Simulation of small-scale hydrogen production

Transcription

1 Simulation of small-scale hydrogen production Tony Persson Department of Chemical Engineering, Lund University, P. O. Box 14, SE-1 00 Lund, Sweden Since the oil prices and the environmental awareness have increased during the past years, hydrogen has gained interest as an alternative clean energy source to existing fossil fuels. This attention is mainly due to hydrogen being a non-polluting energy carrier and having energy saving potential in fuel-cell applications. Hydrogen can be directly combusted in an internal combustion engine, but, if combusted with air producing nitrogen oxides, or electrochemically converted to electricity in a fuel cell system. In this master thesis a small-scale hydrogen production system developed by Intelligent Energy and Catator AB is investigated. The system is set to produce 0 kw hydrogen from several hydrocarbon feeds including natural gas and FT-liquids, and consists of two major parts, a catalytic converter and a PSA unit. The catalytic converter consists of three catalytic steps, steam reforming, water-gas shift and catalytic combustion. The investigation is mainly performed through simulations accomplished in Aspen Plus. The system has an overall system efficiency of 65.6% when at thermo balance with the base configuration. Heat losses amount to 710 W while the parasitic power consumption is 50 W. The overall system efficiency is raised to 73% when the PSA H recovery is 80%. It can be increased even further if right process variables are tuned in right. Introduction Large scale hydrogen generation has been performed for decades within the chemical and petrochemical industry. Small-scale hydrogen generation has not been fully developed but has gained interest over the past few years. There are two main pathways to manufacturing hydrogen in small scale; electrolysis of water and reforming of hydrocarbons. For both methods the hydrogen produced can be made carbon dioxide neutral. For further understanding of small-scale hydrogen generation a simulation model of the system will be investigated. The model is made to investigate how different parameters affect the efficiency of the system. The system will comprehend a steam reformer, a water-gas shift reactor, a pressure swing adsorber and a catalytic combustor. The three catalytic steps, steam reformer, water-gas shift reactor and catalytic combustions are all merged in the catalytic converter. The maximum capacity of the system has been set to 0 kw hydrogen. The technology is further described by Hulteberg et al. [1]. Figure 1 displays a simple flowsheet of the system. Several parameters of the system are investigated including: PSA hydrogen recovery, thermo balance, steam-to-carbon ratio, water-gas shift configuration, system pressure and oxygen level in the exhaust gases. Figure 1. Simple flowsheet of the system. Background The conversion of hydrocarbons to hydrogen can be carried out by several reaction processes, including steam reforming (SREF), partial oxidation (PO), and autothermal reforming (ATR). Steam reforming involves the reaction of steam with hydrocarbon in the presence of a catalyst to produce hydrogen and carbon monoxide. Steam reforming is an endothermic reaction and is often conducted in tube-fired furnaces. In partial oxidation, oxygen reacts with hydrocarbon to produce hydrogen and carbon monoxide in an exothermic reaction. The reaction occurs when the oxygen-to-fuel ratio is less than required for total combustion. The reaction rates are much higher for PO than for SREF, but the hydrogen yield per carbon of fuel is lower. Autothermal reforming involves the reaction of oxygen, steam and hydrocarbons to produce hydrogen and carbon dioxide. This process can be viewed as a

2 combination of PO and SREF. No external heating source is required, because the exothermic reaction provides the heat necessary for the endothermic steam reforming reaction []. The choice of the reaction process to be used for hydrogen production depends on many factors, including the operating characteristics of the application, e.g. varying power demand, rapid start up, frequent shutdowns. Steam reforming is heat transfer limited and therefore does not respond rapidly to changes in power demand. When power demand rapidly decreases, the catalyst can overheat, causing sintering which results in a loss of activity. Autothermal reforming can overcome the limitations of steam reforming regarding changes in power demand, because the heat required for the endothermic reactions is generated within the catalyst bed, a property that allows for more rapid response to changing power demands and faster start up. For stationary applications the steam reformer is often preferred compared to autothermal reforming because of higher efficiency. The concentration of hydrogen in dry reformate is much higher for the steam reformer than for the autothermal reformer. In autothermal reforming the generated hydrogen is diluted with nitrogen from air.[],[3] Seo et al.[4] performed thermodynamic analysis using AspenPlus TM of the three different reforming processes, SREF, ATR and PO. They found that the optimum steam/carbon ratio of the SREF is 1.9, the optimum air ratio of the PO is 0.3, the optimum air ratio and steam/carbon ratio of the ATR reactor are 0.9 and 0.35 respectively. The methane flow rate to generate 1 mol s -1 of hydrogen are mol s -1 for PO, mol s -1 for ATR and mol s -1 for SREF. This result demonstrates that the PO unit has the lowest energy cost to produce the same amount of hydrogen from methane. In this result no care is taken to energy needed to purify the produced gas.[4] The water-gas shift is considered as a secondary hydrogen producer and a primary carbon monoxide clean-up system. The typical industrial operating temperature for the water-gas shift reaction in fixed bed reactor ranges from C, but the reaction is favoured thermodynamically at lower temperatures. Steam need to be supplied in excess in order to yield more hydrogen.[5] Many technologies are available for separating hydrogen from the rest of the gas. Such technologies include Pressure Swing Adsorption (PSA), preferential oxidation of carbon monoxide (PROX), methanation and membrane separation techniques. PROX and methanation are chemical purification techniques while PSA and membrane techniques are physical purification techniques. The chemical purification techniques produce a reformate gas not only containing hydrogen but also carbon dioxide and methane while the physical purification techniques provide a pure hydrogen stream. A reformer membrane reactor uses a dense palladium or platinum alloy membrane to separate hydrogen from the reformate gases within the reformer reactor. A membrane reactor requires a significant hydrogen partial pressure driving force to achieve reasonable permeation rates and high hydrogen recovery.[6] Methanation can be used as a process for removal of traces of carbon monoxide. This is done by using reaction I. CO + 3 H CH 4 + H O (I) Typical methanation conditions are high hydrogen partial pressure, low methane and water content. Under these conditions the equilibrium is shifted far to the methane side, resulting in carbon monoxide concentrations below detection limit. Preferential oxidation oxidises carbon monoxide to carbon dioxide. The reaction that occurs is described as reaction II. CO + 1 O CO (II) The preferential oxidation reaction is often catalyzed by a noble metal catalyst.[7] In a pressure swing adsorption process the reformate goes through an adsorption tower. Hydrogen does not adsorb while other compounds adsorbs in the bed. PSA is currently more mature technology, but for compact, size constrained plants PROX might be a more interesting choice. The advantage in using PROX is that the whole process operates at low pressure, favouring the thermodynamic conversion. All the hydrogen produced in the system with PROX can be utilised while the system with PSA loses at least 0% of the hydrogen to maintain a high purity of product.[8] However, the energy in the lost hydrogen in a PSA system can be used, for example as energy source in the combustion that is needed in a steam reformer. An advantage with a reformer membrane reactor is that it eliminates at least one unit in the system, either a PROX unit or a PSA unit. The permeate that leaves the membrane reactor contains hydrogen at nearly 80% purity, with balance consisting of primarily steam.[6] A system with methanation also has the advantages that it eliminates at least one unit. The technique has the similar disadvantages as PROX, hydrogen is not separated from the other gases. The thermal energy needed for the reforming reactions is generated by the catalytic combustion in the catalytic burner. Providing thermal energy is not the only important function supplied by the catalytic burner, it also converts all gases from the system which still contain burnable fractions, exhaust gas, into carbon dioxide and water.[9] Simulation system The system has four major parts; steam reforming of hydrocarbons to produce hydrogen, water-gas shift reactors to reduce the amount of carbon monoxide and to produce more hydrogen,

3 PSA separation for a pure hydrogen stream and catalytic combustors to provide heat to the steam reforming. The three catalytic steps, steam reforming, water-gas shift rectors and catalytic combustion, are all implemented in one unit, the catalytic converter. The catalytic converter is shown in Figure, it consists of one single reactor with a cross section of 00 by 00 mm. In the first process, the steam reforming stage, natural gas, simulated as pure methane, is reacted with superheated steam to form hydrogen, carbon monoxide and carbon dioxide. The reaction is highly endothermic and heat must be provided for the reaction to reach equilibrium. Figure. The catalytic converter with steam reforming section, water-gas shift section and catalytic combustion section.[1] The process is run at a temperature ranging from 550 C at the inlet to 850 C at the outlet and a pressure of 4 bar. The steam reforming is divided into five different temperature segments, these five segments are physically separated from each other with catalytic combustion segments in-between them. The catalytic combustion segments are used for generating the necessary heat for the desirable reaction. Before the product gas from the steam reforming step enters the water-gas shift step, the gas is quenched/cooled down. This is accomplished by heat exchanging the product gas with the incoming fuel mix i.e. methane and water. In the second catalytic process, the water-gas shift reactors, carbon monoxide levels are reduced to less than 1.5 volume percentage. The exothermic equilibrium reaction where carbon monoxide reacts with steam to produce carbon dioxide and hydrogen is very temperature dependant. The process is run at a pressure of 4 bar and a temperature ranging from 450 C at the inlet to 75 C at the outlet. The excess heat generated from the exothermic reaction must be taken away from the system so that the temperature does not rise in the reactor. The problem is solved by heat exchanging the incoming fuel mixture, methane and water, with the product gas from the water-gas shift reaction. As for the steam reforming the water-gas shift reactions is divided into four temperature segments, the four segments are physically separated with the above mentioned heat exchangers between them. The third and final catalytic process is the catalytic combustor where PSA offgas and added fuel, methane, is catalytic combusted to provide heat for the steam reforming reactors. The combustion is processed at a pressure of 1 bar and at temperatures up to 900 C. The combustion part and the steam reforming part is fully integrated to ensure a compact design and with as few heat losses as possible. The design can be described as a heat exchanger with combustion taking place on one side and the steam reforming reaction taking place on the other side. In the PSA unit, hydrogen is separated from the other gases to produce a high purity hydrogen stream. To yield a high purity hydrogen stream some of the hydrogen will be lost to the offgas stream from the PSA. The offgas stream is used as fuel in the catalytic combustion and therefore the hydrogen will not be lost from the system, the energy in the lost hydrogen is still used for heating the steam reforming reaction. The unit is run at a temperature of 70 C. The PSA unit is extremely sensitive to water; therefore no water can be condensed in the unit. To make sure that no water is condensed in the PSA unit the gas stream is cooled down to 40 C and the condensed water is separated from the gas stream. The gas stream is then heated up to 70 C before it enters the PSA unit. A simple flowsheet of the configuration is shown in Figure 1. For the simulation study in Aspen Plus TM a system that works in the same way as the system described above was created. The steam reforming region is simulated as five different Gibbs reactors. The composition of the outgoing gas is calculated by minimizing Gibbs free energy. The first reactor has a temperature of 550 C and the last one has a temperature of 850 C. After every reactor, except the last one, the temperature of the gas is increased with 75 C via a heat exchanger, to match the temperature of the following reactor. The heat necessary for the reactor and the associative heat exchanger is provided by five combustion reactors. Before the product gas enters the water-gas shift region it must be cooled down. The cooling is accomplished by quenching the gas with incoming fuel mix, methane and water. The gas is cooled to c. 350 C. The water-gas shift region is, like for the steam reforming region simulated, as four Gibbs reactors. The water-gas shift reactors works adiabatic, the first reactor is set to have an outlet temperature of 430 C while the last reactor is set to have an outlet temperature of 75 C. The heat generated from the exothermic water-gas shift reaction is removed from the product gas through heat exchanging with incoming fuel mix. A heat exchanger is placed prior every water-gas shift reactor. After the water-gas shift region the product gas has a carbon monoxide content of less than 1 vol-% on a dry basis. Prior to the PSA the water content in the gas must be reduced, this is achieved through cooling down the gas to 40 C, most of the water is condensed and separated from the gas. To

4 make sure that no water is condensed in the PSA unit, the gas is heated to 70 C before entering the unit. Therefore, the gas is not saturated with water when it enters the PSA unit. In the PSA unit the gas is separated into two streams, one with high purity hydrogen and one with PSA offgas. The PSA unit is simulated as a separator where the hydrogen stream is 100% pure hydrogen. Not all hydrogen is sent to the hydrogen stream, in the base case 65% of the incoming hydrogen is sent to the hydrogen stream. After the PSA unit the hydrogen is cooled to ambient temperature through heat exchanging with incoming water. PSA offgas is sent to a splitter where it is split into five different streams. Each of the streams is sent to a catalytic combustion reactor. There are five catalytic combustion reactors, one for every steam reforming reactor with associative heat exchanger. The catalytic combustion reactors are simulated as Gibbs reactors and are processed at a pressure of 1 bar and a temperature of 900 C. If the energy in the offgas going to the combustion reactor is not enough to provide the necessary heat for the steam reforming reactor, methane can be inserted to the combustion reactor to provide the missing heat. The first combustion reactor has a fresh air injection while the outgoing gases from the reactor are sent to combustion reactor. The outgoing gases from combustion reactor are injected to combustion reactor 3 and so on. The oxygen level of the outgoing gases from the last combustion reactor is set to be 3 vol%, on wet basis, to ensure a high utilization factor. The high temperature of the reactor ensures complete combustion. The outgoing gases from the last combustion reactor is first cooled down by heat exchanging with incoming fuel mix and then further cooled down by heat exchanging with incoming air. Production rate is set to moles hydrogen per minute. Results The system has some drawbacks regarding heat losses, parasitic power consumption and heat utilisation. The heat generated when water is condensed before the PSA unit can not be utilised in the system. Parasitic power consumption accrues especially from the air blower and the PSA motor. Total parasitic power consumption amount to 50 W. Heat losses occur from the two major parts of the system, the catalytic converter and the PSA unit. The heat losses are estimated to 710 W. The first study made was to find at which PSA hydrogen recovery the system has thermo-balance. Thermo-balance is emerging when no methane must be added to the system as fuel to the combustion region. The system reach thermo-balance when the PSA H recovery is 55.7%, the overall system efficiency then amounts to 65.6%. The H recovery of the PSA unit can vary depending on the purity in the pure stream and the size of the unit. If a high pure product stream is wanted the efficiency is lower. For the simulated system, the hydrogen in the offgas stream is not lost for the system because the heating value is used in the combustion reactors. If the PSA H recovery is low, no methane have to be added as fuel to the combustion reactor, but the effect is that the methane converted in the system will lose energy in the form of heat losses. Therefore, the H recovery of the PSA unit has a strong influence on the overall system efficiency. To investigate what consequence the efficiency of the PSA unit has on the overall system efficiency and the methane consumption a simulation study with PSA H recovery varied between 45 and 80 % was carried out. Results from the study are presented in Figure 3. 0,7500 0,7000 0,6500 0,6000 0,5500 0,5000 0,45 0,47 0,49 0,51 0,53 0,55 0,57 0,59 0,61 0,63 0,65 0,67 PSA H recovery 0,69 0,71 0,73 0,75 0,77 0,79 Figure 3. as a function of PSA H recovery. When the PSA H recovery is lower than the value found for thermo balance, 55.7%, there is excess of heat in the combustion reactors. Therefore, an unnecessary high amount of methane is used and much lower overall system efficiency is achieved. The system reaches an efficiency of 7% when the PSA H recovery is 80%. Steam-to-carbon ratio, S/C, is an essential parameter of the system. It effects the coke formation in the steam reforming region, the methane conversion in the steam reforming region as well as the carbon monoxide conversion in the water-gas shift region. These effects make impact on the system efficiency, if a high steam-to-carbon ratio is used an unnecessary amount of water is heated to steam, if a low steam-to-carbon ratio is used the methane conversion in the steam reforming region is low, and some methane will not be converted in the system. To study the effects of the steam-to-carbon ratio the system is simulated with different ratios ranging from 1.8 to 4.0 with increasement of 0. units. Results from the study are presented in Figure 4. The methane conversion is highly reduced when the steam-to-carbon ratio is low. The conversion is raised from 94.7% when the steam-to-carbon ratio is 1.8 to 99.3% when the steam-to-carbon ratio is 4.0.

5 System efficency 0,80 0,78 0,76 0,74 0,7 0,70 0, Steam-to-Carbon ratio Figure 4. as a function of S/C ratio. has a clear optimum at a steam-to-carbon ratio of. where it reaches a value of 79.1%. If a high steam-to-carbon ratio is used the system efficiency decreases rapidly, an unnecessary amount of water is heated to steam. Seo et al[4] found that the optimum steam-to-carbon ratio was 1.9 for a methane steam-reformer. This does not correspond to the results found in this study. But a reason for this can be that when the lower steam-tocarbon ratio is used the system is not in thermo balance and some energy will be lost from the system. To find out if there is even better system efficiency at lower steam-to-carbon ratio with a higher PSA H recovery a study is made where the hydrogen recovery is set above the thermo balance state. The steam-to-carbon ratio is varied from 1.8 to.4 and the PSA H recovery is set to 95%. The result of the study is presented in Figure 5. 0,8 0,815 0,81 0,805 0,8 0,795 0,79 0,785 1,8 1,9,1,,3,4 Steam-toCarbon ratio Figure 5. as a function of S/C ratio at a PSA H recovery of 95%. The results of the study show that a low steamto-carbon ratio leads to higher system efficiency, but only if the PSA H recovery is very high. A PSA H recovery above 90% is not very realistic. The water-gas shift region can be excluded from the system though the hazard carbon monoxide is oxidised to carbon dioxide in the combustion region. Some hydrogen production will be lost, but material costs will be lower since less material is used. A study is made where the water-gas shift region is excluded from the system. Results from the study are shown in Figure 6. The overall system efficiency is lowered with about 4% when the WGS region is excluded. 0,8 0,78 0,76 0,74 0,7 0,7 0,68 0,66 0,64 Without WGS With WGS 1,8, 4 Steam-to-Carbon ratio Figure 6. as a function of S/C ratio with and without the WGS region. System pressure affects the system in various ways, the methane conversion and coke formation in the steam-reforming region, carbon monoxide conversion in the water-gas shift and the PSA H recovery is affected by the system pressure. To see how the overall system efficiency is affected by system pressure a study is made where the pressure is varied between 4 and 14 bar. The catalytic combustion region is always performed at atmospheric pressure. When the system pressure is raised from 4 bar to 14 bar the overall system efficiency is only lowered by one percentage point, see Figure 7. Due to the low decrease of efficiency when the pressure is raised the system can be process at a higher pressure yield a higher PSA H recovery. There is a higher cost both in material and units when a higher pressure is used and this must be considered when selecting system pressure. 0,74 0,7 0,7 0,718 0,716 0,714 0,71 0,71 0,708 0, System pressure (bar) Figure 7. as a function of system pressure. Seo et al[4], predicted that carbon deposits were not present at pressures higher than 10 bar at a steam-to-carbon ratio as low as 1:1. Though the system efficiency is not very affected by the system pressure, the pressure can be raised to make sure that no carbon deposits are present. The oxygen level in the exhaust gases is an important factor for the catalytic combustion region. If high oxygen level is achieved higher amounts of air is used and unnecessary heat is used to warm up the air. If low oxygen level is achieved the

6 temperature may not be consistent in the combustion region and the fuel might not be fully combusted. A study is made where the oxygen level in the exhaust gases is varied between the base case of 3 vol% and 7, 10 and 15 vol%. When the oxygen level in the exhaust gases is varied between 3 vol% and 10 vol% the overall system efficiency is not affected much, there is only a difference of one percentage point between the case with 3 vol% and 10 vol%. The conclusion of this is that even if the oxygen content is 10 vol% the heat needed to warm up the extra amount of air used can be used within the system. If the oxygen level is raised to 15 vol% the overall system efficiency is dramatically lowered, all of the heat in the extra amount of air used can not be utilised within the system. The result of the study is presented in Figure kw is lost. To increase the overall system efficiency it is important to utilize this lost energy. If no heat losses or other drawbacks were present the overall system efficiency is raised with 10%. The overall system efficiency is raised to 73% when the PSA H recovery is 80%. An overall system efficiency above 70% is acceptable but it can be raised even further if right process variables is tuned in right. A lower steam-to-carbon ratio could be used to make the system more efficient, a higher pressure can be used to make sure that no coke formation is present in the system and that the PSA can work with a high hydrogen recovery. A future investigation should involve more studies regarding the coke formation in the steam reforming region. Mainly to see how the steam-to-carbon ratio and the pressure affects the coke formation. 0,75 0,73 0,71 0,69 0,67 0,65 0,63 0,61 0,59 0,57 0,55 3% 7% 10% 15% Oxygen level in exhaust gases Figure 8. Overall system efficiency as a function of oxygen level in exhaust gases. Conclusions The system to produce hydrogen in small-scale developed by Intelligent Energy and Catator AB, has a quite complex configuration. Especially the catalytic converter where heat transfer and reactor configuration has a key role for an efficient production. The simulation system must have the same behaviour as the real system or else the result will have no value. The created simulation system is a decent reflection of the real system. There is no actual heat transfer between the catalytic combustor and the steam reforming region but the combustion is set to always provide the heat required in the reaction zone. The system which mainly has two major components, the catalytic reformer and the PSA unit, has an overall system efficiency of 65.6% when at thermo balance with the base configuration. Estimated heat losses amounts to 710 W while the parasitic power consumption is 50 W. Heat released during the condensation of water in the product gas before it enters the PSA is not available for use within the system, approximately Literature cited [1] Hulteberg P.C., Porter B, Silversand F.A, Woods R, A versatile, steam reforming based small-scale hydrogen production process. WHEC June Lyon France, 006. [] Krumpelt M, Krause T.R, Carter J.D, Kopasz J.P, Ahmed S, Fuel processing for fuel cell systems in transportation and portable power applications. Catalysis Today, : p [3] Hubert C-E, Achard P, Metkemeijer R, Study of a small heat and power PEM fuel cell system generator. Journal of Power Sources, : p [4] Seo Y.S, Shirley A, Kolaczkowski S.T, Evaluation of thermodynamically favourable operating conditions for production of hydrogen in three different reforming technologies. Journal of Power Sources, : p [5] Kamarudin S.K, Daud W.R.W, Som A.Md, Mohammad A.W, Takriff S, Masdar M.S, The conceptual design of a PEMFC system via simulation. Chemical Engineering Journal, : p [6] Lattner J.R, Harold M.P, Comparison of methanolbased fuel processors for PEM fuel cell systems. Applied Catalysis B: Environmental, : p [7] Hulteberg P.C, Brandin J.G.M, Silversand F.A, Lundberg M, Preferential oxidation of carbon monoxide mounted and unmounted noble-metal catalysts in hydrogen-rich streams. International Journal of Hydrogen Energy, : p [8] Dalle Nogare D, Baggio P, Thomas C, Mutri L, Canu P, A thermodynamic analysis of natural gas reforming processes for fuel cell application. Chemical Engineering Science, : p [9] Emonts B, Bögild Hansen J, Loegsgaard Jörgensen S, Höhlein B, Peters R, Compact methanol reformer test for fuel-cell powered light-duty vehicles. Journal of Power Sources, : p Received for review December 0, 007