Sunday, January 12, 2020

Development of Clean, High Efficiency, Higher Capacity, Coal‐fired Power Generation Technology

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.

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.
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 – 950°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, CO2 reduction and much reduced NOx, SOx and particulate emissions. Many Ultra super critical power plants ranging 350MWto 1000MW are 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

Figure 4 Operational parameters vs. Efficiency improvement (%)

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.

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.

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.

Source: S. Bharath Subramaniam - Assistant Professor, Mechanical Engineering, SRM University, Kattankulathur, Tamilnadu, India

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