The London Hydrogen Partnership has released a report that concludes it is theoretically possible for the city of London to generate 141 tonnes of hydrogen per day from waste, using both gasification/pyrolysis and anaerobic digestion of all municipal and commercial waste. Of that 141 tonnes, 68 tonnes could come from just municipal waste—conceivably under the control of a single waste authority.
The daily 141 tonnes could potentially fuel a fleet of 13,750 fuel-cell hybrid and hydrogen hybrid internal combustion engine buses. The current London fleet has slightly more than 8,000 buses. The report notes, however, that since much of the hydrogen would be used for power generation, the 13,750 figure represents the potential, not the expected production.
Gasification would produce a synthesis gas (syngas)—a mixture of hydrogen, carbon monoxide and carbon dioxide, plus small amounts of methane. For hydrogen production, the syngas would be cleaned, reformed and then go through the water-gas shift process to convert carbon monoxide to hydrogen. Further purification would be required for the hydrogen to be of the necessary quality for use in a fuel cell.
...the volume of gas produced is only 20-25% of that produced by incineration, therefore substantially lowering the capital cost of the clean-up equipment. Emissions of local pollutants are substantially lower than incineration, although their exact levels depend on the technology deployed and would have to be monitored in the same way as the emissions from incinerators. Gasifiers are also more modular than incinerators, allowing more flexibility in their use for waste management; the size envisaged in London would treat around 90,000 tonnes of waste per year, compared to 600,000 tonnes per year at the proposed incinerator in Belvedere.
Anaerobic digestion produces bio-methane, which would then be reformed into hydrogen, although not at the site. The biomethane would be fed into the natural gas pipeline system to be routed to a large SMR facility. By contrast, production of hydrogen from the syngas resulting from the gasification process will occur at the gasification site, as transporting the syngas “is not sensible.”
|Bus depots (red) and potential waste sites in London. Click to enlarge.|
The report envisions the possibility of dedicated hydrogen pipelines transporting the hydrogen resulting from gasification to refueling sites. The viability of this scheme would be dependent on locating the hydrogen production facilities close to the potential areas of significant hydrogen demand (refueling depots), to minimize distribution distances and maximize flow rates.
The greenhouse gas emissions resulting from waste-to-energy projects depend on the composition of the waste stream. The biodegradable fraction of the waste is considered to be renewable and the CO2 emissions resulting from treatment of this part of the waste stream are ignored.
Based on an assumed biodegradable fraction of 65%, the non-renewable CO2 emissions of gasification would be 5.6 kg CO2 per kg H2. For reference, this compares to CO2 emissions from reforming of natural gas of around 9 kg CO2 per kg H2, Therefore, whilst gasification-to-H2 is not a zero-GHG emissions process, it compares favorably to fossil fuel-based production routes on this basis. In a situation where purely renewable routes are constrained due to resource availability or cost, this provides the next best option on the basis of GHG emissions.