitor provides an accuracy of 0.2%, probably the highest accuracy in opacity monitors that can be achieved. This can be attributed to the facility of averaging twin measurement, modulation of light source to eliminate interference of ambient light, its periodic automatic lens contamination check and compensation facility on both sides of the optics and with automatic misalignment check and compensation facility. Given that Indian industry is required to measure stack emitted dust density in milligram/m3 ( not microgram/m3) and not opacity of the flue gas, dual beam Opacity monitors with LED operating wavelength of more than 600nm (non EPA) is the ideal solution.

Computation of Total Pollutant Release

Legislation often demands that emission measurements are presented in mg/Nm3 where the expressed volume has been normalized to a standard temperature, pressure and oxygen/CO2 concentration, and where the effects of dilution by water vapour have been removed.

To compute a measurement of the total pollutant release to atmosphere in kg/hr (or tonnes/annum), it is necessary to know the pollutant concentration in mg/m3, the hot gas flow in m/s and cross sectional area at the point of measurement in m².

The total release is then calculated as:

Mass flow (mg/s) = Mass concentration (mg/m3) x Gas velocity (m/s) x Area of Duct (m²)

It is necessary that all measurements are made on the same basis. Attempting to make this calculation using an actual hot wet gas flow in m/s and a normalized gas concentration in mg/Nm3 will produce significant errors

To compute total pollutant release one needs to continuously monitor the velocity of the flue gas. The hostile nature of the flue gas from fossil-fuel fired combustion processes makes flow measurement difficult. Manual or automatic pitot tubes can provide a form of measurement for a short duration, but for reliable, continuous flue gas flow measurement, only non-contact techniques should be considered.

A common technique for non-contact flue gas flow measurement is to use a pair of ultrasonic transceivers (i.e., combined transmitters and receivers) set diagonally across a section of the ductwork. Each transceiver transmits and receives sonic messages out of phase with its opposing transceiver and the difference between the time taken for an upstream and a downstream sonic message is a function of the flue gas flow. Where the flow is perfectly laminar, this cross-duct ultrasonic can be successful.

Unfortunately, perfectly laminar flow is unattainable on a combustion plant; in practice there is always some turbulence. Because cross-duct ultrasonic measurement is made across a diagonal axis, local variations in flow along the direction of that diagonal axis (caused by the turbulent eddies) can produce significant errors in measurement. In practice, where emissions trading is common, many users have fitted two complete sets of sensors in an attempt to minimize these unacceptable errors.

The IR flow monitor utilizes an infra-red correlation technique which requires no contact with the flue gases. The method resembles flow measurement with chemical dye or radioactive tracers, where the velocity is derived from the transport time of the tracer between two measuring points, which are a known distance apart. However, instead of an artificial tracer being added, the naturally occurring fluctuations of the infrared energy in the gas stream are used as the tracer. This technique is suitable for all combustion gases including hot gases and gases with a high dust burden.

IR flow monitor measurement is independent from the turbulence found in the exhaust ducts from combustion processes. During recent trials the IR flow monitor was shown to continuously match the performance of a fully maintained 5-hole pitot on a power plant, whereas other techniques involved with the trial (twin continuous cross-duct ultrasonic devices, and 3-hole pitots) typically overestimated the flow by as much as 15% of measurement.