Encyclopedia
Please note this is an old version of this entry, which may differ significantly from the current revision.
Subjects:
Energy & Fuels
Modern trends in improving environmental safety have determined the urgency in creating innovative technologies that allow the production of electricity and hydrogen without the emission of harmful substances. However, at the moment, there are not so many technical solutions offering the combined production of these useful products with a high degree of efficiency and environmental friendliness. The transition to oxy-fuel combustion power cycles for the co-production of electricity and hydrogen is a prospective way to decrease carbon dioxide emissions into the atmosphere from the energy sector.
- carbon dioxide
- oxygen
- efficiency
- net zero emission
- power generation
1. The Relevance of the Decarbonization of Electricity and Hydrogen Production
The continuous growth of the population and the active industrialization of countries are the reasons for the significantly increasing demand for energy products, both electricity and hydrogen. This is evidenced by the data of the Centre for Energy Economics Research and Policy [1], according to which from 2020 to 2050, the demand for electricity will increase by more than 160% and the demand for hydrogen by 246%. Taking into account the identified trend towards the growth in energy production, one should expect an increase in emissions of toxic substances and greenhouse gases into the atmosphere.
The concern of the global community with regards to global climate change has led to the adoption of a number of international treaties mandating countries to stabilize or reduce greenhouse gas emissions by reforming their energy sectors. In particular, in 1997, the Kyoto Protocol was signed, and in 2015, the Paris Agreement. However, to achieve the desired effect of reducing CO2 emissions, relevant agreements need to be adopted by the largest industrial nations [2].
Decarbonization of the energy industry is one of the key challenges on the way to achieving carbon neutrality [3]. In addition to electric energy, steam, and hot water, hydrogen is another energy product that is in high demand. It is employed across numerous industries. In particular, hydrogen fuel is used in oil refining, metallurgy, and the construction and food industry. In addition, hydrogen is used as a fuel in engines and in standalone power and heat generators; it is a convenient medium to supply heat to distributed consumers, and to transport and store energy [4,5]. An important advantage of hydrogen is its environmental safety; the sole product of its combustion is water vapor. However, since pure hydrogen is almost never found in nature, it needs to be produced first, and the method of its production will largely determine the environmental effect of its use [6,7].
Specifically, approximately 48% of hydrogen is produced by conversion [8]. The conversion of gases involves processing them to change the composition of the initial gas mixture. The conversion is normally applied to hydrocarbon gases (methane and its homologues) and carbon monoxide to produce hydrogen or a mixture of hydrogen and carbon monoxide. The conversion process is carried out using various oxidizing reagents (such as oxygen, steam, CO2, and mixtures thereof).
The most cost-efficient feedstock for conversion is methane (natural gas). Currently, the majority of the industrially produced hydrogen is generated using the steam methane reforming (SMR) process. However, the carbon dioxide formed during the SMR process is not captured and is released into the atmosphere. Such hydrogen is referred to as “gray” hydrogen. To ensure that this process is environmentally neutral, one needs to additionally apply the carbon capture and storage technologies; however, their addition results in the process being considerably more expensive. On average, the cost of SMR production of “blue” hydrogen is 23% higher than that of “grey” hydrogen [9]. The above considerations result in the development of an environmentally safe and economically viable SMR-based hydrogen production process being a high-priority objective.
2. Reducing Carbon Dioxide Emissions from the Steam Methane Reforming Plants
In the SMR process, steam reacts with natural gas at high temperatures and moderate pressure (1.5–2 kgf/cm2) in the presence of a nickel-containing catalyst (up to 20% Ni as NiO). Steam and thermal energy are needed to separate the hydrogen from the methane carbon base.
SMR is currently the cheapest (and most mastered) process for the industrial production of hydrogen. However, it is accompanied by CO and CO2 emissions. There are various solutions available to handle those emissions.
The International Energy Agency (IEA) has provided comparative analysis data for different versions of the combined hydrogen, heat, and electricity generation process in its publication [10], where emerging SMR process configurations are examined alongside their feasibility analysis. The publication has identified several methods of CO2 capture: (a) a method based on chemical absorption technology; (b) a method based on introduction of burners running on H2-saturated fuel; and (c) a method based on low-temperature separation of CO2 and the application of membrane technology.
It should be noted that in SMR plants, three areas are identified where CO2 can be captured: (1) from the syngas stream downstream the water–gas shift reactor; (2) from the tail gas stream of the PSA (pressure swing adsorption) unit; and (3) from SMR flue gases [11].
Obviously, the most efficient option for a SMR unit is the one without any CO2 capture systems, and in the complete absence carbon emissions control, there is no reason to implement such systems.
The introduction of a system for capturing CO2 from the syngas stream using a chemical absorption process results in a 54% reduction in CO2 emissions compared to the version without the capture. Adding the H2-saturated fuel fed to the burners of the reformer furnace further increases the share of prevented CO2 emissions to 64% [12].
There are versions of the process involving purification of the tail gases from the PSA unit. The introduction of capture systems based on chemical absorption technology leads to a reduction in CO2 emissions by 52%, and the introduction of systems based on low-temperature separation of CO2 and the application of membrane technology by 53%. The latter option has been noted to be most effective in the partial implementation of carbon border adjustment [13].
With the full implementation of the carbon border adjustment mechanism, the most efficient option is an SMR unit with CO2 capture from SMR flue gases. The introduction of such a capture system can reduce emissions by as much as 90% [10].
Therefore, most of the known SMR-based hydrogen production processes with minimum greenhouse gas emissions result either in an increase in the present value of hydrogen production by 44% for the most efficient capture option, or in a low percentage of CO2 capture for other options. This means that the development of a highly efficient SMR technology with minimal emissions of harmful substances is a high-priority objective.
3. Oxy-Fuel Combustion Technology for the Combined Production of Electricity and Hydrogen
Today, the main global trend in the development of energy is to reduce the number of toxic substances and greenhouse gases emissions into the atmosphere. Much research has been devoted to investigating the issue of reducing emissions of harmful substances from the power generation units [14,15,16,17,18,19]. Many emission reduction methods are successfully used at the existing thermal power plants. In particular, methods of controls of atmospheric pollutants, such as nitrogen and sulfur oxides, are widely used. At the same time, the prevention of carbon dioxide emissions, which are formed in large quantities during the combustion of fossil fuels, still causes difficulties. The introduction of carbon dioxide capture technologies leads to a significant increase in the electricity cost; therefore, the issue of creating environmentally friendly and economically viable high-capacity energy systems remains open [20,21].
Oxy-fuel energy complexes with a CO2 working flow have great potential as a way of reducing emissions while maintaining a high level of thermal efficiency. Oxy-fuel technologies for power generation are based on the combustion of hydrocarbon fuels in pure oxygen and the capture of carbon dioxide and its disposal. Currently, more than thirty cycles with oxygen fuel combustion are known [22,23,24]. In particular, the following closed thermodynamic cycles are widely known: SCOC-CC, MATIANT cycles, NET Power cycles, Graz cycles, CES cycles, AZEP cycle, and ZEITMOP [25].
The initial versions of such cycles were introduced at the end of the past century. Today, the US, Japan, and European countries are actively developing this avenue. Due to the allocation of grants, active subsidization of “green” technologies for power generation, and the establishment of legislative frameworks facilitating the reduction of CO2 emissions, scientific research is being conducted, experimental plants are being built, and prerequisites are being created for building actual power units with “zero” emission of harmful substances. Large energy corporations are joining their efforts to build demo plants capable of releasing up to 50 MW of electricity to the power grid [26].
Recent developments in the field of creating environmentally friendly energy complexes are aimed at creating power units that simultaneously generate both electric power and hydrogen. Such solutions can simultaneously produce two important energy products at high efficiency rate and without harmful emissions.
In particular, in the work of Zhang N. [27], two novel system configurations were proposed for oxy-fuel natural gas turbine systems with integrated steam reforming and carbon dioxide capture and separation. The steam reforming heat is obtained from the available turbine exhaust heat, and the produced syngas is used as fuel with oxygen as the oxidizer. The authors were focused on the integration of the turbine exhaust heat recovery with both reforming and steam generation processes, in ways that reduce the heat transfer-related exergy destruction. According to the modeling results, a net efficiency of power generation unit is in the range of 50–52%. The key disadvantage of the proposed configuration is the losses occurring in the condenser and the performer.
The article [28] presents a combination of CCGT with steam conversion of methane; however, the reformer is used in this cycle as a method of producing fuel for the power unit. To purify the outgoing gases from the reformer from carbon dioxide, an installation with monoethanolamine is used, which allows the generation of electricity without emissions, with an efficiency of 41.77%. The disadvantage of this technology is the lack of the possibility of producing hydrogen for sale to an external consumer, large heat losses in the condenser of the steam turbine unit and outgoing gases, and large expenses for cleaning carbon dioxide at the outlet of the methane steam conversion plant.
In the work [29], an oxy-fuel combustion power plant for electricity and hydrogen production is presented with near-zero emissions. It represents the combination of two technologies: the Allam cycle and the steam methane reforming plant. According to the modeling results, the power plant efficiency is 54.9% at equal production of the supplied electricity and chemical energy of the produced hydrogen. The absence of a water steam source in the Allam cycle thermal loop is one of the key issues that one faces when trying to integrate a steam methane reformer into the process. It necessitates the supply of low-grade heat to the multi-flow regenerator, which further complicates the issue of controlling the output of electricity and hydrogen in the course of the combined production of energy products.
The above issue can be solved by creating an oxy-fuel energy complex for zero-emission electricity and H2 production based on the SCOC-CC cycle. The fact that this configuration includes a steam turbine circuit results in it being possible to draw steam from its individual branches, which should considerably simplify the load control issue. Moreover, the solution allows an increase in the combined unit efficiency. However, in open sources, no references are available to the results of studies on the energy performance and modes of operation of such power units.
This entry is adapted from the peer-reviewed paper 10.3390/en16165983
This entry is offline, you can click here to edit this entry!
