Accurately Measuring Combustion Gases in Coal Fired Power Plants

Monitoring the carbon monoxide equivalent (COe) in coal-fired power plants is a critical requirement. The focus of coal-fired boilers is shifting towards heat rate efficiency. The future limits that are likely to be laid down by the EPA and the increasing competitiveness in electricity rates are increasing the need for active measurements of COe.

The combustion efficiency of coal-fired plants (Figure 1) can be increased and the CO content can be minimized by monitoring the COe.

Figure 1. Combustion efficiency

The heat rate, efficiency, LOI, total fuel consumption, fouling and localized slagging are all influenced by the localized concentration of COe in the boiler. Active monitoring and measurement of the COe and O₂ levels ensures optimal combustion in coal-fired plants, which as a result helps to minimize fuel costs, maintenance and NO_x.

Optimized combustion has a huge impact on plant economics. A reduction in fuel costs improves the BTU output per kWh (heat rate). Optimizing the combustion process has helped a number of companies to realize huge annual savings, ranging from 60,000 to 2,000,000 USD per unit.

The Challenge

Problems like the insufficient combustion air in furnaces need to be addressed immediately in order to show improvement. Oxygen analyzers are placed at the boiler economizer exit gas ducts in most of the large utility boilers. Due to the reduction in the frequency of overhauling and age of the boiler, there is a substantial amount of air in-leakage between the oxygen analyzers and the furnace exit.

The air that leaks into the furnace after the combustion process is completed does not contribute to combustion, but gets logged in the oxygen analyzers as ‘excess oxygen’. Areas of poor combustion are essentially CO events that are localized and are present throughout the boiler in the form of pockets. A spatial grid developed across the unit can help identify the cause of these pockets.

Figure 2 shows the distinct process zones of the flue gas produced by the furnace that are created by the stream pattern.

Figure 2. Example of multiple flue gas streams

Each combustion stream is compressed by the furnace walls and adjoining combustion streams, as a result stays intact and discrete from other streams. Although slight diffusion of combustion streams may occur in slow-moving regions, they remain intact on the whole due to their high velocity. The only way to optimize the combustion conditions is to inspect the composition of the combustion gases from each burner. A non-uniform combustion pattern in the entire boiler is created by these factors.

The Solution

The AZ40 is a powerful and effective solution that is adopted by most combustion optimization companies to optimize combustion. In order to model the inefficiency of a boiler, a variety of strategies are adopted. These strategies are based on the idea of accurately targeted partial grids to identify pockets of high CO and excess air.

Once the area of poor combustion is identified, the operator can zero in on the asset that is causing inefficiency and then improve the O₂ utilization so the burners are adjusted for optimum combustion.

By creating a matrix of probes that are oriented horizontally and sometimes vertically, projecting at different lengths into the furnace, the poor combustion areas can be monitored effectively. The ultimate aim is to derive samples of flue gas from multiple points using multiple AZ40 analyzers (Figure 3).

Based on advanced features, high precision, and minimum maintenance of the AZ40, it is used in many optimization solutions.

Figure 3. The AZ40 probe and transmitter

The AZ40 scores above other extractive technologies that are available based on the following points:

• Top class accuracy – The AZ40 provides a required resolution at ±20 ppm to measure the CO present in the form of small pockets with a typical concentration range of 150-200 ppm
• Solves common extractive plugging issues – A sample flow uni-block present in the AZ40 makes sure that the sample is heated to temperatures above 204°C so that the dew-point plugging is minimum. Additionally, the advanced programmable blowback option helps minimize the plugging of the probe-filter assembly caused by accumulation of particulates
• Minimum maintenance – orifices are present in the AZ40 and help to maintain a sample flow rate to extend operating conditions. Easy access to these orifices enables easy replacement and cleaning. The lifetime of these orifices is extended to more than a year by the primary and secondary filters. The only maintenance routines required are orifice cleaning and aspirator cleaning.

Conclusion

Substantial improvement in the combustion of gases is brought about by combining a powerful combustion optimization strategy with the AZ40 combustion gas analyzer from ABB. Optimization provides a number of advantages, including mitigation of localized fouling, LOI reduction, better heat rate, improved efficiency and reduced NO_x emissions by lowering boiler excess O₂ and Coe.

This information has been sourced, reviewed and adapted from materials provided by ABB Measurement & Analytics.

For more information on this source, please visit ABB Measurement & Analytics.