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14 vol% to 44 vol%. CO2 is captured in a Selexol plant. The removal efficiency has been assumed to be 87 percent, which is close to the assumption for the IGCC power plant case.
After the CO2 removal, the remaining gas rich in H2 is burned in the gas turbine combustor. The gas turbine has been scaled to the actual fuel gas capacity from the Siemens V 94.4 gas turbine in the IGCC case, assuming unchanged performance. Heat from the gas turbine exhaust gases is utilized to generate steam for the bottoming cycle in a heat recovery boiler.
C-18 Carbon Dioxide (CO2); CO2 Disposal
FIG. C-17 CO2 neutral production of methanol, power, and district heat by gasification of biomass and reforming of natural gas in series combined with CO2 capture. (Source: Vattenfall Utveckling AB.)
Gasification of biomass and reforming of natural gas in series. In this configuration, syngas is produced from biomass, oxygen, and steam in a fluidized bed gasifier at about 950°C, 20 bar. Before being gasified, the biomass is dried in a steam drier, lowering its moisture content from about 50 percent to 15-20 percent. The syngas from the gasifier is cleaned from dust using a ceramic filter at about 500°C and is then mixed with syngas from a natural gas reformer. Since both the gasifier and the reformer are operating at about 20 bar, the syngas mixture must be compressed before entering the methanol synthesis reactor. Since the gas composition is not optimal for a conventional methanol synthesis process, the LPMeOH process has been selected (Fig. C-17).
The unreacted outlet gas from the methanol synthesis reactor is shifted before the CO2 removal. The CO2 content in the gas then increases from 7 to about 18 vol%. Like in the IGCC power plant case, a 90 percent removal has been assumed. The remaining gas, rich in hydrogen (69 vol%), is expanded to the pressure required for the gas turbine combustor. The gas turbine has been scaled to the actual fuel gas capacity from the Siemens V 94.4 gas turbine in the IGCC case, assuming unchanged performance. The heat from the gas turbine exhaust gas is utilized in the steam cycle and for heating the reformer.
Energy efficiencies and costs when capturing carbon dioxide
The calculated efficiencies with and without CO2 capture for the gas turbine-based power plants and for the described examples of CO2 neutral coproduction of methanol, electric power, and district heat are summarized in Table C-2. Additional costs, due to the CO2 capture, were estimated based on data from IEA studies, other literature, and this information source’s in-house information. The results are summarized in Table C-2.
CO2 capture and recovery consumes electricity and energy at high temperatures at the same time as energy at low temperatures can be recovered. This does not
Carbon Dioxide (CO2); CO2 Disposal C-19
TABLE C-2 Calculated Efficiencies and Carbon Dioxide Capture Costs for Electric Power Plants and for Carbon Dioxide Neutral Coproduction of Methanol, Electric Power, and District Heating
Capital costs: 7 percent real interest rate, 20 years economic lifetime
Fuel costs: Natural gas 100 SEK/MWh, coal 50 SEK/MWh, biomass 120 SEK/MWh
District heat credit: 150 SEK/MWh
For Methanol and Electricity
Methanol credit: 230 SEK/MWh (assumed world market price 1 SEK/liter)
Electric power credit: 280 SEK/MWh (calculated production cost from natural gas without CO2 capture)
Power Plant Methanol + Electricity
6000 h/year 8000 h/year
Fossil Fuel, Natural Coal Biomass, MW (LHV) 385 385
Fossil Fuel, Natural Gas Coal
MW (LHV) Gas IGCC MW (LHV) 1245 870
Net electricity, MW 645 870 Methanol prod., MW 280 365
300 320 Net electricity, MW 505 350
District heating, MW 55 85 District heating, MW 90 45
Net efficiencies 56 43 Net efficiencies 17 29
Without CO2 Capture With CO2 Capture
Electric power, % Methanol, %
With CO2 Capture
Electric power, % 47 37 Electric power, % 31 28
District heating, % 7 10 District heating, % 6 4
Total, % 54 47 Total, % 54 61
Captured CO2, tons/h 120 260 Captured CO2, tons/h 260 315
CO2 Capture Costs 220 100 CO2 Capture Costs 340 145