Reuse of Waste Heat as the Endothermic Energy Supply for Methanol Steam Reforming Reaction to Produce Hydrogen

(written by: Willy Yanto Wijaya)

Huge amount of waste heat is being discarded by various industrial sectors, even in the case of Japan only, more than 400 PJ[‡] waste heat in the temperature range 100-150°C is discarded annually [1]. Most of this waste heat is discarded by power plants, followed by chemical and steel companies, as shown in Fig.1.

 

Fig.1. Annual discarded waste heat by various industrial sectors in Japan [1]

 

 

Table 1. Steam reforming reactions of various hydrocarbons [2].

Reactions	Temperature [K]
CH3OH + H2O <-> CO2 + 3H2 (methanol)	344
CH3OCH3 + 3H2O <-> 2CO2 + 6H2 (DME)	400
C2H5OH + 3H2O <-> 2CO2 + 6H2 (ethanol)	592
C2H6 + 4H2O <-> 2CO2 + 7H2 (ethane)	860
CH4 + 2H2O <-> CO2 + 4H2 (methane)	1106

 

At present, hydrogen energy is expected to play more and more important role as the future clean fuel. There are various ways to produce hydrogen. The most common method for hydrogen production is via the reforming of hydrocarbon, as shown in Table 1. Most hydrogen produced today comes from the steam reforming of methane (natural gas). This reforming reaction of methane (CH₄) requires high temperature processing, up to 600-700°C [2]. Therefore, in this writing, we propose steam reforming of methanol (CH₃OH) as an alternative way to produce hydrogen.

The reaction of methanol steam reforming (MSR) is an endothermic reaction, which means that this reaction requires heat (energy) input to proceed. The reaction can be written as follow:

CH₃OH + H₂O <-> CO₂ + 3H₂       dH = 49.5 kJ/mol CH₃OH,     (R1)

where dH means the energy increase from 1 mole of methanol to become 3 moles of hydrogen. This MSR reaction, based on thermodynamics, theoretically requires temperature less than 100°C. Therefore, in this sense, we can make use of the enormous amount of waste heat mentioned above as the heat (energy) supply for the MSR reaction. And by doing so, we could have recovered/ re-used huge amount of otherwise wasted energy.

How much CO₂ emission reduction is expected by recycling the waste heat for the MSR reaction? In this writing, we will present two versions of comparisons. For the first one, we consider that we can get extra energy of around 49.5 kJ for each mole of CH₃OH converted into three moles of H₂, and this extra energy comes from the waste heat energy. If we don’t use the waste heat, but instead combusting the methanol (CH₃OH) to provide the heating (energy input) for the MSR reaction through following reaction:

CH₃OH + 1.5O₂ <-> CO₂ + 2H₂O       dH = -675 kJ/mol CH₃OH,     (R2)

(which means for each mole of CH₃OH combusted (burned), we can obtain 675 kJ of heat energy), we should have burned 0.073 moles of methanol, producing equivalently 0.073 moles of extra CO₂. Thus, by making use of waste heat as the source of endothermic energy input, we can reduce unnecessary extra 0.073 mol (1781 ml) of CO₂ gas emission when producing 3 moles of hydrogen.

One might point out that the MSR reaction described by (R1) still produce 1 mol of CO₂ for every 3 moles of H₂ produced, and thus regard it as unclean way of hydrogen production. However, as a matter of fact, methanol (CH₃OH) can be produced from many sources of biomass as well, and therefore yielding a net carbon cyclic system. Furthermore, if CCS (Carbon Capture and Sequestration) technology is also applied for the MSR system, additional reduction of CO₂ emission can further be achieved.

Another approach of comparison is the comparison with traditional electricity (power) generating plants. The product of MSR reaction (R1) in the form of hydrogen can be used for the fuel cell (FC) system, producing electricity. Currently, the actual efficiency of FC in practice has reached about 50-60%. Taking the modest value of 50% efficiency, the energy gain contained in three moles of hydrogen can be converted into electricity, and thus in the form of eventual electricity energy, the energy gain will become (50%)*49.5 kJ = 25 kJ of additional electricity energy. Then, let’s consider a traditional thermal power plant, where electricity is generated from combustion of coal or natural gas (methane). Since coal is more complex (many varieties), let’s calculate for the case of methane combustion. The combustion of methane can be described in following reaction:

CH₄ + 2O₂ <-> CO₂ + 2H₂O       dH = -800 kJ/mol CH₄,     (R3)

where for each mole of combusted methane, 800 kJ of heat energy can be obtained. However, heat produced by the combustion will go through various heat losses during the combustion, boiler, generator and eventually the electricity form. Assuming a typical power plant having the whole process efficiency of 40%, thus each mole of combusted methane will produce 320 kJ of electricity energy. If the additional electricity energy (25 kJ) obtained from using waste heat for hydrogen production and fuel cell system is taken into account, this means that we have to burn additional 0.078 mol of methane (CH₄) generating equivalent amount of 0.078 mol or around 1900 ml of CO₂. Thus, in a large scale power generating system, tremendous amount of CO₂ emission can be cut by introducing this recycle of waste heat through MSR process.

However, question will arise as why this beneficial system has not become popular/ wide-spread instead of its benefits in the view point of energy conservation and CO₂ reduction? There are still various issues to be solved in order to realize this system. First, the issue with the fuel cell (FC) system. Cost is still the main issue that hampers the wide commercialization of FC in the market. The main parts of FC that make the high cost are the catalysts which use expensive platinum, and the membrane. Researches are being conducted as how to reduce the cost of this FC production.

Another issue regarding the utilization of waste heat for the MSR reaction is that, apparently in actual case, MSR reaction requires temperature about 200°C to proceed. Therefore, the waste heat temperature level of 100-150°C is not sufficient for the MSR reaction. One possible solution we propose is by using absorption heat pump (AHP) system to enhance the temperature level of the waste heat as shown in Fig. 2. As shown in Fig. 2, low temperature waste heat can be enhanced by AHP system to become high temperature steam reaching 200°C which is then to be supplied to MSR reactor to proceed reaction (R1). Of course, some pump work (W_AHP) is needed to boost up the temperature level of the waste heat. But since the working fluid of this AHP system is incompressible fluid, the energy needed for this pump work is relatively small compared to the energy gain obtained by this combined system, as had been shown by Wijaya et al. in their research papers [3-4].

 

Fig. 2. Schematic of combined AHP – MSR system

 

Nevertheless, one obstacle to implement this combined system is again concerning the investment cost for the AHP system. AHP system is a bulk system and is economical only if there is abundant amount of waste heat available. When enormous amount of low temperature waste heat is available, this system is very efficient and advantageous. Therefore, before implementing this combined AHP-MSR system, feasibility study needs to be conducted to probe whether the benefits from this system can cover the additional costs needed.

In summary, our calculations show that significant amount of CO₂ reduction can be achieved by reusing waste heat as the endothermic energy supply for the MSR reaction to produce hydrogen. System improvements of the fuel cell system and absorption heat pump system still have potentials to be carried out, and are expected to play important role for helping to achieve a sustainable energy system in the future.

 

 

References:

[1] Sumitomo H, Kado S, Nozaki T, Fushinobu K, Okazaki K. Exergy enhancement of low temperature waste heat by methanol steam reforming for hydrogen production. In: 8^th Asian Hydrogen Energy Conference, Beijing, China; 2005.

[2] Wijaya WY. Low temperature waste heat recovery through combined absorption heat pump and methanol steam reforming system. Master Thesis, Tokyo Institute of Technology 2010.

[3] Wijaya WY, Kawasaki S, Watanabe H, Okazaki K. Evaluation of combined absorption heat pump – methanol steam reforming system: Feasibility criterion as a measure of system performance. Energy Convers Manage 2011; 52: 1974-82.

[4] Wijaya WY, Kawasaki S, Watanabe H, Okazaki K. Damkohler number as a descriptive parameter in methanol steam reforming and its integration with absorption heat pump system. Applied Energy 2012; 94: 141-147.

 

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[‡] 1PJ = 10¹⁵ Joule