By Brad Buecker, Contributing Editor
During my 27-year career directly with power plants or supporting them, I have had the opportunity to work with wet-limestone scrubbers, selective catalytic reduction units, and other air pollution control (APC) equipment. In the last two or three years, I have been examining APC technologies that also improve furnace combustion, where the proverbial light bulb has gone off in my mind regarding the cost savings that improved combustion can provide.
Unburned Carbon, Bad or Perhaps Good?
The calculations outlined in the next sections are very simple in nature, and those interested in a much more thorough analysis should acquire Babcock & Wilcox’s excellent book Steam, now in its 41st edition. In fact, I utilized it for some of the basic data in this article.
Let’s look at a hypothetical, but by no means weird, 600 MW pulverized-coal plant with the following conditions.
Fuel: Bituminous coal
Fuel higher heating value (HHV): 10,300 Btu/lb
Fuel ash content: 10.8 percent
Fly ash fraction of total ash: 0.8
Percent carbon based on ultimate analyses: 72 percent
Fuel firing rate at full load: 300 tons (600,000 pounds) per hour
Capacity factor: 0.75
Consider combustion improvements that reduce the unburned carbon (determined by loss-on-ignition, LOI, tests) from 15 percent in the fly ash to 5 percent. This 10 percent reduction, when multiplied by the fuel firing rate, ash content, and fly ash fraction equates to almost 5,100 pounds per hour of carbon now being burned to produce heat in the boiler.
Per Steam, the heat of combustion of pure carbon to carbon dioxide (carbon monoxide is another story, as we shall see) is 14,093 Btu/lb, so just this improvement alone would deliver an extra 73,000,000 Btu/hr to the furnace. If one calculates the total theoretical heat input to the furnace (600,000 lb/hr x 10,300 Btu/lb), the reduction in unburned carbon by the figures used in this example represents a 1.2 percentincrease in efficiency. Let us take this a step further. Say the fuel cost is $1.50 per million Btu. Then the cost savings over a year at a capacity factor of 75 percent calculate to $720,000.
Other thoughts to consider regarding the benefits of lower carbon in the ash are the reduced possibility of fires in fabric filter removal devices, a.k.a., baghouses, and improved fly ash quality especially for utilities that sell their ash to cement manufacturers and other industries.
The one positive benefit of unburned carbon is that like carbon added artificially to the flue gas, it can adsorb mercury and remove it with the ash. Much research continues into this issue, and the results are not clear-cut as yet. However, as mercury removal technologies evolve, a balance may develop between the amount of unburned carbon in the ash that provides the best efficiency while also providing good mercury removal.
With regard to measuring unburned carbon, the traditional method has been to collect fly ash samples and then bake them in a muffle furnace at high temperatures to determine loss on ignition (LOI). However, an article in the March 2008 issue of Power Engineering outlined on-line instrumentation to measure LOI.* Reliable on-line readings would allow utility personnel to fine tune combustion parameters in real time, and without the laborious task of collecting fly ash samples and shipping them to the lab.
Don’t Overlook Carbon Monoxide
Carbon monoxide is obviously well known as a hazardous air pollutant, and it is a good indicator of combustion performance problems. When CO levels are high, unburned carbon concentrations are quite likely to be high as well. Yet, those of us in the industry often focus more on the latter.
Whether it be poor boiler design, aging burners and equipment, or other factors, I have seen histories where flue gas CO concentrations (part-per-million [ppm]) have reached triple digits and higher. Ponder this issue from an efficiency standpoint. As was mentioned previously, when carbon is completely combusted to CO2, the heat released is 14,093 Btu/lb. Carbon combustion to carbon monoxide only releases 3,950 Btu/lb, roughly a third of the potential maximum.
How does that equate efficiency-wise? Let’s consider the example from above, where the CO concentration of the flue gas is at or just above 1,000 ppm. Good combustion technology can take this concentration down to just double digits. So, a reduction of CO by 1000 ppm, based on the operating conditions outlined above, results in a heat increase of approximately 33,000,000 Btu/hr. This is less than that for the unburned carbon reduction, but still equates to an efficiency improvement of about 0.5 percent.
A Final Thought
This article is primarily devoted to combustion issues, but because much of my utility work involved and still involves steam surface condenser performance monitoring and cooling water treatment programs, I cannot resist commenting on this issue to utility engineers and chemists. Keep an eye on those condensers! Microbiological fouling or scale formation on the waterside, or air in-leakage on the steam side, can result in efficiency losses similar to or sometimes worse than those outlined above.
When a unit has to be de-rated in the summer due to excessive condenser backpressure, I can think of nobody who is happy, and particularly the maintenance personnel sent in to try to clean tubes with the unit at half load. I have been in condenser waterboxes under these conditions, where the temperature was perhaps 110 F with 100 percent humidity. I could not see with my glasses on or off, just like Mr. Magoo.