Techno-economic comparison of green ammonia production processes

To reduce the fossil-fuel consumption and the impacts of conventional ammonia production on climate change, green ammonia production processes using green hydrogen need to be investigated. For commercial production scale, potential alternatives can be based on biomass gasification and water electrolysis via renewable energy, namely biomass- and power-to-ammonia. The former generally uses entrained flow gasifier due to low CO2 and almost no tar, and air separation units are shared by the gasifier and ammonia synthesis. The latter may use solid-oxide electrolyzer due to high electrical efficiency and the possibility of heat integration with the ammonia synthesis process. In this paper, techno-economic feasibility of these two green ammonia production processes are investigated and compared with the state-of-the-art methane-to-ammonia process, considering system-level heat integration and optimal placement of steam cycles for heat recovery. With a reference ammonia production of 50 kton/year, the results show that there are trade-offs between the overall energy efficiency (LHV) and ammonia production cost for all three cases. The biomass-to-ammonia is the most exothermic but is largely limited by the large heat requirement of acid gas removal. The steam cycles with three pressure levels are able to maximize the heat utilization at the system level. The power-to-ammonia achieves the highest system efficiency of over 74%, much higher than that of biomass-to-ammonia (44%) and methane-to-ammonia (61%). The biomass-to-ammonia reaches above 450 $/ton ammonia production cost with a payback time of over 6 years, higher than those of methane-to-ammonia (400$ /ton, 5 years). The power-to-ammonia is currently not economically feasible due to high stack costs and electricity prices; however, it can be competitive with a payback time of below 5 years with mass production of solid-oxide industry and increased renewable power penetration.

Techno-economic optimization of biomass-to-methanol with solid-oxide electrolyzer

François Maréchal, Maria del Mar Perez Fortes, Jan Van Herle, Ligang Wang, Hanfei Zhang

The purpose of this paper is to assess techno-economically the integration of solid-oxide electrolysis in biomass-to-methanol processes: (1) The hydrogen produced by electrolysis can be used to adjust the composition of syngas from gasification to increase the conversion of carbon in biomass, (2) the oxygen as a byproduct of electrolysis can be used in the gasifier to avoid expensive air separation units, and (3) the overall process can be thermally integrated. Two integration concepts are proposed with different sizing methods of the electrolyzer: (1) the case of full conversion of carbon in biomass, in which a large electrolyzer is driven by the electricity purchased from the grid, and (2) the case of zero power exchange, in which only part of the carbon in biomass is converted reaching self-sufficiency of electricity. The three cases including the state-of-the-art biomass-to-methanol process are investigated to identify (1) possible trade-offs between efficiency and costs, and (2) under which conditions, these concepts become economically viable. With a reference methanol production of 100 kton/year, the results show that there is an optimal design for the state-of-the-art case, which offers an efficiency of 53.3% due to steam cycles and a payback time of 4.8 years. For the integrated concepts, there are sharp trade-offs between the system efficiency and methanol production cost rate. The case of full carbon conversion can reach an energy efficiency of 64.5-66.0% but results in a longer payback time of over 11 years. The case of zero-power exchange can achieve a similar efficiency as the state-of-the-art case with a slightly increased payback time of over 5.5 years. The payback time of the full carbon conversion case can be shorter than 5 years with a reduction in stack cost and electricity price, and an increase in stack lifetime.

ELSEVIER SCI LTD2020

Techno-economic evaluation of biomass-to-fuels with solid-oxide electrolyzer

François Maréchal, Jan Van Herle, Ligang Wang, Hanfei Zhang

Thermochemical biomass-to-fuel conversion requires an increased hydrogen concentration in the syngas derived from gasification, which is currently achieved by water–gas-shift reaction and CO2 removal. State-of-the-art biomass-to-fuels convert less than half of the biomass carbon with the remaining emitted as CO2. Full conversion of biomass carbon can be achieved by integrating solid-oxide electrolyzer with different concepts: (1) steam electrolysis with the hydrogen produced injected into syngas, and (2) co-electrolysis of CO2 and H2O to convert the CO2 captured from the syngas. This paper investigates techno-economically steam- or co-electrolysis-based biomass-to-fuel processes for producing synthetic natural gas, methanol, dimethyl ether and jet fuel, considering system-level heat integration and optimal placement of steam cycles for heat recovery. The results show that state-of-the-art biomass-to-fuels achieve similar energy efficiencies of 48–51% (based on a lower heating value) for the four different fuels. The integrated concept with steam electrolysis achieves the highest energy efficiency: 68% for synthetic natural gas, 64% for methanol, 63% for dimethyl ether, and 56% for jet fuel. The integrated concept with co electrolysis can enhance the state-of-the-art energy efficiency to 66% for synthetic natural gas, 61% for methanol, and 54% for jet fuel. The biomass-to-dimethyl ether with co-electrolysis only reaches an efficiency of 49%, due to additional heat demand. The levelized cost of the product of the integrated concepts highly depends on the price and availability of renewable electricity. The concept with co-electrolysis allows for additional operation flexibility without renewable electricity, resulting in high annual production. Thus, with limited annual available hours of renewable electricity, biomass-to-fuel with co-electrolysis is more economically convenient than that with steam electrolysis. For a plant scale of 60 MWth biomass input with the renewable electricity available for 1800 h annually, the levelized cost of product of biomass-to-synthesis-natural-gas with co-electrolysis is 35 $/GJ, 20% lower than that with steam-electrolysis.

2020

Thermo-environomic evaluation of the ammonia production

François Maréchal, Matthieu Perrenoud, Laurence Tock

Within the global challenge of sustainable energy supply and greenhouse gas emissions mitigation, carbon capture and storage (CCS) and the deployment of renewable resources are considered as promising solutions. In this context, the production of ammonia mainly used in the fertilizer industry that is responsible for a large part of the global CO2 emissions is analyzed in detail in this study. Considering natural gas and biomass as a resource and the option for CO2 capture and storage, different process configurations are systematically compared with regard to energetic, economic and environmental considerations. A consistent thermo-environonomic optimization approach combining flowsheeting, process integration techniques, economic performance evaluation, life cycle assessment and multi-objective optimization is applied for the conceptual process design and the assessment of the competitiveness and the trade-offs. It is highlighted that the quality of the process integration is a key factor for improving the process performance by valorizing the heat excess through electricity cogeneration. Including CO2 mitigation in the ammonia production allows to reduce the emissions, but leads to a slight efficiency decrease due to the additional energy consumption for the compression. For the natural gas fed process yielding an energy efficiency around 65%, the overall life cycle emissions can however be reduced to 0.79kgCO2/kgNH3 with CO2 capture compared to 1.6kgCO2/kgNH3 without capture and to -1.79kgCO2/kgNH3 for the biomass process having an energy efficiency of 50%. The economic competitiveness highly depends on the resource price and the introduction of a carbon tax. This study reveals the potential of the decarbonization of the fertilizer industry.