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Simulation of a Hydrogen-Air Diffusion Flame under Consideration of Component-Specific Diffusivities
Mar 2022
Publication
This work deals with the numerical investigation of a three-dimensional laminar hydrogenair diffusion flame in which a cylindrical fuel jet is surrounded by in-flowing air. To calculate the distribution of gas molecules the model solves the species conservation equation for N-1 components using infinity fast chemistry and irreversible chemical reaction. The consideration of the component-specific diffusion has a strong influence on the position of the high-temperature zone as well as on the concentration distribution of the individual gas molecules. The calculations of the developed model predict the radial and axial species and temperature distribution in the combustion chamber comparable to those from previous publications. Deviations due to a changed burner geometry and air supply narrow the flame structure by up to 50% and the high-temperature zones merge toward the central axis. Due to the reduced inflow velocity of the hydrogen the high-temperature zones develop closer to the nozzle inlet of the combustion chamber. As the power increases the length of the cold hydrogen jet increases. Furthermore the results show that the axial profiles of temperature and mass fractions scale quantitatively with the power input by the fuel.
A Review on Numerical Simulation of Hydrogen Production from Ammonia Decomposition
Jan 2023
Publication
Ammonia (NH3 ) is regarded as a promising medium of hydrogen storage due to its large hydrogen storage density decent performance on safety and moderate storage conditions. On the user side NH3 is generally required to decompose into hydrogen for utilization in fuel cells and therefore it is vital for the NH3 -based hydrogen storage technology development to study NH3 decomposition processes and improve the decomposition efficiency. Numerical simulation has become a powerful tool for analyzing the NH3 decomposition processes since it can provide a revealing insight into the heat and mass transfer phenomena and substantial guidance on further improving the decomposition efficiency. This paper reviews the numerical simulations of NH3 decomposition in various application scenarios including NH3 decomposition in microreactors coupled combustion chemical reactors solid oxide fuel cells and membrane reactors. The models of NH3 decomposition reactions in various scenarios and the heat and mass transport in the reactor are elaborated. The effects of reactor structure and operating conditions on the performance of NH3 decomposition reactor are analyzed. It can be found that NH3 decomposition in microchannel reactors is not limited by heat and mass transfer and NH3 conversion can be improved by using membrane reactors under the same conditions. Finally research prospects and opportunities are proposed in terms of model development and reactor performance improvement for NH3 decomposition.
Green Hydrogen Production by Anion Exchange Membrane Water Electrolysis: Status and Future Perspectives
Jan 2023
Publication
Green hydrogen production i.e. produced on a CO2 -neutral basis through the electrolysis of water employing renewable electricity has attracted increasing attention. The electricity required is generated from Renewable Energy Sources (RES) for example wind energy hydropower or solar energy. Since neither the process of production nor the end products of H2 and O2 are harmful to the environment green hydrogen is climate neutral. Developing electrolysis technology is therefore a research topic to follow. Anion Exchange Membrane (AEM) Water Electrolysis (WE) is an innovative technology that couples the advantages of the more mature technologies of Proton Exchange Membrane (PEM) and conventional alkaline electrolysis with the potential to eliminate the drawbacks of both. AEMWE technology is in an evolutionary stage and involves more investigation on several research topics such as membrane and catalyst development and stability as well as alternative feeding solutions that do not compromise the availability of fresh water. These topics are addressed in this paper mentioning the state-of-the-art materials new promising ones and providing future research directions to improve AEMWE towards a most mature technology.
Optimization of Operating Hydrogen Storage System for Coal–Wind–Solar Power Generation
Jul 2022
Publication
To address the severity of the wind and light abandonment problem and the economics of hydrogen energy production and operation this paper explores the problem of multi-cycle resource allocation optimization of hydrogen storage systems for coal–wind–solar power generation. In view of the seriousness of the problem of abandoning wind and photovoltaic power and the economy of hydrogen production and operation the node selection and scale setting issues for hydrogen production and storage as well as decision-making problems such as the capacity of new transmission lines and new pipelines and route planning are studied. This research takes the satisfaction of energy supply as the basic constraint and constructs a multi-cycle resource allocation optimization model for an integrated energy system aiming to achieve the maximum benefit of the whole system. Using data from Inner Mongolia where wind abandonment and power limitation are severe and Beijing and Shanxi provinces where hydrogen demand is high this paper analyzes the benefits of the hydrogen storage system for coal–wind–solar power generation and explores the impact of national subsidy policies and technological advances on system economics.
Hydrogen-Enriched Compressed Natural Gas Network Simulation for Consuming Green Hydrogen Considering the Hydrogen Diffusion Process
Sep 2022
Publication
Transporting green hydrogen by existing natural gas networks has become a practical means to accommodate curtailed wind and solar power. Restricted by pipe materials and pressure levels there is an upper limit on the hydrogen blending ratio of hydrogen-enriched compressed natural gas (HCNG) that can be transported by natural gas pipelines which affects whether the natural gas network can supply energy safely and reliably. To this end this paper investigates the effects of the intermittent and fluctuating green hydrogen produced by different types of renewable energy on the dynamic distribution of hydrogen concentration after it is blended into natural gas pipelines. Based on the isothermal steady-state simulation results of the natural gas network two convection–diffusion models for the dynamic simulation of hydrogen injections are proposed. Finally the dynamic changes of hydrogen concentration in the pipelines under scenarios of multiple green hydrogen types and multiple injection nodes are simulated on a seven-node natural gas network. The simulation results indicate that compared with the solar-power-dominated hydrogen productionblending scenario the hydrogen concentrations in the natural gas pipelines are more uniformly distributed in the wind-power-dominated scenario and the solar–wind power balance scenario. To be specific in the solar-power-dominated scenario the hydrogen concentration exceeds the limit for more time whilst the overall hydrogen production is low and the local hydrogen concentration in the natural gas network exceeds the limit for nearly 50% of the time in a day. By comparison in the wind-power-dominated scenario all pipelines can work under safe conditions. The hydrogen concentration overrun time in the solar–wind power balance scenario is also improved compared with the solar-power-dominated scenario and the limit-exceeding time of the hydrogen concentration in Pipe 5 and Pipe 6 is reduced to 91.24% and 91.99% of the solar-power-dominated scenario. This work can help verify the day-ahead scheduling strategy of the electricity-HCNG integrated energy system (IES) and provide a reference for the design of local hydrogen production-blending systems.
Electrified Hydrogen Production from Methane for PEM Fuel Cells Feeding: A Review
May 2022
Publication
The greatest challenge of our times is to identify low cost and environmentally friendly alternative energy sources to fossil fuels. From this point of view the decarbonization of industrial chemical processes is fundamental and the use of hydrogen as an energy vector usable by fuel cells is strategic. It is possible to tackle the decarbonization of industrial chemical processes with the electrification of systems. The purpose of this review is to provide an overview of the latest research on the electrification of endothermic industrial chemical processes aimed at the production of H2 from methane and its use for energy production through proton exchange membrane fuel cells (PEMFC). In particular two main electrification methods are examined microwave heating (MW) and resistive heating (Joule) aimed at transferring heat directly on the surface of the catalyst. For cases the catalyst formulation and reactor configuration were analyzed and compared. The key aspects of the use of H2 through PEM were also analyzed highlighting the most used catalysts and their performance. With the information contained in this review we want to give scientists and researchers the opportunity to compare both in terms of reactor and energy efficiency the different solutions proposed for the electrification of chemical processes available in the recent literature. In particular through this review it is possible to identify the solutions that allow a possible scale-up of the electrified chemical process imagining a distributed production of hydrogen and its consequent use with PEMs. As for PEMs in the review it is possible to find interesting alternative solutions to platinum with the PGM (Platinum Group Metal) free-based catalysts proposing the use of Fe or Co for PEM application.
Studies Concerning Electrical Repowering of a Training Airplane Using Hydrogen Fuel Cells
Mar 2024
Publication
The increase in greenhouse gas emissions as well as the risk of fossil fuel depletion has prompted a transition to electric transportation. The European Union aims to substantially reduce pollutant emissions by 2035 through the use of renewable energies. In aviation this transition is particularly challenging mainly due to the weight of onboard equipment. Traditional electric motors with radial magnetic flux have been replaced by axial magnetic flux motors with reduced weight and volume high efficiency power and torque. These motors were initially developed for electric vehicles with in-wheel motors but have been adapted for aviation without modifications. Worldwide there are already companies developing propulsion systems for various aircraft categories using such electric motors. One category of aircraft that could benefit from this electric motor development is traditionally constructed training aircraft with significant remaining flight resource. Electric repowering would allow their continued use for pilot training preparing them for future electrically powered aircraft. This article presents a study on the feasibility of repowering a classic training aircraft with an electric propulsion system. The possibilities of using either a battery or a hybrid source composed of a battery and a fuel cell as an energy source are explored. The goal is to utilize components already in production to eliminate the research phase for specific aircraft components.
A Hydrogen-Fueled Micro Gas Turbine Unit for Carbon-Free Heat and Power Generation
Oct 2022
Publication
The energy transition with transformation into predominantly renewable sources requires technology development to secure power production at all times despite the intermittent nature of the renewables. Micro gas turbines (MGTs) are small heat and power generation units with fast startup and load-following capability and are thereby suitable backup for the future’s decentralized power generation systems. Due to MGTs’ fuel flexibility a range of fuels from high-heat to lowheat content could be utilized with different greenhouse gas generation. Developing micro gas turbines that can operate with carbon-free fuels will guarantee carbon-free power production with zero CO2 emission and will contribute to the alleviation of the global warming problem. In this paper the redevelopment of a standard 100-kW micro gas turbine to run with methane/hydrogen blended fuel is presented. Enabling micro gas turbines to run with hydrogen blended fuels has been pursued by researchers for decades. The first micro gas turbine running with pure hydrogen was developed in Stavanger Norway and launched in May 2022. This was achieved through a collaboration between the University of Stavanger (UiS) and the German Aerospace Centre (DLR). This paper provides an overview of the project and reports the experimental results from the engine operating with methane/hydrogen blended fuel with various hydrogen content up to 100%. During the development process the MGT’s original combustor was replaced with an innovative design to deal with the challenges of burning hydrogen. The fuel train was replaced with a mixing unit new fuel valves and an additional controller that enables the required energy input to maintain the maximum power output independent of the fuel blend specification. This paper presents the test rig setup and the preliminary results of the test campaign which verifies the capability of the MGT unit to support intermittent renewable generation with minimum greenhouse gas production. Results from the MGT operating with blended methane/hydrogen fuel are provided in the paper. The hydrogen content varied from 50% to 100% (volume-based) and power outputs between 35 kW to 100kW were tested. The modifications of the engine mainly the new combustor fuel train valve settings and controller resulted in a stable operation of the MGT with NOx emissions below the allowed limits. Running the engine with pure hydrogen at full load has resulted in less than 25 ppm of NOx emissions with zero carbon-based greenhouse gas production.
An Overview of Promising Alternative Fuels for Road, Rail, Air, and Inland Waterway Transport in Germany
Feb 2022
Publication
To solve the challenge of decarbonizing the transport sector a broad variety of alternative fuels based on different concepts including Power-to-Gas and Power-to-Liquid and propulsion systems have been developed. The current research landscape is investigating either a selection of fuel options or a selection of criteria a comprehensive overview is missing so far. This study aims to close this gap by providing a holistic analysis of existing fuel and drivetrain options spanning production to utilization. For this purpose a case study for Germany is performed considering different vehicle classes in road rail inland waterway and air transport. The evaluated criteria on the production side include technical maturity costs as well as environmental impacts whereas on the utilization side possible blending with existing fossil fuels and the satisfaction of the required mission ranges are evaluated. Overall the fuels and propulsion systems Methanol-to-Gasoline Fischer–Tropsch diesel and kerosene hydrogen battery-electric propulsion HVO DME and natural gas are identified as promising future options. All of these promising fuels could reach near-zero greenhouse gas emissions bounded to some mandatory preconditions. However the current research landscape is characterized by high insecurity with regard to fuel costs depending on the predicted range and length of value chains.
Safe Design of a Hydrogen-Powered Ship: CFD Simulation on Hydrogen Leakage in the Fuel Cell Room
Mar 2023
Publication
Adopting proton exchange membrane fuel cells fuelled by hydrogen presents a promising solution for the shipping industry’s deep decarbonisation. However the potential safety risks associated with hydrogen leakage pose a significant challenge to the development of hydrogen-powered ships. This study examines the safe design principles and leakage risks of the hydrogen gas supply system of China’s first newbuilt hydrogen-powered ship. This study utilises the computational fluid dynamics tool FLACS to analyse the hydrogen dispersion behaviour and concentration distributions in the hydrogen fuel cell room based on the ship’s parameters. This study predicts the flammable gas cloud and time points when gas monitoring points first reach the hydrogen volume concentrations of 0.8% and 1.6% in various leakage scenarios including four different diameters (1 3 5 and 10 mm) and five different directions. This study’s findings indicate that smaller hydrogen pipeline diameters contribute to increased hydrogen safety. Specifically in the hydrogen fuel cell room a single-point leakage in a hydrogen pipeline with an inner diameter not exceeding 3 mm eliminates the possibility of flammable gas cloud explosions. Following a 10 mm leakage diameter the hydrogen concentration in nearly all room positions reaches 4.0% within 6 s of leakage. While the leakage diameter does not impact the location of the monitoring point that first activates the hydrogen leak alarm and triggers an emergency hydrogen supply shutdown the presence of obstructions near hydrogen detectors and the leakage direction can affect it. These insights provide guidance on the optimal locations for hydrogen detectors in the fuel cell room and the pipeline diameters on hydrogen gas supply systems which can facilitate the safe design of hydrogen-powered ships.
Global Potential of Green Ammonia Based on Hybrid PV-wind Power Plants
Apr 2021
Publication
Ammonia is one of the most commonly used feedstock chemicals globally. Therefore decarbonisation of ammonia production is of high relevance towards achieving a carbon neutral energy system. This study investigates the global potential of green ammonia production from semi-flexible ammonia plants utilising a cost-optimised configuration of hybrid PV-wind power plants as well as conversion and balancing technologies. The global weather data used is on an hourly time scale and 0.45◦ × 0.45◦ spatial resolution. The results show that by 2030 solar PV would be the dominating electricity generation technology in most parts of the world and the role of batteries would be limited while no significant role is found for hydrogen-fuelled gas turbines. Green ammonia could be generated at the best sites in the world for a cost range of 440–630 345–420 300–330 and 260–290 €/tNH3 in 2020 2030 2040 and 2050 respectively for a weighted average capital cost of 7%. Comparing this to the decade-average fossil-based ammonia cost of 300–350 €/t green ammonia could become cost-competitive in niche markets by 2030 and substitute fossil-based ammonia globally at current cost levels. A possible cost decline of natural gas and consequently fossil-based ammonia could be fully neutralised by greenhouse gas emissions cost of about 75 €/tCO2 by 2040. By 2040 green ammonia in China would be lower in cost than ammonia from new coal-based plants even at the lowest coal prices and no greenhouse gas emissions cost. The difference in green ammonia production at the least-cost sites in the world’s nine major regions is less than 50 €/tNH3 by 2040. Thus ammonia shipping cost could limit intercontinental trading and favour local or regional production beyond 2040.
Advances in Hydrogen, Carbon Dioxide, and Hydrocarbon Gas Sensor Technology Using GaN and ZnO-Based Devices
Jun 2009
Publication
In this paper we review our recent results in developing gas sensors for hydrogen using various device structures including ZnO nanowires and GaN High Electron Mobility Transistors (HEMTs). ZnO nanowires are particularly interesting because they have a large surface area to volume ratio which will improve sensitivity and because they operate at low current levels will have low power requirements in a sensor module. GaN-based devices offer the advantage of the HEMT structure high temperature operation and simple integration with existing fabrication technology and sensing systems. Improvements in sensitivity recoverability and reliability are presented. Also reported are demonstrations of detection of other gases including CO2 and C2H4 using functionalized GaN HEMTs. This is critical for the development of lab-on-a-chip type systems and can provide a significant advance towards a market-ready sensor application.
Combustion Characterization in a Diffusive Gas Turbine Burner for Hydrogen-Compliant Applications
Jun 2022
Publication
The target of net-zero emissions set by the 2015 Paris Agreement has strongly commissioned the energy production sector to promote decarbonization renewable sources exploitation and systems efficiency. In this framework the utilization of hydrogen as a long-term energy carrier has great potential. This paper is concerned with the combustion characterization in a non-premixed gas turbine burner originally designed for natural gas when it is fed with NG-H2 blends featuring hydrogen content from 0 to 50% in volume. The final aim is to retrofit a 40 MW gas turbine. Starting from the operational data of the engine a CFD model of the steady-state combustion process has been developed with reference to the base load NG conditions by reducing the fuel mass-flow rate by up to 17% to target the baseline turbine inlet temperature. When the fuel is blended with hydrogen for a given temperature at turbine inlet an increase in the peak temperature up to 800 K is obtained if no countermeasures are taken. Furthermore the flame results are more intense and closer to the injector in the case of hydrogen blending. The results of this work hint at the necessity of carefully analyzing the possible NOx compensation strategies as well as the increased thermal stresses on the injector.
On the Climate Impacts of Blue Hydrogen Production
Nov 2021
Publication
Natural gas based hydrogen production with carbon capture and storage is referred to as blue hydrogen. If substantial amounts of CO2 from natural gas reforming are captured and permanently stored such hydrogen could be a low-carbon energy carrier. However recent research raises questions about the effective climate impacts of blue hydrogen from a life cycle perspective. Our analysis sheds light on the relevant issues and provides a balanced perspective on the impacts on climate change associated with blue hydrogen. We show that such impacts may indeed vary over large ranges and depend on only a few key parameters: the methane emission rate of the natural gas supply chain the CO2 removal rate at the hydrogen production plant and the global warming metric applied. State-of-the-art reforming with high CO2 capture rates combined with natural gas supply featuring low methane emissions does indeed allow for substantial reduction of greenhouse gas emissions compared to both conventional natural gas reforming and direct combustion of natural gas. Under such conditions blue hydrogen is compatible with low-carbon economies and exhibits climate change impacts at the upper end of the range of those caused by hydrogen production from renewable-based electricity. However neither current blue nor green hydrogen production pathways render fully “net-zero” hydrogen without additional CO2 removal.
Economic Evaluation of Renewable Hydrogen Integration into Steelworks for the Production of Methanol and Methane
Jun 2022
Publication
This work investigates the cost-efficient integration of renewable hydrogen into steelworks for the production of methane and methanol as an efficient way to decarbonize the steel industry. Three case studies that utilize a mixture of steelworks off-gases (blast furnace gas coke oven gas and basic oxygen furnace gas) which differ on the amount of used off-gases as well as on the end product (methane and/or methanol) are analyzed and evaluated in terms of their economic performance. The most influential cost factors are identified and sensitivity analyses are conducted for different operating and economic parameters. Renewable hydrogen produced by PEM electrolysis is the most expensive component in this scheme and responsible for over 80% of the total costs. Progress in the hydrogen economy (lower electrolyzer capital costs improved electrolyzer efficiency and lower electricity prices) is necessary to establish this technology in the future.
Research on Multi-Period Hydrogen Refueling Station Location Model in Jiading District
Sep 2021
Publication
The construction of hydrogen refueling stations is an important part of the promotion of fuel cell vehicles. In this paper a multi-period hydrogen refueling station location model is presented that can be applied to the planning and construction of hydrogen infrastructures. Based on the hydrogen demand of fuel cell passenger cars and commercial vehicles the model calculates the hydrogen demand of each zone by a weighting method according to population economic level and education level. Then the hydrogen demand of each period is calculated using the generalized Bass diffusion model. Finally the set covering model is improved to determine the locations of the stations. The new model is applied to the scientific planning of hydrogen refueling stations in Jiading District Shanghai; the construction location and sequence of hydrogen refueling stations in each period are given and the growth trend of hydrogen demand and the promoting effect of hydrogen refueling stations are analyzed. The model adopted in this model is then compared with the other two kinds of node-based hydrogen refueling station location models that have previously been proposed.
Recent Development in Nanoconfined Hydrides for Energy Storage
Jun 2022
Publication
Hydrogen is the ultimate vector for a carbon-free sustainable green-energy. While being the most promising candidate to serve this purpose hydrogen inherits a series of characteristics making it particularly difficult to handle store transport and use in a safe manner. The researchers’ attention has thus shifted to storing hydrogen in its more manageable forms: the light metal hydrides and related derivatives (ammonia-borane tetrahydridoborates/borohydrides tetrahydridoaluminates/alanates or reactive hydride composites). Even then the thermodynamic and kinetic behavior faces either too high energy barriers or sluggish kinetics (or both) and an efficient tool to overcome these issues is through nanoconfinement. Nanoconfined energy storage materials are the current state-of-the-art approach regarding hydrogen storage field and the current review aims to summarize the most recent progress in this intriguing field. The latest reviews concerning H2 production and storage are discussed and the shift from bulk to nanomaterials is described in the context of physical and chemical aspects of nanoconfinement effects in the obtained nanocomposites. The types of hosts used for hydrogen materials are divided in classes of substances the mean of hydride inclusion in said hosts and the classes of hydrogen storage materials are presented with their most recent trends and future prospects.
Geological Hydrogen Storage: Geochemical Reactivity of Hydrogen with Sandstone Reservoirs
Jun 2022
Publication
The geological storage of hydrogen is necessary to enable the successful transition to a hydrogen economy and achieve net-zero emissions targets. Comprehensive investigations must be undertaken for each storage site to ensure their long-term suitability and functionality. As such the systematic infrastructure and potential risks of large-scale hydrogen storage must be established. Herein we conducted over 250 batch reaction experiments with different types of reservoir sandstones under conditions representative of the subsurface reflecting expected time scales for geological hydrogen storage to investigate potential reactions involving hydrogen. Each hydrogen experiment was paired with a hydrogen-free control under otherwise identical conditions to ensure that any observed reactions were due to the presence of hydrogen. The results conclusively reveal that there is no risk of hydrogen loss or reservoir integrity degradation due to abiotic geochemical reactions in sandstone reservoirs.
Energy and Utility Skills - Hydrogen Competency Framework Report
Jul 2021
Publication
In 2020 the Department for Business Enterprise and Industrial Strategy (BEIS) commissioned Energy & Utility Skills to develop and deliver a Hydrogen Competency Framework as part of the Hy4Heat programme. The successful completion of this work is detailed in a new report published today.
The work done by Energy & Utility Skills was underpinned by three key pillars:
Collaboration
The resulting outputs of the design development stages are:
More details about this report can be found on the Energy & Utility Skills website.
The work done by Energy & Utility Skills was underpinned by three key pillars:
Collaboration
- Driving growth in engagement levels across the industry
- Designing the framework for both initial trials and any future rollout
- The framework ensures engineers will have all the skills knowledge and understanding they need
The resulting outputs of the design development stages are:
- A Comparative Analysis of Hydrogen and existing hydrocarbon gases
- A Skills Matrix that translates the analysis into skills knowledge and understanding
- An Interim Hydrogen Technical Standard that defines acceptable parameters and requirements for hydrogen installation work
- A Hydrogen Training Specification that will enable training course consistency and facilitate industry recognition
- An independent Hydrogen Assessment Module that will facilitate the addition of a hydrogen competence category on the Gas Safe Register
More details about this report can be found on the Energy & Utility Skills website.
Electrofuels from Excess Renewable Electricity at High Variable Renewable Shares: Cost, Greenhouse Gas Abatement, Carbon Use and Competition
Nov 2020
Publication
Increasing shares of variable renewable electricity (VRE) generation are necessary for achieving high renewable shares in all energy sectors. This results in increased excess renewable electricity (ERE) at times when supply exceeds demand. ERE can be utilized as a low-emission energy source for sector coupling through hydrogen production via electrolysis which can be used directly or combined with a carbon source to produce electrofuels. Such fuels are crucial for the transport sector where renewable alternatives are scarce. However while ERE increases with raising VRE shares carbon emissions decrease and may become a limited resource with several usage options including carbon storage (CCS). Here we perform a model based analysis for the German case until 2050 with a general analysis for regions with a high VRE reliance. Results indicate that ERE-based electrofuels could achieve a greenhouse gas (GHG) abatement of 74 MtCO2eq yearly (46% of current German transport emissions) by displacing fossil fuels at high fuel-cell electric vehicle (FCEV) shares at a cost of 250–320 V per tCO2eq. The capital expenditure of electrolysers was found not to be crucial for the cost despite low capacity factors due to variable ERE patterns. Carbon will likely become a limiting factor when aiming for stringent climate targets and renewable electricity-based hydrocarbon electrofuels replacing fossil fuels achieve up to 70% more GHG abatement than CCS. Given (1) an unsaturated demand for renewable hydrocarbon fuels (2) a saturated renewable hydrogen demand and (3) unused ERE capacities which would otherwise be curtailed we find that carbon is better used for renewable fuel production than being stored in terms of overall GHG abatement.
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