Conversion of waste into hydrogen first requires the production of a synthesis gas (syngas), composed of CO and H2. Gasification is used to directly convert solid waste materials into syngas at high temperatures using controlled conditions. The process of changing over organic or fossil-based carbonaceous materials at high temperatures (>7000C), without combustion, with a controlled measure of oxygen and additionally steam into carbon monoxide, hydrogen, and carbon dioxide is called gasification. The carbon monoxide then, reacts with water to produce carbon dioxide and more hydrogen by means of a water-gas shift response. Hydrogen from this gas stream can be separated by absorbers or special membranes.
An alternate approach, is to partially convert the waste in a pyrolysis unit to produce vapours and char. The char may be combusted separately to provide the energy required for pyrolysis and the volatile vapours can be immediately reformed with steam (or CO2) over a catalyst to produce a H2 rich syngas. The syngas is further conditioned to maximize the H2 content using the water gas shift reaction and is then separated.
Plasma gasification is a relatively new technology, which has advantages over conventional thermal gasification. The plasma is formed using torches and peak temperatures may reach over 5,0000C. The advantages include lower emission, the formation of inert ash, fuel flexibility, very high-temperatures and lower reaction time. Also, the very high temperatures result in a clean syngas composed of almost only CO and H2. Major advantage is that only minimal feedstock preparation or separation is required as it gasify PCB, medical wastes, metallurgical waste, incineration fly ash and low-level radioactive wastes. High electricity is required for plasma gasification and the technology is very expensive.
Anaerobic digestion followed by Methane reforming or Partial Oxidation or Methane Pyrolysis is possible from biomass or waste via anaerobic or aerobic processes. Anaerobic digestion is simple in application and highly efficient, since up to 88% conversion efficiency can be reached with appropriate substrate. Anaerobic digestion consists of a complex series of reactions which are catalysed by a mixed group of microorganisms. In these reactions organic matter is converted in a stepwise fashion to methane and carbon dioxide. Polymers such as cellulose, hemicellulose, pectin and starch are hydrolysed to oligomers or monomers, which are then metabolised by fermentative bacteria with production of hydrogen, carbon dioxide and volatile organic acids. The volatile organic acids other than acetate are converted to methanogenic precursors (H2, CO2 and acetate) by the hydrogen producing organic acid oxidiser. Finally, the methanogenic bacteria produce methane from acetate or H2 – CO2.The generated bio-methane can be converted to hydrogen by steam methane reforming, Partial oxidation of methane or by methane pyrolysis as explained below:
Steam methane reforming
Steam Methane Reforming (SMR) was the first industrial method of hydrogen production, and has been used since 1930. Today, most of the commercially available hydrogen is produced through this matured process, which has an efficiency of 70-80%.
The process takes place in two steps: SMR reaction and water gas shift reaction.
SMR Reaction: CH4 + H2O ® CO + H2
Water gas Shift Reaction: CO + H2O ® CO2 + H2
First, natural gas – which contains hydrocarbons such as methane – is made to undergo a thermochemical reaction in the presence of a catalyst (nickel) using high temperature steam (700-10000C under 14-20 atmosphere of pressure) to produce hydrogen, CO, and CO2. Then, a water-gas shift reaction takes place, and CO and steam are reacted using a catalyst to produce CO2 and more hydrogen. The challenge associated is production of CO2 with this process. Around 1 ton of hydrogen production produces 9 to 12 ton of CO2.
Partial oxidation of methane
In partial oxidation, methane reacts with a limited amount of oxygen, which is not enough to completely oxidize the reactants to carbon dioxide and water . As shown in Reaction (10.9), methane is oxidized to hydrogen and carbon monoxide with less than the stoichiometric amount of oxygen available. Subsequently, the carbon monoxide reacts with water to form carbon dioxide and more hydrogen in the water–gas shift reaction.
CH4 + 0.5 O2 ® CO + 2H2
CO + H2O ® CO2 + H2
…To be continued
Suraj Prasad is working as a QA Automation Engineer at Esko, A Danaher Company. He is a graduate from Manipal Institute of Technology, in Print & Media Technology.
Dr. Bibhu Prasad Rath is a graduate in Mechanical Engineering and has worked in various functions in NTPC – including Operation & Design. He is presently the Additional General Manager in Project Engineering division. His qualifications include ICWA (Inter-1995), M.Tech (IIT Delhi-2002) and PhD (in Business Administration-2015) from Aligarh Muslim University.