Transcript Combustion Based Power Generation : Its Bliss & Curse

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….
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.
CO2 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.
• 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.
IGCC system with dual cycles
IGCC system with steam injected H2/O2 cycle and
CO2 recovery.
Variation of power with steam injection coefficient
(RS)
Reference SOFC-GT system – Regenerative Brayton cycle
Air in
Combustor
Exhaust
Air
Filter
AC
Power
conditioning
system
SOFC
Generator
Compressor
DC
Gas Turbine
Excess air
G
Fuel Air
Recuperator
Pump
Natural Gas
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
Net heat generation (MW)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
1
2
3
stack capacity (MW)
4
5
Stack efficiency…
 Cell voltage is directly proportional to the stack efficiency.
Thus, as the cell voltage is increased stack efficiency is
increased.
60.0
1 bar
Stack efficiency (%)
50.0
40.0
30.0
20.0
10.0
0.0
0.0E+00
2.0E-01
4.0E-01
6.0E-01
Cell voltage (V)
8.0E-01
1.0E+00
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.
UF = 0.7
Bottoming cycle power output
(MW)
0.25
UF = 0.75
UF = 0.8
0.2
UF = 0.85
0.15
0.1
0.05
0
0
2
4
6
Inlet pressure (bar)
From 1 MW SOFC stack
8
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.
TIT 973 K
68
TIT 1073 K
TIT 1173 K
TIT 1273 K
From 1 MW SOFC stack
System efficiency (%)
67
66
65
64
63
62
0
1
2
3
4
Pressure (bar)
5
6
7
8
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.
TIT
Pr = 4 bar, UF = 0.85
1400
Stack Outlet Temp
1350
67.5
Temperature (K)
System efficiency (%)
68
67
66.5
66
1250
1200
65.5
65
950
1300
1000
1050
1100
1150
1200
Turbine Inlet Temperature (K)
1250
1300
1150
0.65
0.7
0.75
0.8
FC Utilization Factor
0.85
0.9
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%.
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%.
THANK YOU