Abstract
The possibility of separating and recovering CO2 in a biogas plant that co-produces electricity, hydrogen, and heat is investigated. Exploiting the ability of a molten carbonate fuel cell (MCFC) to concentrate CO2 in the anode exhaust stream reduces the energy consumption and complexity of CO2 separation techniques that would otherwise be required to remove dilute CO2 from combustion exhaust streams. Three potential CO2 concentrating configurations are numerically simulated to evaluate potential CO2 recovery rates: 1) anode oxidation and partial CO2 recirculation, 2) integration with exhaust from an internal combustion engine, and 3) series connection of molten carbonate cathodes initially fed with internal combustion engine (ICE) exhaust. Physical models have been calibrated with data acquired from an operating MCFC tri-generating plant. Results illustrate a high compatibility between hydrogen co-production and CO2 recovery with series connection of molten carbonate systems offering the best results for efficient CO2 recovery. In this case the carbon capture ratio (CCR) exceeds 73% for two systems in series and 90% for 3 MCFC in series. This remarkably high carbon recovery is possible with 1.4 MWe delivered by the ICE system and 0.9 MWe and about 350 kg day−1 of H2 delivered by the three MCFC.
Highlights
The global impacts of CO2 emissions from power generation have been well established [1]
Use of a MCFC as a carbon dioxide concentrator was investigated experimentally by Sugiura et al [13], where it is showed that the experimental values for the CO2 removal rate from cathode to anode performed in a MCFC almost matched theoretical calculations
The results indicate a co-production flexibility, as it is possible to alter the ratio of hydrogen and electric production without dramatically impacting CO2 capture ratio (CCR)
Summary
The global impacts of CO2 emissions from power generation have been well established [1]. Especially molten carbonate (MCFC) and solid oxide fuel cells (SOFC), have received substantial attention during the past decades and have recently begun to achieve commercial success [7], [8] Both MCFC and SOFC technologies can operate on natural gas and biogas to achieve higher fuel-to-electric (FTE) efficiencies and near zero criteria pollutant emissions (e.g. NOx, SOx, particulate) compared to traditional engines. With natural gas as a feedstock, CO2 emissions can be lowered between 10 and 43.6% with a high temperature fuel cell, depending upon the electric power generating system considered in the comparison, while with renewable biogas feedstock the emissions can be reduced near to zero [11] With this in mind, the current work investigates a further reduction of greenhouse gas emissions adding the recovery of CO2 from a tri-generation MCFC plant. The combination of these techniques with a tri-generation system operated on biogas (such as the one at OCSD) would enable carbon-negative production of electricity and fuel (with the CCS process), since the carbon of the ADG was previously contained in the sewage (before being converted into biogas), which comes from plants that removed CO2 from the atmosphere
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