Coal Combustion By Products are increasingly important. They are a major source of environmental pollution and their treatment affects plant performance.
In this section we will look at formation, treatment and environmental impact of each by-product.
• Fly and Bottom Ash
Formation and Treatment
Bottom ash is removed from bottom of furnace. It is affected by ash fusion temperature. Minimum DT of 1150 to 1200ºC for dry bottom furnace.
Fly ash typically accounts for 80 to 95% of ash formed during combustion of coal in PCC boilers. The ash must be removed from flue gases before release to atmosphere.
Coarser particles can be removed using mechanical cyclones.
Finer particles (< 5µm) are caught in electrostatic precipitators (EP), fabric filters or granular bed filters.
EP operates by passing the fly ash between plate electrodes with a large voltage generated to ionise the air. The particles are charged and migrate towards the plates. When sufficient material has accumulated on the plates it is collected by dislodging from the plates.
The efficiency of EP depends on the resistivity of the ash, which in turn depends on the ash composition. In particular the iron, potassium and sodium contents.
A typical power station of say 1300 MWe fires 4,000,000 tons of coal per annum. At an ash content of 5% this means about 200,000 tons of ash is produced per annum. The disposal or use of the ash is of environmental and economic significance.
Environmental Impact
Trace element content of ash and flue gases is of major environmental concern. Some elements pose a serious threat to plant and animal life whereas some are beneficial.
Example is build up of As, Cd and Hg in the food chain is considered harmful to human health.
The US EPA has classified trace elements of environmental concern as shown in below table.
Trace elements are distributed between the bottom ash, fly ash and flue gases. The mineralogical association of a particular trace element in the coal structure determines its behavior during combustion.
In general elements associated with sulphide or organic matter in coal tend to volatilise. Examples are Hg, As, Se, Ni, Pb, Cu and Zn. This means these elements tend to exit with flue gases. The metals are present in the flue gas itself or as condensate on surface of fine ash particles.
Most of the trace elements in fly ash are higher concentration than in bottom ash due to vaporisation-condensation mechanisms.
Mostly fly ash particles and a proportion of trace elements are removed by EP or fabric filters. However most EP systems are ineffective in collecting the finest particles that has high trace element surface concentrations due to condensation.
FGD and NOx cleaning systems appear to further reduce the trace element levels in flue gas (As, Hg, B and Se).
Active carbon filters can be used to reduce As, Pb, Cd and Se.
An understanding of the partitioning of trace elements is essential to understand potential environmental impact.
Radionuclides
The impact of radionuclides is another consideration. Coal contains trace levels of Th, Ra and U.
The US EPA has determined average values of U and Th as 1.3 and 3.2 ppm respectively in USA coals. It is established that fissionable U-235 (that used for nuclear power) is 0.7% of U content.
ORNL presented data for expected world coal use for period 1937 to 2040.
Expected consumption of coal is 637,409,000,000 tons (637.409 billion tons)
Based on above radionuclide average contents
Uranium in coal ash is 828,632 tons (containing 5883 tons of U-235)
Thorium in coal ash is 2,039,709 tons.
(Note assuming USA levels for world coal usage).
For comparison operation of 1000 MWe nuclear power plant amounts to 4.8 person-rem/year whereas a 1000 MWe coal power plant amounts to 490 person-rem/year. Data from NCRP reports no. 92 and 95.
So if the data is correct the radiation exposure from coal is greater than nuclear.
Potential problems discussed by ORNL are:
- Long term radiation effects
- Possible use of coal ash as nuclear weapons source material
- Waste of coal ash as feed source for nuclear power
• Sulphur Dioxide SO2
Formation
Majority of sulphur in the coal is converted to SO2 some is converted to SO3 and some reacts with the ash.
Strict environmental standards apply for flue gas SO2 levels.
Treatment
SO2 can be removed by flue gas desulphurisation (FGD).
- Wet scrubbers using a calcium based sorbent producing a gypsum by-product
- Spray dry scrubbers
- Sorbent injection
The most effective and widely used treatment is wet scrubbing. Limestone slurry is injected into the flue gas to produce calcium sulphate and sulphite. About 90% of the SO2 can be removed in this process.
• Nitrous Oxides NOx
Formation
NOx is produced from nitrogen in coal and combustion air. It is a direct cause of acid rain.
Treatment
The formation of NOx can be reduced by using special burners, reducing excess air, staging fuel burning and decreasing flame temperatures.
NOx can be removed from flue gas using catalytic reduction, ammonia injected into flue gas with a titanium catalyst.
• Carbon Dioxide CO2
Formation
This is the main component of flue gases. The Greenhouse Gas effect is the concern with increased emissions.
Treatment
There are no commercial treatments in use to reduce carbon dioxide emissions.
The direction of control is by lowering the amount of coal used as a fuel by increasing the efficiency of power generation.