Emissions Monitoring: Measuring unburned carbon in flyash online

By: David Clifford & Tom Hope, Thermo Fisher Scientific, USA

The results of a project conducted at two coal fired power plants in the United States to gauge the effectiveness of an on-line, carbon-in-flyash monitor that takes advantage of recent advances in particle and gas measurement technologies and electronics are presented here by Thermo Fisher Scientific.

Regulations that limit the carbon content of flyash are proving a challenge for plant operators. NOx regulations that require plant operators to modify traditional combustion processes. SOx regulations that have, along with deregulation of the power industry, forced plant operators to diversify their coal sources; new mercury control requirements by USEPA.

Furthermore, there is the European Trading Scheme, which will drive all power plants to extract more energy from fuel inputs between 2008 and 2012 to avoid costly emissions credits; and the Kyoto Protocol, which sets binding targets on industrialized countries for the reduction of greenhouse gas emissions by an average of 5.2 per cent below 1990 levels between 2008 and 2012. An added challenge that plant operators face is the requirement that flyash that is to be sold to the concrete industry worldwide must have a low carbon level.

One method that plant operators can employ to help control the carbon content of flyash is to use an accurate, automated, monitor of flyash loss of ignition (LOI) to more frequently report unburned carbon measurements to plant operators than would be possible using traditional, manual LOI measurement methods.

Thermo Fisher Scientific’s Air Quality Instruments division, has carried out a project to determine the effectiveness of a version of its Series 4100 automated, on-line carbon-in-flyash monitor that takes advantage of recent advances in electronics and analysis technologies. The project gathered recent operating experience of the new device, the Series 4200, under actual plant operating conditions in the boiler exhaust ducts at USA coal fired utilities. The project showed how the monitor can improve the credibility and use of on-line, automated combustion efficiency data, and provided a measurement solution to help plant operators worldwide. The findings are applicable not only for plants in the United States but also for coal fired plants worldwide.

Monitor benefits

On-line, carbon-in-ash (CIA) monitors increase the availability of data that tracks combustion efficiency. The benefits of this can include:

  • Increased combustion efficiency
  • Greater amounts of low-carbon fly ash available for sale
  • Reduction in the amount of fly ash that goes to landfill
  • Increased consistency and availability of LOI-type measurements
  • Elimination or reduction of manual LOI procedures
  • Improved mercury capture and control efficiency, while balancing plant operating efficiency

Identification of equipment failures

Plant operators can use data on residual carbon in flyash to assess the effectiveness of the actions they have taken to optimize plant performance. Because reduction of flyash carbon content is a direct effect of improved combustion efficiency, plant operators employ the CIA measurement as a key component in their strategy to reduce fuel use. By tracking changes in the level of flyash carbon, plant operators can monitor coal mill performance, optimize burner configuration and track furnace performance. Also, lower carbon levels in the flyash can increase the availability of low-carbon flyash for sale. The impacts of these benefits, either alone or collectively, can result in a significant contribution to a plant’s financial position. In fact, simply using industry average prices for fuel and the average cost of ash disposal in combination with increased revenue from improved flyash sales, the payback time for this carbon-in-ash monitor can be as little as a month or two.

Performance evaluation

The first objective of the test in the coal-fired power plants was to study the impact of the design changes to the Series 4200 on system performance under real-world conditions. To this end, Thermo Scientific installed monitors in several locations. One monitor was installed in a Midwest power plant that burns local sub-bituminous coals. A second monitor was installed at a utility station in the northeast that burns bituminous coals from eastern USA and South America. Both plants obtain fuel from multiple sources.

After a short startup/conditioning period, Thermo Scientific tracked operating reliability, maintenance requirements and measurement performance at the installations over three months. At both locations, the monitor was installed just downstream of the economizer and the monitors. Plant operations staff performed routine operations and weekly maintenance, while Thermo Fisher Scientific carried out non-routine maintenance and repairs.

The second objective of the test programme was to qualify the monitor’s measurement accuracy during typical plant operating conditions by periodically collecting and analyzing samples with the Series 4200 and time correlated samples taken from the flyash collection hoppers.

Operating performance

The Midwestern plant operates in base-load supply mode, so it typically runs at a high rate during the day and reduces load in the early morning hours. The plant blends incoming coal with stockpiled fuel.

The monitor was allowed to operate just as it would when part of normal plant operations. The plant engineer responsible for the environmental monitoring programmes at the plant was the monitor operator and was provided support when required by Thermo Fisher Scientific’s R&D engineering staff.

Over the time evaluation period shown in Figure 1, the monitor operated properly and with only weekly maintenance (filter cartridge exchange, function checks) required. Over the two month evaluation period, only two instrument malfunctions occurred that resulted in unexpected downtime.

Figure 1: Performance evaluation at coal fired power plant in the MidWest of the United States
Click here to enlarge image

The first malfunction was caused by the operator inadvertently positioning the mass transducer incorrectly, which resulted in damage to the transducer mounting. When repairs were made, the monitor’s operating firmware was updated with a revision that will prevent this from occurring again. The only other unexpected downtime occurred when the instrument air supply pressure exceeded the safe operating range of the monitor’s linear slide mechanism. The monitor’s operating firmware properly and automatically put the monitor in suspend mode and issued an alarm warning to the operator, who responded to reset the pressure regulator to the proper setting.

At the second test site, after a short startup period, the monitor was allowed to operate as it would if it were part of normal plant operations. Ongoing operations and maintenance support was provided by plant operations personnel with assistance by the Thermo Fisher Scientific R&D team, when required. Figure 2 summarizes the CIA measurement data and duct velocity and temperature data that can be used to help identify changes in plant load and operations.

Figure 2: Performance evaluation at coal fired power plant in the Mideastern United States
Click here to enlarge image


Measurement accuracy

The Series 4200 sampling point was upstream of the plant’s flyash collection hoppers and downstream of the economizer. Manual samples were collected from the hopper with timing of sample collection consistent with times that the Series 4200 automatic samples were taken. Manual samples were split into three aliquots. One of the splits was analysed in the plant’s main chemistry laboratory. One sample was analyzed by the generating unit plant laboratory and one sample was subsequently analyzed using the Series 4200 manual analysis mode.

The plant laboratories used the standard LOI method (ASTM C311) in which a sample is weighed gravimetrically then introduced into a high temperature furnace in which any unburned carbon in the sample is oxidized. The sample is then removed from the furnace, allowed to cool and then reweighed. Oxidation results in the loss of the carbon present in the flyash sample due to its conversion to carbon dioxide, which is lost to atmosphere. Consequently, the analysis reflects the weight percentage of carbon in the original sample. Figure 3 shows the results of the analysis comparisons.

Figure 3: Accuracy comparison between Series 4200 and bin samples
Click here to enlarge image

On average, the difference between the plant lab analysis and the Series 4200 analysis was less than 0.5 per cent actual CIA. Secondly, note that even though the automatic sample taken by the Series 4200 was collected by extracting the sample from a single fixed point in a very large duct, the automatic analysis tracked almost perfectly with the time-consistent, manually collected samples analyzed by the plant lab.

SCoring goals

During the tracking period, Thermo Fisher Scientific found that the monitor can meet its goals for maintainability and reliability. With the exception of the operator-induced malfunction, which was subsequently addressed through a change of software to prevent an operator from inadvertently positioning the sensor in an undesired position, the monitor operated as designed. In the instance where the air pressure was above the proper operating range, the monitor properly responded by automatically entering its protective standby mode and issued a fault alarm.

Monitor accuracy as determined by comparison with manual samples shows that the monitor can meet accuracy goals regardless of plant operating conditions or the type of coal being used. The accuracy results also show that the new electronics, the linear slide mechanism and the software platforms improved reliability without negatively affecting measurement performance of the basic inertial mass measurement/thermal oxidation design.

No posts to display