1. A novel IGCC system with steam injected H2/O2 cycle and CO2 recovery P M V Subbarao Professor Mechanical Engineering Department Low Quality Fuel but High Efficiency Unit….

2. The Curtain Raiser The integrated gasification combined cycle (IGCC) is one of the advanced clean coal power generation systems. Compared with the conventional coal fired power plant, it has lower emissions of SO2, NOX and particle pollutants. Though it is reputed to be the cleanest coal fired power plant, CO2 emission cannot be greatly reduced by this technology. Only proportionally reduced with improvement of the IGCC system efficiency. How to reduce CO2 emission effectively from the IGCC system becomes the main subject of researchers.

3. CO 2 Recovery Generally, there are five ways to separate and recover CO2 from the IGCC system. (1) CO2 separation and recovery from the exhaust fuel gas; (2) CO2 sequestration before combustion; (3) CO2 sequestration by a polygeneration system that combines the IGCC system with chemical processes; (4) CO2 recovery using integrated thermal cycles with fuel oriented transfer; and (5) CO2 separation and recovery based on a novel thermal cycle, the semi-closed O2/CO2 cycle IGCC.

4. IGCC combustion products mainly consist of CO2 and H2O, and hence, it separates CO2 without extra energy consumption. However, the O2 production and CO2 recovery demand large energy consumptions. The energy penalty for separating and recovering CO2 will bring an efficiency decrease of about 7 percentage points. The IGCC system with dual cycles (DC-IGCC) and less CO2 emission is another example. Its efficiency decrease is less than 4 percentage points after separating and recovering CO2.

5. IGCC system with dual cycles

6. IGCC system with steam injected H2/O2 cycle and CO2 recovery.

7. Variation of power with steam injection coefficient (R S )

9. Reference SOFC-GT system – Regenerative Brayton cycle SOFC Generator DC AC Power conditioning system Combustor Recuperator Air Filter Pump G Natural Gas Air in Compresso r Air Fuel Exhaust Gas Turbine Excess air

10. Net heat produced in the fuel cell stack Vs Stack capacity  Methane and air as a fuel.  Operates at 4 bar, 1073 K  Fuel utilization factor of 80%  Heat generated from the stack is approximately 25% of the stack capacity.  Heat generation is almost linear  Stack effluent consist of 20% of unutilized fuel and products

11. Stack efficiency…  Cell voltage is directly proportional to the stack efficiency. Thus, as the cell voltage is increased stack efficiency is increased.

12. Bottoming cycle power output  After reaching the optimum pressure, turbine power output decreases asserting that this is the optimized value.  This is because of the decrement in TIT due to the shift of heat recovery. From 1 MW SOFC stack

13. Effect of operating parameters  At low pressure ratios, primary fuel in the gas turbine must be reduced to meet constant exhaust temperature and more fuel can be sent to the SOFC stack. This means less output from the gas turbine and more from the SOFC, thus increasing the efficiency.  After reaching optimum pressure, overall cycle efficiency gets reduced due to high power consumption because of pressurization. From 1 MW SOFC stack

14. Effect of operating parameters…  Increasing the TIT does not lead to much improvement in the efficiency of the system. Indeed, more fuel consumption at high inlet temperature leads to less utilization of fuel in the fuel cell stack.  The temperature of the working fluid entering the expander of the GT increases as the excess air decreases.  Thus, for the same utilization and electrochemical production, the requirement to heat less air results in higher TIT and higher overall efficiency.

15. Findings  From the above it is clear that electrical efficiency of the system can be as high as 65%.  Increase in operating pressure increases the overall system efficiency. But, high pressures lead to cost, so, there should be a balance between the development cost and the efficiency.  Decreasing the excess air in the SOFC-GT has a positive effect on the overall efficiency.  Also, simulation results show that the cell voltage is about 0.7V with electrical efficiency of the plant better than 65%.

16. Findings…  Low air flow or no supplementary fuel is also beneficial.  Pressure ration optimum was found to be 4.5 for the referred SOFC-GT cycle.  The advantage of incorporating the gas turbine engines to form the bottoming cycle of the plant can be realized from the increase in overall system efficiency.  In this case, the ratio of the power turbine AC output to the SOFC stack AC output is about 25%.

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