• Energy Research

Proper selection of the operating conditions of molten carbonate fuel cells (MCFCs) can not only effectively improve the efficiency of power generation, but also reduce carbon emissions and help achieve the great goal of “carbon neutrality”.

Abstract This paper presents a model-based investigation to handle the fundamental issues for the design of co-electrolysis based power-to-methane at the levels of both the stack and system: the role of CO2 in co-electrolysis, the benefits of employing pressurized stack operation and the conditions of promoting internal methanation. Results show that the electrochemical reaction of co-electrolysis is dominated by H2O splitting while CO2 is converted via reverse water-gas shift reaction. Increasing CO2 feed fraction mainly enlarges the concentration and cathode-activation overpotentials. Internal methanation in the stack can be effectively promoted by pressurized operation under high reactant utilization with low current density and large stack cooling. For the operation of a single stack, methane fraction of dry gas at the cathode outlet can reach as high as 30 vol.% (at 30 bar and high flowrate of sweep gas), which is, unfortunately, not preferred for enhancing system efficiency due to the penalty from the pressurization of sweep gas. The number drops down to 15 vol.% (at 15 bar) to achieve the highest system efficiency (at 0.27 A/cm2). The internal methanation can serve as an effective internal heat source to maintain stack temperature (thus enhancing electrochemistry), particularly at a small current density. This enables the co-electrolysis based power-to-methane to achieve higher efficiency than the steam-electrolysis based (90% vs 86% on higher heating value, or 83% vs 79% on lower heating value without heat and converter losses).

To efficiently and environment friendly combust the domestic garbage, sludge, and swill waste fuels, five different fuels are prepared by mixing the waste fuels together with coal, and grass biomass at different mixing ratios, and finally those fuels were combusted in a circulating fluidized bed (CFB) reactor. The emission performances of NOx, dioxin, and heavy metal during the combustion tests are studied. The results showed that a stable furnace temperature can be reached at approximately 850 °C when combusting all studied mixed fuels, benefiting the thermal processes of sludge and domestic garbage and thus realizing the purpose of waste-to-fuel. In addition, the dioxin emissions are much lower than the emission standards, and NOx emissions could be reduced significantly by adjusting the ratio of waste fuels. However, the emissions of mercury, lead, and the combinations of chromium, tin, antimony, cupper and manganese components all exceeded the pollution control standard for hazardous wastes incineration, a further technology is required for heavy metal reductions to achieve the emission standards.

Abstract 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.

Abstract 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.

Abstract Urea is widely used in agriculture, industry, and food, while it is also a potential fuel. Large-scale urea production relies on fossil fuels, thus there is a strong need for green urea given the increasing penetration of renewable energy sources. A potential alternative is biomass-to-urea; however, it cannot fully convert the biomass carbon into urea. To achieve full carbon conversion, innovative integrated biomass- and power-to-urea processes are designed conceptually. The two green urea production processes are evaluated techno-economically and compared with state-of-the-art methane-to-urea. The results show that the methane-to-urea achieves a system efficiency of 58% (LHV), while biomass-to-urea only has 39% (LHV) with unconverted biomass carbon of up to 60%. The integrated power- and biomass-to-urea has outstanding heat integration performance which fixes all biomass carbon into urea, with an efficiency enhanced up to 53%. Due to the electricity demand, the levelized cost of the urea of integrated biomass- and power-to-urea is 15 – 38 and 58 – 87% points higher than those of the biomass-to-urea and methane-to-urea for the scale of 10 – 60 MWth urea production. The available annual hours and electricity price of renewable electricity have a significant impact on the levelized cost of the urea. When the available annual hours decrease from 7200 to 3600 with an electricity price of 73 $/MWh, the levelized cost of urea increases on average by 13% from 51 $/GJ with the plant capacity being 10 – 60 MWth urea. However, when electricity price is reduced from 73 $/MWh to 35 $/MWh with available annual hours of 3600, the levelized cost decreases on average by 15% from 59 $/GJ with the same plant capacity.

Abstract Biomass-derived fuels are attractive due to the reduced greenhouse gas emissions and the potential contribution to the development of the agricultural industry. Particularly, 2nd generation biofuels, e.g., synthetic biodiesel as a high-performance and alternative mobility fuel, can be produced via biomass-gasification based processes. There are mainly three types of biomass gasification processes: (1) moving- or fixed-bed gasifier for coal gasification with oxidizing blast gas (air + hot syngas) (2) fluidized-bed gasifier that uses air (oxidant agent) to fluidize the bed and the added carbon-containing particle, and (3) entrained flow gasifier that uses pure oxygen to reach high operating temperature. The entrained flow gasifier seems to be a promising choice with high scale-up potential, due to the high-pressure operation and none N2 diluted syngas production, which can lead to the compact design of down-stream equipment. Particularly, the syngas produced contains no tar, and low methane and CO2. The disadvantage of this gasification technology is the need of high-purity O2 supply of, usually from an air separation unit (ASU). Therefore, solid-oxide electrolysis offers very good opportunity of integrating with entrained-flow gasifier, due to that (1) possible pure oxygen production to avoid the ASU, (2) high operating temperature for better heat integration with the original gasification process, and (3) hydrogen production via steam electrolysis for adjusting the syngas composition. In this paper, the integration of the SOE in the EFG-based biomass to methanol systems (SOEC case) is investigated and technically compared with the traditional biomass-to-liquid system (base case), whose syngas composition is adjusted by water-gas-shift reactors. The results show that, the mass yield of the methanol is set as around 69.4 t/hr, SOEC case can achieve higher energy efficiency, the energetic efficiencies of the base case and SOEC case were 47.95% and 59.1%, respectively.