Carbon recycling for zero-emission carbon-neutral transportation

Abstract

Converting current internal combustion engines to operate in closed-cycle, lean-burn mode with pure oxygen could generate an exhaust stream of >99% carbon dioxide. The ease of capture of this carbon, relative to the dilute CO₂ in current exhaust gases, points the way to a feasible route to zero-emission transportation. Prospects for implementing such an approach, which would include re-synthesis of fuel from the captured CO₂ and water using renewable power, are considered.     

Critical flow rate of anode fuel exhaust in a PEM fuel cell system

A manual purge line was added into the exterior fuel exhaust stream of a Ballard PEM stack in a Nexa™ power module. With the addition of manual exhaust purge, high levels of inert gases were intentionally added to the anode feed without changing normal operational procedures. A new method of determining the critical minimum flow rate in the anode exhaust stream was given by an anode mass balance. This type of operation makes dual use of membranes in the MEAs as both gas purifiers and as solid electrolytes. The PEM stack was successfully operated with up to ca. 7% nitrogen or carbon dioxide in the absence of a palladium-based hydrogen separator at ca. 200 W power level. Nitrogen in the anode stream was concentrated from 7.5% to 91.6%. The system maintained a fuel efficiency of 99% at a manual purge rate of 2.22 ml s⁻¹ and no auto purge. The fuel cell stack efficiency was 64% and the stack output efficiency was 75%. The overall system efficiency was 39%. After troublesome CO and H₂S poisons were removed, a hydrocarbon reformate containing high levels of CO₂ and H₂O was further used in the Nexa™ stack. The size and complexity of the fuel processing system may be reduced at a specified power level by using this operational method.     

Study of the cell reversal process of large area proton exchange membrane fuel cells under fuel starvation

In this research, the fuel starvation phenomena in a single proton exchange membrane fuel cell (PEMFC) are investigated experimentally. The response characteristics of a single cell under the different degrees of fuel starvation are explored. The key parameters (cell voltage, current distribution, cathode and anode potentials, and local interfacial potentials between anode and membrane, etc.) are measured in situ with a specially constructed segmented fuel cell. Experimental results show that during the cell reversal process due to the fuel starvation, the current distribution is extremely uneven, the local high interfacial potential is suffered near the anode outlet, hydrogen and water are oxidized simultaneously in the different regions at the anode, and the carbon corrosion is proved to occur at the anode by analyzing the anode exhaust gas. When the fuel starvation becomes severer, the water electrolysis current gets larger, the local interfacial potential turns higher, and the carbon corrosion near the anode outlet gets more significant. The local interfacial potential near the anode outlet increases from ca. 1.8 to 2.6 V when the hydrogen stoichiometry decreases from 0.91 to 0.55. The producing rate of the carbon dioxide also increases from 18 to 20 ml min⁻¹.     

Hydrogen production by methanol–water solution electrolysis

The production of hydrogen by methanol–water solution electrolysis was investigated. Hydrogen and carbon dioxide were contained in the cathode exhaust gas and the hydrogen concentration was 95.5–97.2 mol%. The hydrogen flow rate in the cathode exhaust gas increased in proportion to the current density and almost agreed with the theoretical hydrogen-production rate. The voltage and electrical energy needed to produce hydrogen were less than that for water electrolysis. The electrical energy needed in methanol–water solution electrolysis was less than 60% of that required in water electrolysis. Permeation of methanol, water and carbon dioxide from the anode to the cathode of the electrolytic cell occurred with hydrogen production. The permeation rate of methanol became greater than that of water as the current density increased. When the current density was constant, the permeation rate of water did not depend on the methanol concentration in the methanol–water solution supplied to the anode, and that of methanol increased while that of carbon dioxide decreased as the methanol concentration increased.     

MCFC-based CO2 capture system for small scale CHP plants

Carbon dioxide emissions into the atmosphere are considered among the main reasons of the greenhouse effect. The largest share of CO₂ is emitted by power plants using fossil fuels. Nowadays there are several technologies to capture CO₂ from power plants' exhaust gas but each of them consumes a significant part of the electric power generated by the plant. The Molten Carbonate Fuel Cell (MCFC) can be used as concentrator of CO₂, due to the chemical reactions that occurs in the cell stack: carbon dioxide entering into the cathode side is transported to the anode side via CO₃⁼ ions and is finally concentrated in the anodic exhaust. MCFC systems can be integrated in existing power plants (retro fitting) to separate CO₂ in the exhaust gas and, at the same time, produce additional energy. The aim of this study is to find a feasible system design for medium scale cogeneration plants which are not considered economically and technically interesting for existing technologies for carbon capture, but are increasing in numbers with respect to large size power plants. This trend, if confirmed, will increase number of medium cogeneration plants with consequent benefit for both MCFC market for this application and effect on global CO₂ emissions. System concept has been developed in a numerical model, using AspenTech engineering software. The model simulates a plant, which separates CO₂ from a cogeneration plant exhaust gases and produces electric power. Data showing the effect of CO₂ on cell voltage and cogenerator exhaust gas composition were taken from experimental activities in the fuel cell laboratory of the University of Perugia, FCLab, and from existing CHP plants. The innovative aspect of this model is the introduction of recirculation to optimize the performance of the MCFC. Cathode recirculation allows to decrease the carbon dioxide utilization factor of the cell keeping at the same time system CO₂ removal efficiency at high level. At anode side, recirculation is used to reduce the fuel consumption (due to the unreacted hydrogen) and to increase the CO₂ purity in the stored gas. The system design was completely introduced in the model and several analyses were performed. CO₂ removal efficiency of 63% was reached with correspondent total efficiency of about 35%. System outlet is also thermal power, due to the high temperature of cathode exhaust off gases, and it is possible to consider integration of this outlet with the cogeneration system. This system, compared to other post-combustion CO₂ removal technologies, does not consume energy, but produces additional electrical and thermal power with a global efficiency of about 70%.     

Characterisation of direct methanol fuel cells under near-ambient conditions

Direct methanol fuel cells have been characterised under ambient conditions. By operating the cathode with hydrogen as well as air, the half-cell and full-cell overvoltages have been found. Both the anode and the cathode kinetics improve with increasing temperature, but the cathode suffers from additional losses due to methanol crossover and water flooding. The methanol concentration, however, mainly influences the cathode. Impedance spectroscopy measurements on single cells and stacks confirm these results, and indicate that water removal from the cathode is important for stable operation of direct methanol fuel cells. Carbon dioxide (CO₂) in the cathode outlet was measured by mass spectroscopy (MS). The amount of carbon dioxide crossover is noticeable compared to the methanol crossover, but decreases with temperature and methanol concentration. Almost 100% of the crossover methanol is converted to carbon dioxide at the cathode.     

Effect of gas composition on Ru dissolution and crossover in polymer-electrolyte membrane fuel cells

Pt-Ru-based anodes are commonly used in polymer-electrolyte membrane fuel cells (PEMFCs) to provide improved CO tolerance for reformate fuel applications. However, Ru crossover from the anode to the cathode has been identified as a critical durability problem that has severe performance implications. In the present study, an anode accelerated stress test (AST) was used to simulate potential spikes that occur during fuel cell start-ups and shutdowns to induce Ru crossover. The effects of fuel gas composition, namely hydrogen and carbon dioxide concentrations, on Ru dissolution and crossover were investigated. The cell performance losses were correlated with the degree of Ru crossover as determined by the changes in cathode cyclic voltammetry (CV) characteristics and neutron activation analysis (NAA). It was found that higher hydrogen concentration in the fuel accelerated Ru crossover and that the presence of carbon dioxide hindered Ru crossover. In particular, the injection of 20 vol.% carbon dioxide during potential cycling resulted in very minor Ru crossover, which showed essentially identical performance losses and CV characteristic changes as a fuel cell composed of a Ru-free anode. The experimental results suggest that the Ru species in our Pt-Ru metal oxide catalysts need to go through a reduction step by hydrogen before dissolution. The presence of carbon dioxide may play a role in hindering the reduction step.     

Thermodynamic analysis of a new process for hydrogen and electric power production by using carbonaceous fuels and high-temperature process heat

A new process for the simultaneous production of hydrogen and electrical power by using carbonaceous fuels and high-temperature process heat is presented in this paper. In an electrolytic cell, sulfur dioxide dissolved in an aqueous solution of sulfuric acid is electrochemically oxidized to sulfuric acid at the anode, while hydrogen gas is evolved at the cathode. The sulfuric acid produced in the cell provides the oxygen for the fuel combustion which subsequently takes place at high pressure. The combustion gas consisting mainly of CO₂, SO₂ and H₂O expands in a turbine in order to produce electrical power. After the expansion, the components sulfur dioxide and water are separated from the combustion gas and fed together with added water into the electrolysis cell.The process shows some advantages compared with already existing or proposed processes for the production of hydrogen or electric power. The influence of the sulfuric acid concentration and some other important process parameters on the energetic and exergetic efficiency of the total process is shown. The results shown in this paper have been obtained by using carbon (as a substitute for coal which is the preferred fuel) and a nuclear heat production plant (as an example of providing the required high-temperature process heat).     

A review of hydrogen usage in internal combustion engines (gasoline-Lpg-diesel) from combustion performance aspect

Demand for fossil fuels is increasing day by day with the increase in industrialization and energy demand in the world. For this reason, many countries are looking for alternative energy sources against this increasing energy demand. Hydrogen is an alternative fuel with high efficiency and superior properties. The development of hydrogen-powered vehicles in the transport sector is expected to reduce fuel consumption and air pollution from exhaust emissions. In this study, the use of hydrogen as a fuel in vehicles and the current experimental studies in the literature are examined and the results of using hydrogen as an additional fuel are investigated. The effects of hydrogen usage on engine performance and exhaust emissions as an additional fuel to internal combustion gasoline, diesel and LPG engines are explained. Depending on the amount of hydrogen added to the fuel system, the engine power and torque are increased at most on petrol engines, while they are decreased on LPG and diesel engines. In terms of chemical products, the emissions of harmful exhaust gases in gasoline and LPG engines are reduced, while some diesel engines increase nitrogen oxide levels. In addition, it is understood that there will be a positive effect on the environment, due to hydrogen usage in all engine types.     

Electrochemical separation of hydrogen from reformate using PEM fuel cell technology

This article is an examination of the feasibility of electrochemically separating hydrogen obtained by steam reforming a hydrocarbon or alcohol source. A potential advantage of this process is that the carbon dioxide rich exhaust stream should be able to be captured and stored thereby reducing greenhouse gas emissions. Results are presented for the performance of the anode of proton exchange membrane (PEM) electrochemical cell for the separation of hydrogen from a H₂–CO₂ gas mixture and from a H₂–CO₂–CO gas mixture. Experiments were carried out using a single cell state-of-the-art PEM fuel cell. The anode was fed with either a H₂–CO₂ gas mixture or a H₂–CO₂–CO gas mixture and hydrogen was evolved at the cathode. All experiments were performed at room temperature and atmospheric pressure. With the H₂–CO₂ gas mixture the hydrogen extraction efficiency is quite high. When the gas mixture included CO, however, the hydrogen extraction efficiency is relatively poor. To improve the efficiency for the separation of the gas mixture containing CO, the effect of periodic pulsing on the anode potential was examined. Results show that pulsing can substantially reduce the anode potential thereby improving the overall efficiency of the separation process although the anode potential of the CO poisoned and pulsed cell still lies above that of an unpoisoned cell.     

Behaviour modelling of a PEMFC operating on diluted hydrogen feed

The polymer electrolyte membrane fuel cell (PEMFC) using reformate gas as fuel is regarded as an attractive solution for the near‐term introduction of fuel cells in stationary or mobile power generation market. With respect to hydrogen feeding, the reformate gas fuelling involves additional polarization losses because of the hydrogen dilution and the impurities contained in the gas. In this paper a one‐dimensional model has been developed to investigate the behaviour of a PEMFC operating with reformate gas mixture. The model, based on a semi‐empirical approach, considers the kinetic reactions in the anode side taking into account the effect of reverse water–gas shift (RWGS) due to the presence of CO₂ in the fuel. As it is well known, the exhaust stream from fuel reformers can contain a high carbon dioxide concentration (>20%) that can have a detrimental effect on the fuel cell performance because of the combination of the dilution and the formation of CO by the RWGS reaction. The numerical simulation results have been compared with the experimental data, obtained in the test room of Industrial Engineering Department of Cassino University, and a good match has been observed. The model has been developed by using a simplified approach that, nevertheless, can allow to obtain a good numerical prediction of the fuel cell performance reducing the simulation time and computational efforts. Copyright © 2008 John Wiley & Sons, Ltd.     

Sensitivity analysis of a polybenzimidazole‐based polymer fuel cell and insight into the effect of humidification

The present work describes a systematic investigation of the effect of operating temperature, cathode stoichiometry, anode stoichiometry and reactants humidification rate on the behavior of a polybenzimidazole‐based high temperature polymer fuel cell. The effect of reactants humidification was also considered; actually, in real applications, the syngas holds great amounts of water. Furthermore, water diffuses through the membrane and reaches the cathode side where it adds to the water produced by the electrochemical reaction. The investigation is based on the analysis of polarization curves measured under different operating conditions. Anode stoichiometry has no impact on the fuel cell voltage, while cathode stoichiometry and fuel cell temperature are relevant. When the anode stream is humidified, negligible effects take place; conversely, when the cathode stream is humidified, a consistent drop in the fuel cell voltage is observed, with a consequent drop in the power output. When air is saturated at 70 °C, a power loss of 8% and 27% takes place at 0.55 A cm^?2 and 0.9 A cm^?2, respectively. Such a finding might represent an issue when high power densities are pursued. The effect of cathode humidification was further investigated by means of electrochemical impedance spectroscopy and cyclic voltammetry. Thanks to dedicated tests, the effect of water in the cathode feed stream was clarified. Cathode humidification increases the electrode catalyst active area due to the dilution of the phosphoric acid retained in the electrode. Conversely, the presence of water hinders the oxygen mass transport to the catalyst active sites. Copyright © 2013 John Wiley & Sons, Ltd.     

A review of hydrogen as a compression ignition engine fuel

Diesel fuelled engines emit higher levels of carbon dioxide and other harmful air pollutants (such as noxious gases and particulates) per litre of fuel than gasoline engines. This fact, combined with the recent diesel emission scandal and the rumours of more widespread cheating by automotive manufacturers have initiated a long discussion about the future and sustainability of diesel engines.Improving the compression ignition engine is a direct way of going green. Reducing the harmful emissions can be achieved by future developments in the engine technology but also the implementation of alternative fuels. Hydrogen is a renewable, high-efficient and clean fuel that can potentially save the future of diesel-type engines. The evolution of high-efficiency renewable hydrogen production methods is the most important path for the start of a new hydrogen era for the compression ignition engine that can improve its sustainability and maximum efficiency.This paper provides a detailed overview of hydrogen as a fuel for compression ignition engines. A comprehensive review of the past and recent research activities on the topic is documented. The review focuses on the in-cylinder combustion of hydrogen either as a primary fuel or in dual fuel operation. The effects of injection strategies, compression ratio and exhaust gas recirculation on the combustion and emission characteristics of the hydrogen fuelled engine are fully analysed. The main limitations, challenges and perspectives are presented.     

Effect of sulfur contaminants on MCFC performance

Molten carbonate fuel cells (MCFC) used as carbon dioxide separation units in integrated fuel cell and conventional power generation can potentially reduce carbon emission from fossil fuel power production. The MCFC can utilize CO₂ in combustion flue gas at the cathode as oxidant and concentrate it at the anode through the cell reaction and thereby simplifying capture and storage. However, combustion flue gas often contains sulfur dioxide which, if entering the cathode, causes performance degradation by corrosion and by poisoning of the fuel cell. The effect of contaminating an MCFC with low concentrations of both SO₂ at the cathode and H₂S at the anode was studied. The poisoning mechanism of SO₂ is believed to be that of sulfur transfer through the electrolyte and formation of H₂S at the anode. By using a small button cell setup in which the anode and cathode behavior can be studied separately, the anodic poisoning from SO₂ in oxidant gas can be directly compared to that of H₂S in fuel gas. Measurements were performed with SO₂ added to oxidant gas in concentrations up to 24 ppm, both for short-term (90 min) and for long-term (100 h) contaminant exposure. The poisoning effect of H₂S was studied for gas compositions with high- and low concentration of H₂ in fuel gas. The H₂S was added to the fuel gas stream in concentrations of 1, 2 and 4 ppm. Results show that the effect of SO₂ in oxidant gas was significant after 100 h exposure with 8 ppm, and for short-term exposure above 12 ppm. The effect of SO₂ was also seen on the anode side, supporting the theory of a sulfur transfer mechanism and H₂S poisoning. The effect on anode polarization of H₂S in fuel gas was equivalent to that of SO₂ in oxidant gas.     

Cogeneration of power and substitute of natural gas using electrolytic hydrogen,biomass and high temperature fuel cells

Energy storage from renewable sources is one of the main current goals for the energy sector, and the production of a substitute of natural gas could be a good solution to solve the problem in the short term, helping the transition to hydrogen in the long term.Renewable energy sources usually generate variable electric power or medium/low energy content gas. This paper proposes a way to upgrade these products through the use of electrolytic hydrogen. By using electrolytic oxygen as an oxidant for biomass partial oxidation and for high temperature fuel cells, the exhaust gas after post-combustion is an almost pure mixture of water and carbon dioxide. Once such a gas is dehydrated, the carbon dioxide can be mixed with electrolytic hydrogen to obtain methane through the Sabatier process.Four layouts based on molten carbonate fuel cells and solid oxide fuel cells has been investigated. The results obtained are very similar: the conversion efficiency is close to 60% and the mix of energy output consist of fuel for about 75% and electric power for about 25%.     

Performance analysis of hybrid systems incorporating high temperature fuel cells and closed cycle heat engines at part-load operation

This work presents a comparative analysis of hybrid systems based on molten carbonate fuel cells and making use of closed-cycle externally-heated bottoming systems. Two options are considered: reciprocating (Stirling) engines and supercritical carbon dioxide turbines. These two engines share the common feature of working on closed cycles with pure fluids (H₂ and CO₂ respectively) but, at the same time, they differ in their internal structure: Stirling engines make use of volumetric machinery whereas the supercritical carbon dioxide gas turbine (SCO₂) is composed by turbomachinery. In both cases, the working fluid is subjected to very high pressures and temperatures in the range of 50–200 bar and 40–650 °C.     

Combined Hydrogen,Heat and Power (CHHP) pilot plant design

The Combined Hydrogen, Heat and Power (CHHP) system consists of a molten carbonate fuel cell, DFC300. DFC300 consumes biogas, and produces electricity and hydrogen. The high temperature flue gas can be recovered for useful purposes. During the hydrogen recovery process, the anode exhaust gas (37.1% H₂O, 45.9% CO₂, 5.7% CO, and 11.2% H₂) is sent through a water gas shift (WGS) reactor to increase the hydrogen and carbon dioxide composition, and then water is removed in a vapor–liquid separator. The remaining hydrogen and carbon dioxide mixture gas is separated using a 2-adsorber pressure swing adsorption unit under 1379 kPa. Resulting hydrogen can achieve 99.99% purity, and it can be stored in composite hydrogen storage tanks pressurized at 34,474 kPa. Hydrogen is produced at a rate of 2.58 kg/h. The produced hydrogen is filled into transportable hydrogen cylinders and trucked to a residential community 7.5 km away from the CHHP site. The community is powered by fuel cells to supply electricity to approximately 51 apartments. A heat recovery unit to produce steam and hot water recovers hot air exhaust from the DFC300, having a total heating value of 405 MJ/h. The greenhouse employs a two-phase steam heating system. Hot water supply is mainly needed for the CHHP education center. DFC300 produces electricity at a maximum capacity of 280 kW. A substation is built to set up the interconnections. Power poles and power lines are built to distribute electricity to the CHHP system, the education center, and the greenhouse. The overall electricity consumption of the CHHP system is 86 kW, and the greenhouse consumes 40 kW. Therefore, an aggregate of 154 kW of power can be used to provide power to the UC Davis campus.     

Transient analysis of multicomponent transport with carbon monoxide poisoning effect of a PEM fuel cell

A one-dimensional, two-phase, transient mathematical model was developed to analyze how carbon monoxide poisoning affects the performance of a PEM fuel cell. This work examines both vapor and liquid water transport inside the cell. The theoretical results indicate that a higher CO concentration results in less hydrogen coverage and a large drop in the time to reach steady state t_ss. The slowing of the reactions at both the anode and the cathode reduce the saturation of liquid water in the catalytic layers. The distribution of liquid water depends more strongly on the CO concentration than on dilution of hydrogen in the MEA of the fuel cell. Increasing the amount of pure hydrogen drastically increases t_ss for a wide range of CO contents. At a relatively low CO content, a long t_ss can be achieved using pure hydrogen, especially at high cell voltage, promoting the tolerance for CO and providing the desired performance of the fuel cell.     

Experimental investigation of CO2 separation from lignite flue gases by 100 cm single Molten Carbonate Fuel Cell

The paper presents an experimental investigation of using a Molten Carbonate Fuel Cell (MCFC) to reduce CO₂ emission from the flue gas of a lignite fired boiler. The MCFC is placed in the flue gas stream and separates CO₂ from the cathode side to the anode side. As a result, a mixture of CO₂ and H₂O is obtained from which pure CO₂ can be obtained through condensation of water and carbon dioxide. The main advantages of this solution are: additional electricity generated, reduced CO₂ emissions and higher system efficiency. The results obtained show that the use of an MCFC could reduce CO₂ emissions by 90% with over 30% efficiency in additional power generation.     

Development of a high-efficiency hydrogen generator for fuel cells for distributed power generation

A collaborative effort between Intelligent Energy and Cal Poly Pomona has developed an adsorption enhanced reformer (AER) for hydrogen generation for use in conjunction with fuel cells in small sizes. The AER operates at a lower temperature (about 500 °C) and has a higher hydrogen yield and purity than those in the conventional steam reforming. It employs ceria supported rhodium as the catalyst and potassium-promoted hydrotalcites to remove carbon dioxide from the products. A novel pulsing feed concept is developed for the AER operation to allow a deeper conversion of the feedstock to hydrogen. Continuous production of near fuel-cell grade hydrogen is demonstrated in the AER with four packed beds running alternately. In the best case of methane reforming, the overall conversion to hydrogen is 92% while the carbon dioxide and carbon monoxide concentrations in the production stream are on the ppm level. The ratio of carbon dioxide in the regeneration exhaust to the one in the product stream is on the order of 10³.