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Recovery of un-combusted fuel in a gas turbine power cycle with Chemical Looping Combustion.

Gunnarsson, Jonas (2010)
Department of Energy Sciences
Abstract
Chemical Loping Combustion (CLC) is a novel combustion technology in which the exhaust gas is inherently divided into oxygen depleted air in one stream and CO2 and water vapour in another. This is however under the ideal condition that the fuel conversion is complete, i.e. all the hydrocarbons in the fuel is transformed to heat, water vapour and CO2. The CO2 capture rate can under such ideal conditions be almost 100%. But both thermodynamic calculations as well as practical experiments show that complete fuel conversion might not always be expected. The present work examines different ways of recovering unconverted fuel from the gas conditioning process to a CLC reactor and simulates a number of power cycles with CO2 conditioning and... (More)
Chemical Loping Combustion (CLC) is a novel combustion technology in which the exhaust gas is inherently divided into oxygen depleted air in one stream and CO2 and water vapour in another. This is however under the ideal condition that the fuel conversion is complete, i.e. all the hydrocarbons in the fuel is transformed to heat, water vapour and CO2. The CO2 capture rate can under such ideal conditions be almost 100%. But both thermodynamic calculations as well as practical experiments show that complete fuel conversion might not always be expected. The present work examines different ways of recovering unconverted fuel from the gas conditioning process to a CLC reactor and simulates a number of power cycles with CO2 conditioning and compression to see how incomplete fuel conversion (and recycling) can affect cycle efficiency. Three main ways of recovering the unconverted fuel has been investigated when varying the fuel conversion between 100-90%. From the CLC reduction reactor three different compositions of the unconverted fuel has been investigated 1) only methane, 2) a mix of hydrocarbons as in the NG, 3) H2 and CO. The different cases were tested in two different cycles, one, ‗A-‗, without CO2 turbine and one, ‗B-‗ with CO2 turbine. The efficiency for cycle ‗A-‗ at 100% fuel conversion and CO2 compression to 200 bar was 50,37% while it for ‗B-‘ was 51,84%. The unconverted fuel can be separated in a distillation column and recirculated to the CLC reactor. Two main drawbacks were noticed when simulating those cases, accumulation of incombustible volatiles in the recirculation loop and a large fraction of CO2 recycled to the reactor, i.e. large additional CO2 compression work. The accumulation of incombustible volatiles, in the simulations N2 from the fuel, was handled by a bleed stream which introduced CO2 emissions to the atmosphere and had a negative impact on cycle efficiency. With a fuel conversion rate at 95% the CO2 capture rate was 95%, and the cycle electrical efficiency was 48,85% (compared to 47,75 without any recovery) in cycle ‗A-‘. Simulations were also performed without any N2 in the fuel and hence without bleed stream. At 95% fuel conversion rate the CO2 capture was 100% and the electrical efficiency was 49,45% which shows the influence small fractions of inerts can have as they accumulate. The unconverted fuel can instead be separated in a distillation column and brought to a burner prior the air turbine. This way no accumulation of inerts occurs and the design and control of the CLC reactor becomes simpler. There will however be CO2 emissions to the atmosphere. At 95% fuel conversion the CO2 capture rate was around 90% and the electrical efficiency was 49,57% in cycle ‗A-‘. One alternative mentioned in the literature is the possibility to inject oxygen from an ASU directly after the CLC reduction reactor and thus oxidize the unconverted fuel. Simulations show that this alternative has around double ―extra work/recovered energy‖ ratio (which is the work for the ASU plus the extra CO2 compression work divided by the recovered fuel energy), meaning that it is less efficient than the other alternatives. It is also questionable if it is possible to oxidize such small amount of hydrocarbons in a CO2 stream, and if it is possible, how much excess air would be necessary? The excess air has to be taken care of, most probably in a distillation column which will lead to further efficiency penalties and CO2 emissions which will probably make this alternative less attractive. This case was tested in cycle ‗B-‗ but due to bad utilization of the heat the efficiency at 95% conversion rate was no higher than 49,26% but with 100% capture rate. (Less)
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author
Gunnarsson, Jonas
supervisor
organization
year
type
H1 - Master's Degree (One Year)
subject
keywords
Chemical Looping Combustion (CLC) un-combusted fuel gas turbine power cycle
other publication id
ISRN LUTMDN/TMHP--10/5222--SE
language
English
id
1698092
date added to LUP
2010-10-21 10:51:35
date last changed
2010-10-21 10:51:35
@misc{1698092,
  abstract     = {Chemical Loping Combustion (CLC) is a novel combustion technology in which the exhaust gas is inherently divided into oxygen depleted air in one stream and CO2 and water vapour in another. This is however under the ideal condition that the fuel conversion is complete, i.e. all the hydrocarbons in the fuel is transformed to heat, water vapour and CO2. The CO2 capture rate can under such ideal conditions be almost 100%. But both thermodynamic calculations as well as practical experiments show that complete fuel conversion might not always be expected. The present work examines different ways of recovering unconverted fuel from the gas conditioning process to a CLC reactor and simulates a number of power cycles with CO2 conditioning and compression to see how incomplete fuel conversion (and recycling) can affect cycle efficiency. Three main ways of recovering the unconverted fuel has been investigated when varying the fuel conversion between 100-90%. From the CLC reduction reactor three different compositions of the unconverted fuel has been investigated 1) only methane, 2) a mix of hydrocarbons as in the NG, 3) H2 and CO. The different cases were tested in two different cycles, one, ‗A-‗, without CO2 turbine and one, ‗B-‗ with CO2 turbine. The efficiency for cycle ‗A-‗ at 100% fuel conversion and CO2 compression to 200 bar was 50,37% while it for ‗B-‘ was 51,84%. The unconverted fuel can be separated in a distillation column and recirculated to the CLC reactor. Two main drawbacks were noticed when simulating those cases, accumulation of incombustible volatiles in the recirculation loop and a large fraction of CO2 recycled to the reactor, i.e. large additional CO2 compression work. The accumulation of incombustible volatiles, in the simulations N2 from the fuel, was handled by a bleed stream which introduced CO2 emissions to the atmosphere and had a negative impact on cycle efficiency. With a fuel conversion rate at 95% the CO2 capture rate was 95%, and the cycle electrical efficiency was 48,85% (compared to 47,75 without any recovery) in cycle ‗A-‘. Simulations were also performed without any N2 in the fuel and hence without bleed stream. At 95% fuel conversion rate the CO2 capture was 100% and the electrical efficiency was 49,45% which shows the influence small fractions of inerts can have as they accumulate. The unconverted fuel can instead be separated in a distillation column and brought to a burner prior the air turbine. This way no accumulation of inerts occurs and the design and control of the CLC reactor becomes simpler. There will however be CO2 emissions to the atmosphere. At 95% fuel conversion the CO2 capture rate was around 90% and the electrical efficiency was 49,57% in cycle ‗A-‘. One alternative mentioned in the literature is the possibility to inject oxygen from an ASU directly after the CLC reduction reactor and thus oxidize the unconverted fuel. Simulations show that this alternative has around double ―extra work/recovered energy‖ ratio (which is the work for the ASU plus the extra CO2 compression work divided by the recovered fuel energy), meaning that it is less efficient than the other alternatives. It is also questionable if it is possible to oxidize such small amount of hydrocarbons in a CO2 stream, and if it is possible, how much excess air would be necessary? The excess air has to be taken care of, most probably in a distillation column which will lead to further efficiency penalties and CO2 emissions which will probably make this alternative less attractive. This case was tested in cycle ‗B-‗ but due to bad utilization of the heat the efficiency at 95% conversion rate was no higher than 49,26% but with 100% capture rate.},
  author       = {Gunnarsson, Jonas},
  keyword      = {Chemical Looping Combustion (CLC)
un-combusted fuel
gas turbine power cycle},
  language     = {eng},
  note         = {Student Paper},
  title        = {Recovery of un-combusted fuel in a gas turbine power cycle with Chemical Looping Combustion.},
  year         = {2010},
}