CLEAN COAL TECHNOLOGIES, CHALLENGES AND FUTURE SCOPE

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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 8, Issue 2, February 2017, pp. 34–40, Article ID: IJMET_08_02_005
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=8&IType=2
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
CLEAN COAL TECHNOLOGIES, CHALLENGES AND
FUTURE SCOPE
S. Bharath Subramaniam
Assistant Professor, Mechanical Engineering, SRM University,
Kattankulathur, Tamilnadu, India
ABSTRACT
Clean Coal Technologies (CCT) are technological developments that lead to efficient
combustion of coal with reduced emissions. It is achieved through combustion or gasification. A
combination of clean coal technologies is necessary to achieve maximum power with enhanced
energy conversion. The efficiency and quality of the power generation depends upon the coal
content. Clean coal technologies, challenges and the future scope are summarized in this paper.
Key words: Clean coal technologies (CCT), combustion, gasification, coal.
Cite this Article: S. Bharath Subramaniam. Clean Coal Technologies, Challenges and Future
Scope. International Journal of Mechanical Engineering and Technology, 8(2), 2017, pp. 34–40.
http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=8&IType=2
1. INTRODUCTION
The coal power plants are considered clean when combined with certain advanced technologies.
Supercritical and Ultra-Supercritical steam cycle, Circulating Fluidized bed combustion (CFBC), and
Integrated Gasification Combined Cycle (IGCC) are some of the advanced coal technologies. Coal is the
most widely available fossil fuel. However burning the coal can pollute the environment. Clean coal
technologies addresses the issue.
2. TECHNOLOGIES FOR COAL
Coal is burnt for generating electricity. Increased usage of coal will result in pollution unless cleaner and
efficient coal technologies are incorporated. Efficient usage of coal is by means of reducing coal’s
Greenhouse gas (GHG) emissions. Another way to utilize coal plants is through co-firing techniques. In a
pulverized coal fired power plant water is converted to steam which after undergoing various states i.e.
superheating, reheating etc. drives the steam turbine which is coupled to a generator to generate electricity.
Nowadays pulverized coal fired power plant is not considered environment friendly as emissions are more.

Clean Coal Technologies, Challenges and Future Scope
Figure 1 Schematic of pulverized coal fired plant
2.1. Clean Coal usage
Clean coal usage starts from coal washing and upto efficient combustion in the combustor. Coal washing
reduces the ash content. Appropriate fuel preparation method is employed. Particulate control depends on
the Electrostatic Precipitators (ESP). Flue gas desulphurization (FGD) units can remove major portion of
the SO2. Low-NOx burners, over-fire air etc. are used for NOx reduction.
2.2. Fluidized bed combustion (FBC)
Figure 2 Schematic of a Circulating Fluidized bed Combustor (CFBC)

Fluidized bed combustion (FBC) significantly reduces SOXa nd NOx emissions. Sulphur emissions
from the coal suc has SO2 is absorbed by a sorbent (limestone), which is fed into the combustion chamber
along with the coal. Major portion of the sulphur can be removed in the combustor itself. Fluidized bed
combustors operate at a relatively low temperature (800 – 9500
C).Fluidized bed combustion is mainly
suited for low quality fuels. Their relatively lower-cost, clean and efficient combustion makes it a
recognizable technology. Circulating Fluidized Bed Combustion (CFBC) has gained more acceptances but
it is mainly used with low quality fuels and the plant efficiency is similar to subcritical plants. CFB can be
designed for supercritical conditions, but only fewer plants are in operation currently.
2.3. Ultra – Supercritical and Supercritical Technologies
There is no distinction between the liquid and the vapour phase in the supercritical state. Water / steam
reaches this state at a pressure of about 221.2 bar. Above this pressure the cycle becomes supercritical and
the fluid is in single phase. As a result no water / steam separating device is required. Supercritical units
have higher plant efficiency than that of Subcritical units because of higher steam parameters. The Gross
plant efficiency is around 40‐41% for supercritical units which are higher than the Sub‐critical unit. The
Ultra-supercritical units have an overall plant efficiency of 46% to 49%.Some of the advantages of Ultra
supercritical and Supercritical units are reduced fuel costs due to higher plant efficiency, CO2reductionand
much reduced NOx, SOx and particulate emissions. Many Ultra super critical power plants
ranging350MWto 1000MWare under operation/construction. The energy conversion of steam power plant
can be enhanced by increasing the main steam parameters. The water in the supercritical boiler is
pressurized by the feed pump, sensible heat is added until water attains saturation temperature and changes
instantaneously to dry saturated steam followed by superheating.
Figure 3 Efficiency vs. Emissions
Fig. 4 Operational parameters vs. Efficiency improvement (%)
Figure 4 Operational parameters vs. Efficiency improvement (%)
0
200
400
600
800
1000
1200
30 36 38 42 43 46 50 55
CO2emissions(g/kWhr)
Net plant efficiency (% LHV basis)
Efficiency vs. Emissions

Figure 5 Plant capacity and Parameters vs. Coal consumption
2.4. Integrated Gasification Combined cycle (IGCC)
At high temperatures the carbon in the coal reacts with steam and produces a combustible gas, which is a
mixture of hydrogen (H2) and carbon monoxide (CO).The gas is cleaned and is used to drive a gas turbine
and generate power. The high temperature combustion gases leaving the gas turbine can be used to
produce steam, which in turn can be used to obtain steam power.
Figure 6 Schematic of Integrated gasification combined cycle (IGCC) plan
2.5. Oxyfuel Technology
This technology is for CO2 capture. The nitrogen present in the air reduces the CO2 concentration in the
flue gas. In the oxy-fuel combustion a combination of oxygen and the flue gas is used for the combustion
of the coal. A gas consisting mainly of CO2 and water vapour is formed. Concentrated CO2 is produced and
is captured. The flue gas controls the flame temperature in the boiler.
0.56
0.58
0.6
0.62
0.64
0.66
Coalconsumption(kg/kWhr)
Plant capacity and parameters
Plant capacity and Parameters vs. Coal consumption

Figure 7 Schematic of Oxy fuel technology
3. INDIAN ENERGY SCENARIO
Figure 8 India’s estimated energy mix by 2030
4. CHALLENGES TO CLEAN COAL TECHNOLOGIES
4.1. Issues with Ultra Supercritical / Supercritical technologies
For incorporating USC/SC parameters, advanced materials are required. Some of the materials are P91
piping and quality boiler plates. High thermal stresses and fatigue in the boiler sections of a Supercritical
plant occur and lead to relatively higher maintenance costs. Thermal stresses in the turbine blade, solid
particle erosion and complicated start-up procedures are required in USC / SC plants.USC units are more
sensitive to feed-water quality. Lower operational availability and reliability of steam turbines as compared
with sub-critical units.
62%14%
10%
8%
6%
India's estimated energy mix by 2030
Coal
Renewables
Hydro
Nuclear
Gas

Figure 9 Wall corrosion and thermal stresses in Supercritical technology
4.2. Issues with Fluidized Bed Combustion (FBC)
Water wall tube Failures, clinker Formation, refractory damages, and air pre heater tubes choke up and
tube failures occur due to accumulation of bed material in the combustor. Refractory damage occurs in the
combustor area, fluidized bed heat exchanger (FBHE) area and cyclone area.
Figure 10 Refractory damage in the cyclone
5. CONCLUSION
Development of clean, high efficiency, higher capacity, coal‐fired power generation technology is a
strategic task. In order to meet the increasing demand for electric power, improve the utilization efficiency
and reduce the pollutant emissions, we have to develop Ultra Supercritical / Supercritical units. For high
ash and sulphur content coals Fluidized bed combustion (FBC) can be employed. Coal can also be co-fired
with biomass which offers many benefits and leads to better biomass utilization. The development of
supercritical steam cycles with higher steam temperatures, combined with modern plant design and
automation, leads to significant efficiency improvement and CO2 reduction.
6. FUTURE SCOPE
IGCC plants are more flexible for environmental requirements on pollutants because today IGCC plants
operate lower cost for Carbon capture and Sequestration (CCS).The coal power plants both existing and
future has to be more flexible in response to the changing electricity demand.

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CLEAN COAL TECHNOLOGIES, CHALLENGES AND FUTURE SCOPE

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CLEAN COAL TECHNOLOGIES, CHALLENGES AND FUTURE SCOPE