Coal to SNG: Technical progress, modeling and system optimization through exergy analysis

SNG production with CO2 capture based on coal-steam gasification and one-step methanation

Synthetic natural gas production by sorption enhanced steam hydrogasification based processes for improving CH4 yield and mitigating CO2 emissions

The production of synthetic natural gas in the context of power-to-gas is a promising technology for the utilization of CO2. Ni-based catalysts supported on carbon nanotubes (CNTs) were prepared through incipient wetness impregnation and characterized using N2 adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and temperature-programmed reduction (TPR). The catalysts were tested for CO2 methanation in the 200–400 °C temperature range and at atmospheric pressure. The results demonstrated that the catalytic activity increased with the addition of the CNTs and Ni loading. The selectivity towards CH4 was close to 100% for the Ni/ZrO2/CNT catalysts. Reduction of the calcined catalyst at 500 °C using H2 modified the surface chemistry of the catalyst, leading to an increase in the Ni particles. The CO2 conversion was dependent on the Ni loading and the temperature reduction in the NiO species. The 10Ni/ZrO2/CNT catalyst was highly stable in CO2 methanation at 350 °C for 24 h. Thus, CNTs combined with Ni and ZrO2 were considered promising for use as catalysts in CO2 methanation at low temperatures.

The natural gas distribution infrastructure is well developed in many countries, enabling the fuel to be transported long distances via pipelines and easily delivered throughout cities. Using the existing pipeline to transport renewably generated synthetic natural gas (SNG) can leverage the value of the product. While the price of natural gas is near record lows in the United States, many other countries are working to develop SNG as an alternative fuel for transportation markets, especially in Europe and for island nations. This study presents an SNG plant design and evaluates its performance for producing SNG by reacting renewably generated hydrogen with carbon dioxide. The carbon dioxide feedstock is assumed to be captured and scrubbed from an existing coal fired power plant at the city-gate, where the SNG plant is co-located. Historically, methanation has been a common practice for eliminating carbon monoxide and carbon dioxide in various chemical processes such as ammonia production and natural gas purification; for these processes, only small amounts (1–3% molar basis) of carbon oxides need to be converted to methane. A “bulk” methanation process is unique due to the high concentration of carbon oxides and hydrogen. In addition, the carbon dioxide is the only carbon source, and the reaction characteristics of carbon dioxide are much different than carbon monoxide. Thermodynamic and kinetic considerations of the methanation reaction are explored to model and simulate a system of reactors for the conversion of hydrogen and carbon dioxide to SNG. Multiple reactor stages are used to increase temperature control of the reactor and drain water to promote the forward direction of the methanation reaction. Heat recuperation and recovery using organic Rankine cycle units for electricity generation utilizes the heat produced from the methanation reaction. Bulk recycle is used to increase the overall reactant conversion while allowing a satisfactorily high methane content SNG product. A hydrogen membrane separates hydrogen for recycle to increase the Wobbe index of the product SNG by increasing the methane content to nearly 93% by volume. The product SNG has a Wobbe index of 47.5 MJ/m3 which is acceptable for natural gas pipeline transport and end-use appliances in the existing infrastructure. The overall plant efficiency is shown to be 78.1% HHV and 83.2% LHV. The 2nd Law efficiency for the SNG production plant is 84.1%.

Synthetic Natural Gas (SNG) is the most researched option for a Power-to-Fuel pathway in Germany after hydrogen, having the advantage of being compatible with the existing infrastructure. However, it is not clear under which conditions SNG is economically and environmentally advantageous compared to natural gas usage, since this is determined by a complex interplay of many factors. This study analyzes the technical, economic and environmental aspects of a pilot SNG plant to determine the key parameters for profitable and sustainable operation. The SNG plant was simulated in Aspen Plus® with CO2 from biogas production as a feedstock and with hydrogen provided by a 1 MWel electrolyzer unit. A life cycle analysis (LCA) was undertaken considering several impact categories with a special focus on global warming potential (GWP). An SNG cost of 0.33–4.22 €/kWhth was calculated, depending on factors such as operational hours, electricity price and type of electrolyzer. It was found that the CO2 price has a negligible effect on the SNG cost, while the electricity is the main cost driver. This shows that significant cost reductions will be needed for SNG to be competitive with natural gas. For the investigated scenarios, a CO2 tax of at least 1442 €/t was determined, calling for more drastic measures. Considering the global warming potential, only an operation with an emission factor of electricity below 121 g CO2-eq/kWhel leads to a reduction in emissions. This demonstrates that unless renewable energies are implemented at a much higher rate than predicted, no sustainable SNG production before 2050 will be possible in Germany.

Radiocarbon-based determination of biogenic and fossil carbon partitioning in the production of synthetic natural gas

After a brief review of the current research on the production of synthetic natural gas (SNG) from lignocellulosic biomass by gasification and methanation, this paper presents detailed thermo-economic process optimisation of the polygeneration of SNG, power and heat. Based on a previously developed model, all suitable candidate configurations of a superstructure of promising technologies for the individual conversion steps are optimised with respect to the overall efficiency and investement cost with an evolutionary, multi-objective algorithm. In an extensive analysis, the influence of process technology, operating conditions and process integration on the thermo-economic performance is discussed and the best technology matches are determined. Systematically optimised flowsheets might thereby convert 66 to 75% of the dry wood’s lower heating value to SNG while cogenerating a considerable amount of power and/or industrial heat. In order to provide a general database of optimal plant configurations, cost exponents that quantify the economies of scale are regressed, and the most profitable flowsheets are identified for different energy price scenarios and scale. A comparison with current literature on SNG production from biomass reveals the potential of applying such systematic process systems engineering approaches for the design of energy- and cost-efficient biofuel plants.

Life cycle assessment of SNG from wood for heating, electricity, and transportation

A transition towards synthetic fuels such as synthetic natural gas (SNG) is a requirement that can contribute to future energy security and sustainable development. Conventional coal gasification could be replaced with biomass gasification to produce synthetic natural gas that can be used in existing natural gas distribution infrastructure, transport, domestic and industrial sectors with almost no or little modifications. This study evaluates the integration of synthetic natural gas production using biomass gasification with combined heat and power plant (CHP). Various system performance indicators are used for comparing different systems such as carbon conversion, energy efficiencies, economic feasibility and commercial viability of technologies. For the evaluation of studied system, 100 MW of dried biomass is used as input for synthetic natural gas production together with power and district heating. The results obtained by integration of process showed a substantial synthetic natural gas production potential i.e. about 74.3 MW based on 100 MW biomass input. The process heat requirement of the system is about 26.60 MW and 0.60 MW for electrification. The results indicate that 27.35 MW heat is available for district heating and about 3.4 MW net amount of heat is available for power production by organic Rankine cycle (ORC). The results also show that the system is 74.3 % efficient based on lower heating value. Poly generation concept of SNG production along with power and district heating system claims its economic viability.

In this work, we have considered a non-canonical scalar field dark energy model in the framework of flat FRW background. It has also been assumed that the dark matter sector interacts with the non-canonical dark energy sector through some interaction term. Using the solutions for this interacting non-canonical scalar field dark energy model, we have investigated the validity of generalized second law (GSL) of thermodynamics in various scenarios using first law and area law of thermodynamics. For this purpose, we have assumed two types of horizons viz apparent horizon and event horizon for the universe and using first law of thermodynamics, we have examined the validity of GSL on both apparent and event horizons. Next, we have considered two types of entropy-corrections on apparent and event horizons. Using the modified area law, we have examined the validity of GSL of thermodynamics on apparent and event horizons under some restrictions of model parameters.

Biomass is a renewable, yet scarce resource since land is limited. This thesis addresses the question how to efficiently convert the available lignocellulosic biomass and biomass wastes to fuel and other useful energy services. In particular, it presents a systematic methodology for the conceptual design of thermochemical processes and demonstrates it at the production of Synthetic Natural Gas (SNG) through conventional biomass gasification and methanation, or hydrothermal gasification of biomass wastes. Through an appropriate mathematical decomposition of the design problem, thermo-economic process modelling, advanced process integration techniques and multi-objective optimisation are combined to provide a set of parameter- and scale-independent flowsheets for the optimal trade-off between several design targets. The results of various design studies consistently demonstrate that process integration plays a critical role in the synthesis of energy- and cost-efficient processes. It allows both for a rational energy recovery by cogeneration and process intensification. Considerable potential is furthermore assessed for combining several complementary processes for an appropriate and complete conversion of the resource, which might decidedly improve the environmental performance of fuel production from biomass and stresses the importance of a systematic process design.

Production of synthetic natural gas (SNG) offers an alternative way to valorize captured CO2 from energy intensive industrial processes or from a dedicated CO2 grid. This paper presents an energy-efficient way for synthetic natural gas production using captured CO2 and renewable hydrogen. Considering several renewable hydrogen production sources, a techno-economic analysis was performed to find a promising path toward its practical application. In the paper, the five possible renewable hydrogen sources (photo fermentation, dark fermentation, biomass gasification, bio photolysis, and PV electrolysis) were compared to the two reference cases (steam methane reforming and water electrolysis) from an economic stand point using key performance indicators. Possible hydrogen production capacities were also considered for the evaluation. From a technical point of view, the SNG process is an efficient process from both energy efficiency (about 57%) and CO2 conversion rate (99%). From the evaluated options, the photo-fermentation proved to be the most attractive with a levelized cost of synthetic natural gas of 18.62 €/GJ. Considering the production capacities, this option loses its advantageousness and biomass gasification becomes more attractive with a little higher levelized cost at 20.96 €/GJ. Both results present the option when no CO2 credit is considered. As presented, the CO2 credits significantly improve the key performance indicators, however, the SNG levelized cost is still higher than natural gas prices.

Analysis of the integrated system of electrolysis and methanation units for the production of synthetic natural gas (SNG)

The purpose of this study is to investigate the performance features of coal-to-fuel systems based on different gasification technologies. The target products are the Fischer–Tropsch synthetic crude and synthetic natural gas. Two types of entrained-flow gasifierbased coal-to-fuel systems are simulated and their performance features are discussed. One is a single-stage water quench cooling entrained-flow gasifier, and another one is a two-stage syngas cooling entrained-flow gasifier. The conservation of energy (first law of thermodynamics) and the quality of energy (second law of thermodynamics) for the systems are both investigated. The results of exergy analysis provide insights about the potential targets for technology improvement. The features of different gasifier-based coal-to-fuel systems are discussed. The results provide information about the research and development priorities in future.

Based solely on the First Law of Thermodynamics, which addresses energy efficiency, heat pumps are considered to be a major asset for achieving the decarbonization goals of the Paris Agreement. Yet, this approach alone overestimates the potential for decarbonization and conceals certain root causes of carbon dioxide emissions arising from exergy destruction. This paper redefines the decarbonization potential of heat pumps holistically by applying the First and Second Laws of thermodynamics together. This research shows that power-to-heat energy conversion systems are responsible for exergy destruction, leading to additional carbon dioxide emissions, as the destroyed exergy must be offset by other means, possibly by burning fossil fuels. An exergy-based model was developed and used for different scenarios. Results were compared with boilers in heating mode fueled by fossil fuels or biogas, and with adsorption machines in cooling mode. Thermal energy storage systems and their optimal operational regimes for minimum environmental footprint have also been considered and modeled in accordance with the Second Law of Thermodynamics. A correction factor for the coefficient of performance of heat pumps has been developed to account for exergy destruction and emission avoidance associated with typical heat pump operation under electrical and ambient thermal inputs. Results show that today’s coefficient of performance must be higher to meet the emissions-mitigation and total electrification targets to mitigate global warming. The Second Law of thermodynamics shows that the case study about the comfort cooling system of a nature center building with ground source heat pumps and photovoltaic panels is responsible for emissions by 0.81 kg CO2 emissions/kWen-h, rather than saving 1.62 kg CO2/kWen-h by a no-heat pump case, comprising solar photovoltaic panels, adsorption cooling, and desiccant wheel. This case study also shows that the carbon dioxide emission responsibility resulting from exergy destruction during the power-to-heat process is as important as the direct emissions from electric power use. This paper concludes that the exergy destruction-related carbon footprint should not be neglected across all design and application phases to foster better awareness and establish exergy-rational strategy planning towards the Paris Agreement goals.