CAM ALLAM Hydrogen Generator Edward Chesky Major (Retd) US Army, USA Mohideen Ibramsha 1968 Alumni of Thiagarajar College of Engineering, Madurai, TN, India 1974 intellectual son of PhD guide Prof. V.Rajaraman & Mrs. Dharma Rajaramn, CS, EE, IIT, Kanpur, UP, India 1991 First HOD of CSE, CEC [now BSAU] Chennai, TN, India Associate Professor (Retd), Computer Science, Framingham, MA, USA Consultant R&D, M A M College of Engineering, Trichy, TN, India Advisor, HyDIGIT Pte Ltd, Singapore
This article was published elsewhere on Wednesday, May 16, 2018 - 02:38 pm: It is posted here without any modification in contents but for minor formatting.
Introduction: ALLAM Cycle uses CH4 as fuel. The supercritical CO2 is the turbine fluid in ALLAM Cycle and thus the CO2 produced by burning CH4 in pure Oxygen is captured without any penalty incurred by other power plants. In order to make the cost of electricity produced to be cheaper than the cost from other fossil fuel plants, the ALLAM Cycle uses a Recuperator made of four Printed Circuit Heat Exchangers (PCHE) manufactured by Heatric of UK. The demonstration plant built at La Porte, TX, USA is the first plant using supercritical CO2 as the working fluid. The whole world is awaiting the ‘First Fire’ of this plant to exploit the benefits of supercritical CO2 as working fluid. A search on – La Porte TX ALLAM first fire – was done at 11:30 am EDT, April 26, 2018. The search produced 10 URLs. None of them reported that the ‘First Fire’ has occurred. The ‘Allam Cycle Pressure-Enthalpy Diagram’ given in page 6 of the above PDF was exported to the Paint program for analysis. The turbine exhaust at 30 bar is cooled in the heat exchanger. The pixel coordinates of the arrow for this cooling are: (713,416) for tail, and (380, 416) for head. The working fluid is heated at 310 bar in the heat exchanger. The pixel coordinates of the corresponding arrow are: (369, 268) for tail, and (705, 268) for the head. The heat extracted from the working fluid corresponds to 713 – 380 = 333 pixels. The heat supplied to the working fluid is 705 – 369 = 336 pixels. In other words the heat exchanger is expected to work at 100% efficiency. We analyzed the Recuperator at https://mohideenibramsha7.wixsite.com/website/single-post/2018/08/06/Hydrogen-Economy-Now-3 and found that the design expects the Recuperator to work at the absolute extremes of the material characteristics. Instead of expecting the PCHE to work at extreme limits, we suggest using the heat energy of the turbine exhaust in the high temperature reforming of CH4 to produce Hydrogen. We add a second combustor and heat exchanger to reheat the working fluid. The high temperature reformation of CH4 produces CO and Hydrogen. To get CO2, we use a water shift reactor to convert the CO to CO2 and produce additional Hydrogen. We describe the ‘CAM ALLAM Enhanced Hydrogen Generator’ in the following sections. We use the CAM ALLAM Cycle in our design expecting that all ALLAM Cycle plants using CH4 as fuel would be converted to CAM ALLAM Cycle plants using Hydrogen as fuel as soon as the Hydrogen economy is realized. The CAM ALLAM Cycle is described at https://mohideenibramsha7.wixsite.com/website/single-post/2018/07/25/Hydrogen-Economy-Now-1 Onsite H2 generation: The Hydrogen is currently generated by reforming CH4. Many process industries have their own CH4 reformers so that there is no need to transport Hydrogen from its place of generation to use. We would like to adopt the same strategy of reforming CH4 onsite until we wean away from fossil CH4 for the CAM ALLAM Power plants as well. Steam Reforming: At high temperature, Hydrogen is produced using the following reaction. CH4 + H2O ‘gives’ CO + 3H2 Sixteen Kg of CH4 and eighteen Kg of steam produce twenty eight Kg of CO and six Kg of Hydrogen. The CO is oxidized in the water shift reactor using more steam. Water shift reaction: The water shift reaction follows CO + H2O ‘gives’ CO2 + H2. Twenty eight Kg of CO and eighteen Kg of steam produce forty four Kg of CO2 and two Kg of Hydrogen. Pressure Swing Absorption: The final product of forty four Kg of CO2 and eight Kg of Hydrogen is separated using pressure swing absorption that absorbs CO2 leaving Hydrogen as output. The PSA [ Pressure Swing Absorption ] is used to purify a mixed gas of CO2 and H2 with 5.5 Kg of CO2 for every Kg of Hydrogen. Hydrogen from ALLAM Cycle: Rodney Allam and his co-authors report the fuel consumption for a 500 MWth plant in a paper, the PDF of which is available at https://www.sciencedirect.com/science/article/pii/S187661021731932X . The plant uses 10 Kg/s of CH4. The demonstration plant of 50 MWth would use just 1 Kg/s of CH4. Burning 1 Kg/s of CH4 produces 36/16 = 2.25 Kg/s of steam. The turbine exhaust of 92.3 Kg/s then has 2.25 Kg/s of steam and 90.05 Kg/s of CO2. The steam is cooled to water and is discarded by the ALLAM Cycle. Instead of discarding the water we could reheat the water to steam in the Recuperator and use it to reform CH4 to produce Hydrogen. Such a redesign of the ALLAM Cycle with added Hydrogen generation would work with the existing PSA systems. Because the ‘First Fire’ of the demonstration plant at La Porte, TX has not occurred so far, we consider using the hot turbine exhaust itself for producing Hydrogen assuming that the Recuperator is at fault. In what follows, we consider the CAM ALLAM Cycle. Hydrogen from CAM ALLAM: In the CAM ALLAM Cycle we burn less Hydrogen in the main Combustor to match the steam produced. As 1 Kg/s of CH4 produces 2.25 Kg/s of steam, the turbine exhaust of 92.3 Kg/s has 2.25 Kg/s of steam and 90.05 Kg/s of CO2. The LHV of CH4 is 50 MJ/Kg while that of H2 is 120 MJ/Kg. To match the heat produced by 1 Kg/s of CH4 we need 50/120 = 0.4167 Kg/s of H2. As 1 Kg of H2 produces 9 Kg of steam, 50/120 Kg/s of H2 produces 450/120 = 3.75 Kg/s of steam. To supply the same amount of steam to the turbine, we burn 2.25/3.75 x 50/120 Kg/s of H2 in the main Combustor. We burn 0.25 Kg/s of H2 in the main Combustor. We burn the remaining 0.167 Kg/s of H2 in an auxiliary Combustor mixing the appropriate amount of CO2 to match the main Combustor. This CO2 and steam is not sent to the turbine. Instead a part of the 90.05 Kg/s of CO2 is heated through appropriate heat exchanger and supplied to the turbine. We mix CO2 to burn the H2 in the auxiliary Combustor so that the exhaust of the auxiliary Combustor could be added to the exhaust of the turbine for reforming CH4. We defer the calculation except to maintain the same ratio of H2 and CO2 in the auxiliary Combustor as in the main Combustor. The main Combustor burns 0.25 Kg/s of H2 along with 90.05 Kg/s of CO2. To burn 0.167 Kg/s of H2 we need (50/120 – 0.25)/0.25 x 90.05 Kg/s of CO2. That is we need 60.03 Kg/s of CO2 in the auxiliary Combustor. In total we burn 0.4167 Kg/s of H2 in both the Combustors. The total steam produced is 3.75 Kg/s. The total CO2 is 90.05 + 60.03 = 150.08 Kg/s. The 3.75 Kg/s of steam is used to reform CH4 following the reaction ‘CH4 + H2O giving CO + 3 H2’. Thus we reform 16 Kg of CH4 using 18 Kg of steam to produce 28 Kg of CO and 6 Kg of Hydrogen. We produce 3.75/18 x 28 Kg/s of CO and 3.75/18 x 6 Kg/s of H2 by adding 3.75/18 x 16 Kg/s of CH4 to the 150.08 Kg/s of CO2. The output of the steam reformer has 150.08 Kg/s of CO2, 5.83 Kg/s of CO and 1.25 Kg/s of H2. The water shift reaction follows ‘CO + H2O giving CO2 + H2’ . We add 18 Kg of steam for 28 Kg of CO and get 44 Kg of CO2 and 2 Kg of H2. For 3.75/18 x 28 Kg/s of CO we add (3.75/18 x 28)/28 x 18 = 3.75 Kg/s of steam. The Hydrogen produced by the water shift reaction is 2/28 x (3.75/18 x 28) = 7.5/18 = 2.5/6 = 0.4167 Kg/s. The CO2 produced is 44/28 x (3.75/18 x 28) = 44 x 3.75 /18 = 22 x 3.75 / 9 = 22 x 1.25 / 3 = 27.5/3 = 9.167 Kg/s. The output of the water shift reactor has 150.08 + 9.167 = 159.247 Kg/s of CO2 and 1.25 + 0.4167 = 1.6667 Kg/s of H2. The CO2:H2 ratio is 159.247: 1.6667. That is for every Kg/s of H2 we have 95.55 Kg/s of CO2. The existing PSA systems separate Hydrogen from a mix of 5.5 Kg of CO2 for every Kg of H2. To separate 1 Kg of H2 from 95.55 Kg of CO2 we could pass the output of the water shift reactor through a series of 95.55/5.5 = 17.37 or 18 PSA systems. Even though it is technically feasible, we do not expect this approach to be economically feasible. Recent developments filtering H2 through porous ceramics offer scope. In particular getting H2 at 0.999 purity from a mixed 20% H2 and 80% CO2 quoted below is very attractive. https://www.mdpi.com/1996-1944/9/11/930/pdf === 16 November 2016 Separation of Hydrogen from Carbon Dioxide through Porous Ceramics The Al2O3 and YSZ samples were formed into disk shapes 20 mm in diameter and 3 mm in thickness. The diameter and thickness of the SiC disk sample were 10 and 3 mm, respectively. … …The fraction of H2 gas through the Al2O3 and YSZ compacts became higher than the mixing ratio of the supplied gas and increased with decreasing pressure gradient, reaching 0.966 at 6.16 MPa/m in Al2O3 and 0.844 at 3.13 MPa/m in YSZ. … the dependence of the H2 fraction on the pressure gradient was also observed for the mixed gases with different inlet gas compositions of 20 vol % H2–80 vol % CO2 and 80 vol % H2–20 vol % CO2. When 80% H2–20% CO2 mixed gas was permeated, the H2 fraction was 0.976 at 7.61 MPa/m in Al2O3, 0.936 at 3.13 MPa/m in YSZ, and 0.987 at 3.06 MPa/m in SiC, respectively. In the case of 20% H2–80% CO2 mixed gas, the H2 fraction was 0.972 at 6.88 MPa/m in Al2O3, 0.504 at 8.04 MPa/m in YSZ and 0.999 at 3.40 MPa/m in SiC, respectively. === The pressure differential between the input and output of the SIC filter is 3.40 MPa/m. The filter is just 3 mm thick. Thus the actual pressure difference is 3.4 x 3/1000 Mpa = 10.2/1000 Mpa = 0.0102 Mpa = 0.0102 x 10 bar = 0.102 bar. Until the field develops to have reasonable pressure difference between the input and output, these filters could not be used for industrial applications. We are left with just one option: cool the hot turbine exhaust and Combustor output to collect the steam as water and reheat the collected water to steam in the Recuperator. This CO2 free steam is used for SMR and we use existing industrial equipment to separate the Hydrogen from the CO2. As of now the CO2 is let off. We consider the optimization of the Steam Methane Reforming process to identify the additional inputs to be given, if any. Optimization of SMR: Researcher Sinaei Nobandegani and five coauthors have reported on “An Industrial Steam Methane Reformer optimization using response surface methodology” published in the Journal of Natural Gas Science And Engineering 36 (2016) pp 540 – 549 the free PDF of which could be downloaded from https://www.researchgate.net/publication/309381179_An_industrial_Steam_Methane_R eformer_optimization_using_response_surface_method We use the data given in the above PDF in generating the following table.
No Variable Greater than Less than or equal
1 Tube Wall Temperature 800 K 1100 K
2 Input Feed temperature 650 K 815 K
3 Input Feed Pressure 23 bar 27 bar
4 Steam / Methane in input 3.15 7.00
5 Hydrogen / Methane in input 0.15 0.50
6 Feed Flow Rate 2800 kmol/hour 9000 kmol/hour
They used the above six parameters as variables to simultaneously optimize the following two objective functions. 1. Minimize CH4 2. Maximize H2 They found the following optimum values for the variables.
No Variable Optimal value Constraint limited?
1 Tube Wall Temperature 1100 K Upper limited
2 Input Feed Temperature 625.12 K No. Less than lower
3 Input Feed Pressure 26.32 bar No
4 Steam / Methane in input 4.03 No
5 Hydrogen / Methane in input 0.15 Lower limited
6 Feed Flow rate 2800.07 kmol/hour Lower limited
The Steam Methane Reforming equation is CH4 + 2 H2O ‘gives’ CO2 + 4H2. In terms of mole fractions, one mole of Methane with two moles of steam produces Hydrogen. The optimization has indicated that we need to supply four moles of steam for one mole of Methane to get just 0.0095 mole of untreated Methane. The input feed pressure of 26.32 bar could be easily supported by the Heatric PCHE. We use a PSA [Pressure Swing Adsorption] to collect Hydrogen at 99.999% purity with 0.001% contaminant as water as described at http://inside.mines.edu/~jjechura/EnergyTech/07_Hydrogen_from_SMR.pdf The excess two moles of steam supplied for every mole of CH4 is recovered as water and reused for further reforming. This excess two moles of water is supplied just once and thus all steam produced by the CAM ALLAM Power Plant is used in producing Hydrogen. Conclusion: Steam from 50 MWth CAM ALLAM Power Plant was calculated as 3.75 Kg/s earlier. From this 3.75 Kg/s of steam we produce 3.75 x 8/36 Kg/s of Hydrogen as 36 Kg of steam produces 8 Kg of Hydrogen by CH4 + 2 H2O ‘gives’ CO2 + 4H2. Thus the steam from the 50 MWth CAM ALLAM Power Plant produces 0.83 Kg/s of H2. To produce 3.75 Kg/s of steam we burn 3.75/9 = 0.4167 Kg/s of H2. Thus a 50 MWth CAM ALLAM Power Plant produces an additional 0.4167 Kg/s of H2 for every 0.4167 Kg/s of H2 consumed. We conclude that every CAM ALLAM Power Plant produces in excess the same amount of Hydrogen consumed when the required CH4 is reformed. If we use fossil CH4, the resulting CO2 is a green house gas. However we could use CH4 from biomass ensuring that the production of Hydrogen is Carbon neutral.