From wet waste to green gas

At 600 degrees and 250 bars, supercritical water rips organic molecules apart, producing gas from wet biomass. PhD candidate Onursal Yakaboylu modelled the process.

Too much manure and dwindling gas production – that sums it up for the Netherlands in 2016. A new technology called ‘supercritical water gasification of wet biomass’ has the potential to restore the equilibrium. The technique converts manure, but also sewage sludge and food industry waste, into biogas (hydrogen and methane). The conversion efficiency is potentially high (up to 95%) but limited by the costs of the equipment.

At the conditions of use (600 degrees celsius and 250-300 bars) water gets into a supercritical (SC) state with a gas-like viscosity and a liquid-like density. Supercritical water behaves like an organic solvent. Thus, any mixed-in biomass becomes completely dissolved, leading to a fast and near-complete hydrolysis (for which water acts both as a catalyst and as a reactant). The outcome of the process, after cooling down and separation of vapour and liquid, is a mix of hydrogen and methane (natural gas), the latter of which can be fed into the Dutch gas infrastructure. By-products such as CO₂, CO, H₂S and NH₃ are filtered out and can be used for other processes.

The Dutch firm Gensos in Delft has developed a full scale (0.5 ton/hour) pilot plant for supercritical water gasification, as well as a smaller experimental installation. Founder Dr. John Harinck said that Gensos is running duration tests with wet waste from prospective customers. Gensos needs to verify gas conversion (currently at 75%) and system reliability in preparation for market introduction.

Yakaboylu, who did his PhD research on this topic at the process & energy department of the Mechanical, Maritime and Materials faculty, puts the supercritical water gasification next to conventional gasification for dry biomass and anaerobic digestion for wet biomass. Advantages of supercritical water gasification over digestion for wet biomass are the process speed (minutes instead of days), higher gas conversion (almost double) and, hence, the lower amount of residue.

Yakaboylu has modelled the thermodynamics of the process, with a special interest in the formation (precipitation) of salts, since these can clog up the equipment. “Not only do our models predict gas composition but also the concentration of minerals. This knowledge helps to move to a place of precipitation of salts from, say, a heat exchanger into the reactor instead, since it’s easier to clean,” he said.

Validation

The practical tests he performed, as a validation of his models, gave him a feel for the technical challenges for this promising technology. Think only of the pumps that have to compress a non-homogenous mass to 250-300 bars of pressure. Or consider the fittings that have to withstand pressures of hundreds of bars or the pipes and fittings that get exposed to extremely corrosive supercritical solutions.

Despite the challenges, Yakaboylu thinks supercritical water gasification is a promising technology for the processing of wet biomass since the product gases can serve all kinds of energy markets: heat, power and transportation.

Harinck said that thanks to Yakaboylu, engineers at Gensos could optimise the gasification process. His models also allow making predictions on the gas production from different feedstock without having to do time-consuming experiments.

• Onursal Yakaboylu, Supercritical Water Gasification of Wet Biomass, PhD supervisors Prof. Bendiks Jan Boersma and Dr. Wiebren de Jong (Faculty of 3mE), March 17th 2016.