(707f) Bio-Energy with Carbon Capture and Storage (BECCS): Opportunities for Efficiency Improvement

Bio-energy with carbon capture and storage (BECCS) will be a key mitigation technology in reducing atmospheric CO₂ concentrations (Fuss et al., 2014; Fajardy and Mac Dowell, 2017; Mac Dowell and Fajardy, 2017). There are opportunities to improve efficiency and enhance commercial viability of BECCS. One approach is to recover waste heat from the power plant boiler system to supply thermal energy for the amine-based CO₂ capture process (Harkin et al., 2009, 2010). The amount of heat available from the furnace exhaust gas is a function of adiabatic flame temperature (AFT), which in turn depends on fuel properties (Flagan and Seinfeld, 2012). Compared to higher quality coal, biomass tends to have lower heating value, lower ash and variable moisture content (5–60 wt% wet). The selection of a high quality biomass (e.g. low moisture and low ash) can have a positive impact on the combustion efficiency by increasing AFT (Sami et al., 2001), which in turn enhances heat recovery and energy efficiency (Bui et al., 2017a; Bui et al., 2017b). Additionally, biomass has lower sulphur and nitrogen content compared to coal, thus, co-firing can reduce the emissions of SO_X and NO_X (Spliethoff and Hein, 1998; Spliethoff et al., 2000).

This study evaluates the energy efficiency, recoverable heat and carbon intensity of a 500 MW pulverised fuel BECCS system for different biomass co-firing proportions and capture solvents. The modelling procedure used for this analysis is as follows:

1. Selection of coal type (high and medium sulphur content), biomass type (wheat straw and clean wood chips), and CO₂ capture solvents (MEA, Cansolv, “new solvent”).
2. 500 MW ultra-supercritical power plant model: Calculates the fuel firing flow rate and net power output for different biomass co-firing proportions.
3. Chemical equilibrium model of biomass and coal co-combustion: Determines the flue gas composition, flow rate, thermodynamic properties and adiabatic flame temperature (AFT).
4. Heat recovery model: Calculations to determine the effect of flue gas heat recovery on the overall plant efficiency and carbon intensity.

The opportunities to improve BECCS energy efficiency include higher heat recovery (HR) and using high performance capture solvents, which can increase efficiency from 31%_HHV (conventional MEA system) to 38%_HHV (new solvent + HR). Significant reductions to SO_X emissions can be obtained by increasing biomass co-firing proportion or using coals with low sulphur content. The level of NO_X emissions depends on combustion conditions, and will increase with temperature. The low efficiency MEA system was more carbon negative on a per MWh basis compared to the high efficiency system. This was due to higher fuel consumption per MWh of electricity generated, thus, more CO₂ is captured from the atmosphere. However, it is important to consider that the dispatch of higher efficiency systems is favoured due to their lower marginal cost of electricity generation. The annual carbon removal of a BECCS system depends on the efficiency and annual capacity of the power plant. Thus, there is a trade-off between efficiency and carbon intensity that needs to be considered when evaluating BECCS potential for climate mitigation.

References

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