(263c) Coal Gasification by Conventional Versus Calcium Looping Process – A Life Cycle Energy, Global Warming, Land Use and Water Assessment

In this work, we evaluate two coal gasification processes: conventional and calcium looping (CLP) implemented with CO₂ sequestration, according to their environmental and energetic performances. Environmental factors considered are energy use, water use, land use and global warming potential (GWP) of the processes. Net energy analysis is performed for energetic evaluation. We calculate the energy return of investment (EROI)  at three scales: a traditional equipment scale that only considers the gasification process, a value chain scale analogous to a process-LCA, and an economy scale tiered hybrid LCA approach based on combining process data with economic data. At the economic scale we used two different softwares, ECOLCA and EIOLCA for EROI calculations to compare the impact of models on results. At the equipment scale, we consider only the energy used directly in the process to produce the energetic outputs, namely, electricity and fuel grade hydrogen. At the value chain scale, we account for the energy spent in coal mining, transportation of coal to the plant, limestone mining and transportation, gasification, calcination and solid disposal for CLP. For the conventional process at this scale, the steps considered are coal mining and transportation, gasification and solid disposal. Relevant information about these steps was gathered from the National Renewable Energy Laboratory life cycle inventory database. For the economy scale, we used the Carnegie Mellon University EIO-LCA on-line software and Ohio State University ECO-LCA on-line software. These models rely on aggregated sectors, which we combine with process data and further include the energy for equipment production, plant infrastructure and chemical production for both processes.

            Global warming potential is also calculated in process, value –chain and economic scales only by using EIO-LCA model. Water use and land use assessments are performed using both EIO-LCA and ECO-LCA models and compared results are presented. Since the amounts of electricity, hydrogen and carbon dioxide are different in conventional and CL processes, we use an allocation strategy to allocate the amount of energy spent to produce each product based on their monetary values for fair comparison of processes. We calculate the product EROIs for electricity and hydrogen and MJ/kg carbon dioxide value for CO₂  

In calculation of EROI, two cases are considered depending on whether differences in energy quality are considered or not.  Without accounting for differences in energy quality, EROIs calculated for CLP are 3.36, 2.96 yafor the equipment, value chain respectively, and for economy scale 2.68 with EIO-LCA model and 2.79 with ECO-LCA model. For conventional process, they are 7.68, 6.16, 5.30 and 5.64, for the respective scales and models. When we account for energy quality of electricity, results are 2.85, 2.63, 2.47 with EIO-LCA model and 2.54 with ECO-LCA model for CLP and 4.35, 3.89, 3.57 with EIO-LCA and 3.70 with ECO-LCA model for the conventional process. These results show that for all the cases, regardless of the scale and whether energy quality is considered or not, EROI for CLP is lower than for the conventional process. This is due to the large energy consumption for regenerating the absorbent and the much higher electrical energy spent in CLP compared to the conventional process.

CLP looks better from global warming potential point of view. In process scale, CLP is an emission-free process whereas conventional process emits 1417.7 metric tonnes of CO₂ per year. For only value-chain scale CLP is more CO₂ emitting process (28733.8 for conventional process vs. 53199.5 metric tonnes of CO₂ for CLP). This situation is mainly due to higher input amounts of CLP. In economy scale, these quantities are found to be 1.85 and 4.08 MtCO₂/MWe for CLP and conventional process, respectively.    

Water use is calculated in process scale and economic scale. In process scale, 1638385.2 kgal/year and 2026326.3 kgal/year are the water consumption results for CLP and conventional process respectively. In economic scale, conventional process consumes less water compared to CLP (3123704 kgal/year vs 3437373 kgal/year) according to EIO-LCA results. In ECO-LCA calculations, conventional process consumes 3063407.9 k gal/year water and 3180661.4 CLP consumes kgal/year. In both cases in which ECO-LCA and EIO-LCA models are used CLP is the more water using process, and consumption differences are 10% for both cases. In the meantime, calculation results are consistent for both models.

Land use assessment is performed using ECO-LCA model. Despite CLP seems to use more land, the additional land needed to offset the CO₂ emitted by the conventional process causes much more land to be affected by the conventional gasification process (25839.43 acres for CLP and 189288.53 acres for conventional process).

Lastly we found the following results according to our allocation calculations. In conventional process, EROI of electricity is 21.51 and 25.60 for energy quality excluded and considered cases respectively. These values are 14.50 and 19.52 for CLP. EROI of hydrogen for conventional process is calculated 31.61 and 18.31 for energy quality excluded and included cases respectively. The same results for CLP are 18.39 and 12.02. Amount of energy spent for carbon dioxide production are found to be 1.29 and 2.23 MJ/kg CO₂ in conventional process for cases of energy quality correction exclusion and inclusion. These values are 1.91 and 2.92 MJ/kg CO₂ in CL process. From these results, we concluded that more energy is used for production of certain amount of product in CLP.

Further process optimization studies on energy aspect of CLP can make it a favorable process in adoption of CO₂ capture in coal gasification processes leading to an emission-free power generation technology which we expect will be the main goal of future studies.