Definition of gasification (hydrogen production): Gasification is a method of producing hydrogen by heating coal, biomass, or a heavy hydrocarbon at high temperatures (500-1400°C) and high pressures (>33 bar). (Garcia 2015)
[I’m in the process of picking up on this part. My sources equated coal gasification with pyrogasification, which is not correct, both technically (the former tech is mature, the latter has yet to scale up) and ecologically (the former is highly polluting, the latter is carbon-negative …). Moreover, it would be necessary to check if the hydrogen produced is valorized as such or simply burned, as it is often the case it seems to me in the steel industry: one gasifies the coal to produce coke, then one burns the co-produced coal gas, rich in hydrogen, to feed the other processes with heat].
The gasification process (coal, biomass …)
A combination of several reactions
The gasification would be in fact a combination of several reactions:
The chemistry of gasification is quite complex, involving a combination of cracking, partial oxidation, steam gasification, WGS and methanation reactions (Velez, Chejne, Valdez, Emery, & Londono, 2008).
Navarro et al. 2015
The global reaction is:
Feedstock + H2O + O2 = H2 + COx + CH4 + HCs + char + tars + others (HCN, NH3, NCl …)
This reaction would be endothermic and its enthalpy would be 120 to 160 kJ/mol. ( Navarro et al. 2015, p.27)
Garcia (2015) and Navarro et al. (2015) do not seem to agree on the heat/pressure. While according to the former, the process would take place at 500-1400°C and >33 bars, for the latter it would be 926-1126°C and 5-10 bars … I obviously do not have the knowledge to decide between them.
The process to produce hydrogen though gasification
Navarro et al. 2015 propose us a scheme with the following stages:
The basic elements (steam, oxygen and the raw material) are prepared, then injected into a gasifier. The result is cleaned, and then passes into a reformer. Then, there are two units to make the catalytic conversion (CO+H2O = CO2+H2), one at high temperature, 400°C (HTWGS), the other around 200-300°C (LTWGS). Finally, the purification.
On the preparation, upstream, of the biomass, I refer you to Garcia (2015, p.143-144) Ditto for the different types of gasifiers (with fixed bed, fluid bed, guided flow, plasma, supercritical water …). ( Garcia 2015, p.145-155) Ditto for the different variables (gasifying agent, temperature, catalyst …) (Garcia 2015, p.155-160).
Assets and challenges of this hydrogen production
According to Garcia (2015, p.138, 161), this production method would have several advantages:
- It would produce “fewer contaminants while producing more fuel than conventional combustion processes.
- It would achieve a higher electrical performance, 30-32% compared to 22% with a conventional Rankine cycle (??) Would produce less NOX and SOX (= compounds of sulfur oxides and nitrates, which I guess are toxic).
It would also have several challenges (Navarro et al. 2015; Garcia 2015, p.162): mainly removing tars and ashes and avoiding that they deactivate the catalyst.
Gasification in practice
The price of producing hydrogen from coal gasification is between $0.83 and $1.7 per kg of H2. Carbon capture would raise the price by 10% (Fossil Fuel Hydrogen, Technical, Economic and Environmental Potential).
There have been two projects in France to produce hydrogen from biomass:
- “Wood-hy/hy-boy” carried by the communes of the Landes d’Armagnac will produce hydrogen from the pulpwood of the Landaise forest. It has been developed with Enosis and is supported by Engie. The site should start production in 2022 and produce 1000 tons of hydrogen per year.
- VitrHydrogen aims to develop Hynoca (HYdrogen NO CArbon), an industrial process of Haffner Energy to convert various biomass into hydrogen. A first demonstrator was to start in 2019 in Vitry-le-François.
Bibliography
- Département américain de l’énergie, « Hydrogen Production: Natural Gas Reforming »
- Navarro R.M., Guil R., Fierro J.L.G. (2015), Introduction to hydrogen production, in Compendium of hydrogen energy, Vol. 1 Hydrogen production and purification, éd. Woodhead Publishing, Kidlington, 2015
- Garcia L. (2015), Hydrogen production by steam reforming of natural gas and other nonrenewable feedstocks, in Compendium of hydrogen energy, Vol. 1 Hydrogen production and purification, éd. Woodhead Publishing, Kidlington, 2015