Enhancing Efficiency of Corncob-Fired Power Generation with Carbon Capture and Storage

. Bioenergy from biomass wastes with carbon capture and storage (CCS) is an important way to compensate for hard-to-abate emissions and collaborate with decarbonizing the energy industry. This work evaluates a corncob-fired power generation with CCS regarding overall energy efficiency in two process alternatives: (a) post-combustion CO 2 capture by an aqueous blend of methyl-diethanolamine and piperazine; and (b) oxy-combustion coupled to state-of-art air separation unit. The alternatives are simulated in Aspen HYSYS and compared with a conventional plant to evaluate the energy penalty of capturing CO 2 . The lean solvent composition is optimized for the lowest regeneration heat demand (2.92 GJ/t CO2 ). Post-combustion capture designed for 90% CO 2 abatement presents an efficiency penalty of 7.96%LHV. In contrast, Oxy-combustion has zero CO 2 emissions and outperforms Post-combustion with a lower penalty of 6.77%LHV, given a chance to have oxygen supplied at an energy cost of 139 kWh/tO 2 . To render Post-combustion the most efficient route, it would be necessary to have its reboiler heat ratio reduced to 2.30 GJ/t CO2 .


Introduction
One of the main challenges of the century is to decarbonize the economy to limit global warming according to the Paris Agreement targets while meeting the increasing global energy demand.Energy transition towards Net-Zero Emissions (NZE) moves the energy industry toward a renewable-based matrix.Although noticeable advancements are in course, fossil fuel substitution is not occurring due to the steep expansion in energy demand and energy security concerns.Decarbonization of fossil energy is required while renewable energy supply and storage are not widely stablished, and the most attractive alternative is the use of carbon capture, utilization, and storage (CCUS).The concept can be applied to power generation with fossil fuels, allowing ≥90% CO2 abatement, and to biomass-based processes, where net negative CO2 emissions is achievable, by considering the life cycle of carbon [1].The latter pathway is also known as bioenergy with carbon capture and storage (BECCS), considered by IPCC as an essential tool to compensate hard-to-abate emissions and thus allow limiting global temperature increase below 2°C [2].
In the context of BECCS systems, using biomass wastes reduces environmental and social issues related to biomass cultivation for energy purposes [3].Among possible waste biomass resources, corncob stands out with reduced mineral [4] and nitrogen contents [5], which favor its use for combustion [4], besides presenting large availability in many countries.The resource is not always well availed, left to decay in crop fields [5] when it is not used as fuel or crushed to serve as animal food.When it is consumed for heat and power generation, the current practice is to release the exhaust gas into the atmosphere.However, the process could be adapted to mitigate emissions through the BECCS concept, avoiding the return of biogenic CO2 to the environment.
This work evaluates corncob-fired power generation as a waste-to-power BECCS system capable of carbon dioxide removal from the atmosphere.Post-combustion CO2 capture with aqueous-blended amines is compared in terms of overall efficiency with oxy-combustion coupled to a standalone air separation unit (ASU).Heating demand sensitivity is evaluated for solvent composition and CO2 mass fraction in lean amine for a given capture efficiency and minimum approach in the rich/lean-solvent heat exchanger.

Methods
Figure 1 presents an overview of the considered BECCS alternatives: (a) post-combustion CO2 capture via chemical absorption with aqueous blended-amine solution (Fig. 1a); and (b) oxy-combustion CO2 capture (Fig. 1b), where gaseous oxygen (GOX) is supplied by different alternatives of air separation unit.The conventional CO2-emitting process is also simulated, comprising only the first block of Fig. 1a.In case (a), the lean (treated) flue gas from the absorption plant is released into the atmosphere containing 10% of generated CO2 (90% capture efficiency), while pure CO2 is sent to compression, dehydration, and pumping.In case (b), part of the flue gas is recycled to the burners to keep the combustion temperature the same as in air-blown case (a), and the remaining part is compressed, dehydrated, and pumped.No further purification is considered.In all cases, the CO2-fluid is dehydrated by triethylene glycol (TEG) at 60 bar and exported with 150 ppm(mol) H2O at 150 bar.The process alternatives are simulated with Aspen HYSYS v12.1.The main simulation premises are summarized in Table 1.Cubic-Plus-Association Equation-of-State is utilized for thermodynamic modeling, except in the following special cases: biomass combustion, free H2O systems, chemical absorption plant, and CO2 dehydration unit, where Peng-Robinson, ASME Table, Acid-gas and Glycol property package are utilized, respectively.The thermodynamic packages are employed with the binary interaction parameters available from the software database.The same biomass feed of 96.81 t/h of grinded corncobs is applied to all processes, of which 8.64%w is moisture and 2.41%w is ash [6].To represent the biochemical composition of corncobs, the same procedure described in [7] is availed, with a basis on the lower heating value and elemental composition of biomass from [6].It is assumed that the particle size distribution of input biomass is suitable for complete conversion in the boiler, which is approached with 10% of combustion air excess.A simple Rankine cycle is adopted, with superheated steam generation at 560°C and 30 bar.The high-pressure turbine discharge is at 4 bar, and a part of this steam is utilized as a heating utility in the plant, supplying reboiler heat duties in amine and TEG regeneration.CO2 absorption by an aqueous solution of methyl-diethanolamine (MDEA) blended with piperazine (PZ) in different proportions is considered, given the relatively high CO2 content in the flue gas and the better stability in the presence of O2 and SO2 comparatively to monoethanolamine (MEA) [8].

Results and Discussion
Table 2 presents the composition of selected process streams, and Table 3 summarizes the main results from the process simulation.The flue gas leaving the HRSG to the directcontact-cooler (DCC) in the Post-combustion case has the same composition and flowrate as its counterpart in the Conventional process since the only upstream modifications for CO2 capture are applied in the configuration of the steam cycle for the supply of steam to the solvent regeneration reboilers.The chemical absorption plant receives cooled gas from the DCC and produces lean gas with ≈90% less CO2 for atmospheric emission.The captured CO2 is compressed, dehydrated, and pumped, resulting in 127.7 t/h of exported CO2-fluid (99.9%mol).In contrast, the oxy-combustion case has zero emission to the atmosphere and a less pure exported CO2-fluid (93.9%mol), as CO2 end-purification is not included.Since GOX feed is assumed at 95%mol purity with slight excess of 1% and gas recycle is utilized for combustion temperature abatement, as usually prescribed for oxyfuel conditions, the flue gas is constituted mainly by CO2 (70.9%mol) and H2O (24.5%mol), with air species present in minor but relevant contents.These latter are due to 1% excess and use of impure GOX 95%mol, which originates ≈90% of the N2 in exported CO2-fluid.Utilization of GOX at higher purities can be considered for a greater CO2 purity and improved performance if N2free cryogenic separation of O2/Ar species is avoided due to close boiling points and significantly higher power consumption above 97%mol O2 [9][10][11].

Post-combustion
Table 3 reveals a major reduction of turbines' output power in the Post-Combustion case comparatively to the conventional process.This results from extracting a significant portion of low-pressure steam (LPS) from the Rankine cycle to meet solvent regeneration requirements, totaling 103.5 MW, of which only ≈0.1 MW is for TEG, due to the low purification service of the dehydration unit.The high requirement of the amine system is compatible with large CO2 flow to the chemical absorption plant, as revealed by the relatively high CO2 content of 18.0%mol for flue gas.The value is compatible with a heat ratio of 2.92 GJ/t for recovering 127.6 t/h CO2 (90% of absorber inlet CO2).This ratio was obtained after optimization of lean amine composition.To illustrate the heat ratio response to this composition, Figs.2a-d present a sensitivity analysis to CO2 and PZ mass percentages for 4 levels of H2O content.The best lean amine composition has 2.85%w CO2 in MDEA/PZ/H2O solvent with mass proportion 50/5/45, which results in 2.92 GJ/t.For the sake of heat ratio comparison with a conventional solvent, aqueous monoethanolamine (MEA) was also simulated for current assumptions, and after optimization of lean amine composition for treating cooled gas leaving DCC (composition shown in Table 2) for the minimum heat ratio, 3.53 GJ/t was found for 35%wMEA (solute-free basis) with loading of 0.397 molCO2/molMEA.The relatively high loading is mostly explained by the high CO2 content in flue gas, which facilitates separation and allows the lean gas to leave the absorber at a proportionally higher CO2 content.With some modifications in the amine process configuration, a lower heat ratio is likely achievable, as Zhou et al. investigated [12].Their work indicated a higher PZ content for optimum efficiency, probably due to a lower CO2 content in their flue gas.It should yet be noted that all results of sensitivity analysis and optimization of lean amine composition relies on Aspen HYSYS v12.1 data and its updated version of Acid-Gas property package.For a future work in determination of optimum amine blend composition, the operation pressure of regeneration column could also be subject of optimization, for the highest overall efficiency [13].

Oxy-Combustion
Table 3 evinces significantly greater overall efficiency of the oxyfuel mode compared to postcombustion case.The result would not be possible with a 2-column air separation unit and is an outcome of adopting a state-of-the-art GOX production plant, which demands 139 kWh/t O2 (1 atm) [9][10].Out of 14.30 MW shown for GOX supply, 14.11 MW is derived from air separation, and the balance is due to equivalent compression.Although GOX is likely to be supplied directly in the required pressure, 1 atm is assumed to with a common standard basis for expressing air separation power requirement [9][10].
Unlike the usual 2-column and 3-column processes, the selected air separation unit (TVR-2REB) is based on single-column near-atmospheric distillation with cryogenic top vapor recompression.Currently, most cryogenic plants for GOX production employ additional pressurized columns to generate liquid-nitrogen reflux to the main fractionation tower.The TVR-2REB solution avoids unnecessary compression of O2 in air without nitrogen compression at near-ambient temperature.The distillation column has an intermediate reboiler, where partial vaporization of O2-rich liquid is driven by condensation of compressed nitrogen.The bottom reboiler is heated by the liquefaction of pressurized air, which is then subcooled and fed to the column.According to [9], the separation power demand is significantly lower than other known standalone plants for either 95% and 99.5%mol O2 purity standards.For instance, for atmospheric GOX 95%, while TVR-2REB demands 139 kWh/tO2, 2-column and 3-column designs are known to require 200 [10] and 158 kWh/tO2 [14], respectively.The product stream leaves the process at a pressure slightly above 1 atm, but the equivalent compression power is discounted.
In this application to the corncob BECCS system, if TVR-2REB is considered for GOX supply, it would be necessary to reduce the post-combustion heat ratio to 2.30 GJ/t to make its overall efficiency greater than Oxy-combustion.If a 3-column plant [14] were applied, air separation demand would be 1.93 MW higher, implying 0.51%LHV lower efficiency (23.96%LHV).The corresponding break-even heat ratio for an equal efficiency via chemical absorption would be 2.60 GJ/t, which is more easily achievable with advanced amine blends and process configurations.Although the development of TVR-2REB process allows a significant reduction in power requirement for oxygen production, there is still a wide opportunity for further enhancements.Air separation technology can make oxy-combustion even more efficient, since from a Thermodynamic theoretical viewpoint it is possible to reduce GOX specific power down to 50 kWh/t O2 [15].
Corncob-fired power generation is evaluated for three process alternatives, aiming the reduction of the energy-efficiency penalty for carbon capture and storage: (i) conventional CO2-emitting process; (ii) post-combustion CO2 capture by chemical absorption with mixed amine (MDEA+PZ); and (iii) oxy-combustion with flue gas total compression to storage and GOX supply from state-of-the-art air separation unit.The conventional process exhibits overall efficiency of 31.24%LHV,emitting 1.20 kg/kWh of biogenic CO2.Post-combustion is designed for 90% CO2 capture, presenting net efficiency of 23.28%LHV and specific emission of 0.161 kg/kWh, if the lean solvent composition is optimized for the lowest regeneration heat, which results in 2.92 GJ/tCO2 for the adopted premises.Without CO2 endpurification, Oxy-combustion has no emissions and outperforms Post-combustion with higher efficiency of 24.47%LHV if only 139 kWh/tO2 is required for GOX production.To make Post-combustion the most efficient alternative, reducing its heat ratio of regeneration to 2.30 GJ/t of captured CO2 is needed, requiring a more efficient solvent formulation and advanced process configurations.Regardless of the chosen alternative, the concept configures a BECCS solution and can remove carbon dioxide from the atmosphere, given the occurrence of CO2 biofixation in the crops.

Fig. 1 .
Fig. 1.Overview of considered power generation alternatives with carbon capture and storage: (a) post-combustion with chemical absorption; (b) oxy-combustion coupled to air separation unit.

Table 2 .
Molar composition of process streams.

Table 3 .
Summary of main simulation results.