Coal quality control

Daniel Mahr, P.E of US based Energy Associates, P.C, gives us an in depth look at the major issues relating to coal quality from the perspective of thermal power generation.

Why Coal?

The power industry is confronting challenges with seemingly conflicting goals – affordable rates, dependable service, and reduced impacts. Different energy conversion technologies have their applications, but no single option does it all. Wind, solar, and hydro options don’t use any fuel, so shouldn’t we just rely on these technologies? Wind power is best sited where the duration/velocity makes sense, away from migration pathways, and away from neighbours who would object to noise and fluttering shadows. It will likely require 100 per cent back-up or additional energy storage systems, and new longer transmission lines to load centres are often required. There are similar requirements for solar power, just substitute lumens for velocity. With hydro power, there are concerns for fish migration/spawning, land use, geological concerns for supporting the weight of a new lake, stability concerns for newly saturated perimeter hills that can result in landslides, and the impact of a drought on production. Its the delivered cost on your utility bill that counts; the capital cost of the plant itself is only a single component.

Large, central power plants provide the reliability and flexibility utilities require for baseload, cycling, and on-demand situations. They can be strategically located near load centres or along transmission corridors and provide the economy of scale needed to minimize the cost of production.

The single, largest, operating cost for a gas, oil, or coal -fired electrical power generating station is fuel. In the simplest terms, the power plant is converting the chemical energy stored in fuel to electrical energy. Plant design, process requirements, and efficiency goals make fuel quality an issue. A high performance engine needs a high quality fuel.
So as we see, when we purchase a fuel, we are purchasing energy value. Coal is the most difficult to extract and burn, but as a source of energy, it is also the most economical. That’s the reason it fueled the industrial revolution and has historically been the fuel of choice in many countries for power generation. It continues to be the fuel of choice for new power generation for counties with high growth, India and China for instance.

What’s Coal?

Coal is formed from organic plant matter. It is the stored product of the photosynthesis of solar energy that has transformed carbon dioxide and water molecules into compounds containing carbon, hydrogen, and oxygen. Coal is created over aeons with favourable geologic and climate conditions. The results for each individual deposit is time and process dependent, so coal properties vary from region to region, mine to mine, and even seam to seam. Parameters such as heating value, moisture content, sulphur content, ash composition, and ash quantity are important in maintaining boiler rating, reliability, and performance. The absorption of nutrients by plants and the geological sediments/conditions introduce non-combustible minerals to coal, which for combustion purposes are impurities. The combustion residue of this mineral matter is ash.

Combustion Technology Issues

As power plants face a growing need to reduce costs and environmental impacts, coal quality is increasingly an issue of interest, as a means to do more with less. Coal quality affects plant performance in efficiency, emissions, and availability. At high combustion temperatures, fractions of ash can become partially fused and sticky. Depending upon a particular coal’s ash fusion temperature, it can adhere to heating surfaces building up as slag on water-walls and bridging tubes to obstruct the flow of combustion gases. Tube/refractory erosion and corrosion are issues too.

Recognizing the importance of fuel quality, coal specifications have become more restrictive, monitoring more intensive, and penalties more expensive. This can lead to increasing fuel cost as the demand for the most desirable sources escalates.

For large, central power stations, pulverized coal-fired (PC) boilers have evolved as the technology of choice. PC boilers combust a suspension of finely ground coal that is blown into the furnace in a gaseous matrix to form a large, stable flame vortex. Fine coal particles react similarly to atomized particles of liquid fuels. The reaction time is measured in seconds. The amount of coal, its heating value, and the impurities determine the size and design of the furnace/boiler and placement of the heating surfaces. Coal ash/impurities can form deposits on heat transfer surfaces and the ash itself must be collected. Products of combustion including SO_X and NO_X compounds must be controlled. The amount of ash and its constituents are basic design parameters for the boiler and the back-end air quality control systems.

Circulating fluidized bed (CFB) boilers are a more recent design option. Their size has gradually increased since the technology was first commercially demonstrated at the Nucla Station, in an EPRI (Electrical Power Research Institute) sponsored program. In a CFB boiler, fuel is combusted at lower temperatures in an aerated/fluidized bed of material that typically includes crushed limestone. The lower combustion temperatures and calcium content of the limestone reduce the formation/discharge of SO_X and NO_X compounds, so emission controls start in the combustion zone. Air quality control systems can further reduce emissions. The relatively long residence time for fuel within the combustion zone makes this combustion technology useful for lower quality fuels – fuels that are difficult to ignite, take longer to fully combust, and contain large quantities of impurities that are problematic for suspension firing in a  PC boiler.

Other combustion technologies, like coal gasification and pressurized fluidized bed boilers are being developed/demonstrated. The co-firing of coal with biomass and other solid fuels is also practical. Each technology has its own, unique requirements. So while coal quality control is an issue of importance, its means and methodology cannot be separated from its utilization. Combustion technology and fuel quality are coalescent issues.

Resource Management

Coal quality control begins at the mine. The mining engineer is responsible for developing the mining plan, monitoring production, and managing operations. One objective of any mining plan is to maximize recovery of the deposit of suitable quality coal. This is an economic issue; it’s cost effective to retrieve as much of a given resource that is economically possible. Mine development has sunk costs that should be “spread” over as much coal as possible. There are economic “cut-off” parameters that impact the mine plan. For open cast mines, the issues include strip ratios, how much overburden or interburden must be removed to expose a given quantity contained in a coal seam. For underground mines, it can be the seam height, pitch, depth, roof stability, etc.