Hydrogen Economy Now 6

Enhancing the yield of Hydrogen using Hydrogen for heat Using Heat Exchangers to recover waste heat 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 Email: ibramsha7@yahoo.com

This article was posted elsewhere on Sunday, June 03, 2018 - 06:27 pm, and is re-posted here without any change in contents.

Introduction: We found that Steam Methane Reforming of 1 Kg of CH4 gives 0.3621 Kg of Hydrogen when we burn 0.1379 Kg of Hydrogen to heat the CH4 and H2O in

https://mohideenibramsha7.wixsite.com/website/single-post/2018/08/07/Hydrogen-Economy-Now-5

We ignored the heat in the output of the reforming reactor. Instead of wasting the heat in the output, we could reduce the Hydrogen fuel by recovering the heat using heat exchangers. We investigate this aspect now. Heat Exchanger Efficiency: In the ALLAM Cycle demonstration plant at La Porte, TX which achieved ‘First Fire’ of its 50 MWth combustor on May 30, 2018 as reported in http://www.powermag.com/pioneering-zero-emission-natural-gas-power-cycle-achieves-first-fire/?pagenum=2 the Printed Circuit Heat Exchangers made by Heatric are used. From https://www.heatric.com/typical_characteristics_of_PCHEs.html we find that Heatric designs the exchangers to match the customer specifications with 99% efficiency. To find out the amount of heat that could be recovered, without getting into intricate details of the specifications of the heat exchangers to be used, we calculate the heat energy of the output of the Optimal Steam Methane Reformer and deduce that the difference between the heat of the input and output is available for recovery using heat exchangers. As the actual design of the Printed Circuit Heat Exchangers are not attempted now, we ignore the 1% inefficiency of the heat exchanger and treat the heat content of the output to be recovered fully. Output of Optimal Steam Methane Reformer: The Reformer uses 4 moles of H2O for every mole of CH4. The output has 2 moles of H2O, one mole of CO2 and 4 moles of H2. Eventually all the output is cooled to atmospheric temperature of 25 C. In terms of weight, for 16 Kg of CH4 and 72 Kg of steam, we get 44 Kg of CO2, 36 Kg of H2O and 8 Kg of Hydrogen. In https://mohideenibramsha7.wixsite.com/website/single-post/2018/08/07/Hydrogen-Economy-Now-5

we calculated the heat input starting with the initial temperature of 25 C. Accordingly the heat content of the above quantities of CO2, H2O, and H2 is zero. The heat of reaction of the Steam Methane Reformer of the two reactions is lost in the process of reforming. Hence we need to calculate the Hydrogen required to supply the heat of reaction only when the waste heat is fully recovered by the heat exchanger. Heat of reaction: The heat of reaction for CH4 + 2H2O ‘giving’ CO2 + 4H2 is given as 29% of the energy (HHV) of CH4 in http://inside.mines.edu/~jjechura/EnergyTech/07_Hydrogen_from_SMR.pdf . The HHV of CH4 is given as 55.50 MJ/Kg in https://en.wikipedia.org/wiki/Heat_of_combustion . Thus the heat of reaction of Steam Methane Reforming is 0.29 x 55.5 MJ/Kg = 16.095 MJ for 1 Kg of Methane. Thus for 16 Kg of Methane producing 8 Kg of Hydrogen, the heat of reaction is 16.095 x 16 = 257.52 MJ. Hydrogen for the heat of reaction: The HHV of Hydrogen is 141.80 MJ/Kg. To supply 257.52 MJ we need 257.52/141.8 = 1.816 Kg of Hydrogen. When we burn Hydrogen in pure oxygen we get steam only. The burning of 1.816 Kg of Hydrogen produces 16.344 Kg of steam. This steam is part of the 72 Kg of steam supplied to the optimal Steam Methane Reformer. Net Hydrogen: The Steam Methane Reformer supplies 8 Kg of Hydrogen, of which 1.816 Kg is used to supply the heat of reaction. Since we have used the HHV of Hydrogen, the steam from burning of Hydrogen is also part of the 72 Kg of steam used in https://mohideenibramsha7.wixsite.com/website/single-post/2018/08/07/Hydrogen-Economy-Now-5

Heating the inputs by Hydrogen and recovering the heat from the products of Steam Methane Reformer gives 6.184 Kg of Hydrogen for every 16 Kg of Methane. Thus we get 0.3865 Kg of Hydrogen for every Kg of Methane when we use heat exchangers to recover the heat from the products of the Steam Methane Reformer. Conclusion: We found that we get 0.3621 Kg of Hydrogen for every Kg of Methane ignoring the heat content of the output of the Steam Methane Reformer in https://mohideenibramsha7.wixsite.com/website/single-post/2018/08/07/Hydrogen-Economy-Now-5

Now we find that using heat exchangers to recover the heat from the output of the Steam Methane Reformer we get 0.3865 Kg of Hydrogen for every Kg of Methane. The improvement is just 6.738% compared to not recovering the heat using heat exchangers. It looks like the additional effort needed in recovering the heat from the output of the Steam Methane Reformer might not be financially justified.

The other articles in this series are: