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Biomass and waste conversion in supercritical water for the production of renewable hydrogen


The separation and purification of hydrogen is an essential component in the exploitation of hydrogen as an energy carrier in stationary and transport applications, particularly for fuel cells. Smaller scale and distributed applications require miniaturised technologies. Palladium metal and its alloys can uniquely purify hydrogen by membrane diffusion, but palladium is very expensive. To be competitive, the membrane should be thinner than 7 microns and pin hole free. The proposer has developed a process for the production of 3 -10 micron membranes supported on porous ceramic and tested prototype hydrogen separators that meet relevant permeation, purity and cost targets for economic acceptance. It is recognised that surface perfection of the ceramic support is critical to reliable manufacture of thin membranes and their reliability in use. For widespread exploitation, development is required to produce packaging designs that are reliable and economic to manufacture.
The proposer has developed a prototype catalytic membrane reformer that produces pure hydrogen as part of a gas upgrading process for hydrogen rich product streams. In addition to the targeted biomass gasification process, additional opportunities exist for upgrading other process gas streams in competition with existing technologies. In principle the current design may be modified to apply to a range of scales, pressures and gas compositions. Multifuel reformers are considered to have significant potential in a future hydrogen economy.
The result is a method for the preparation of a slurry from wet biomass (organic waste, herbaceous plant-like material) that can be pumped into ( high pressure) reactors for an energy conversion process like supercritical gasification, if necessary via a heat exchanger. Conversion process like supercritical gasification are of special interest for the utilization of the energy content of wet biomass because a drying step for the biomass is not necessary. The pumping tests are carried out for two capacity ranges; Small feed capacity pumps, only for pilot scale testing and large scale pumps, used for carry out pumping tests. Plunger, piston pumps found the most suitable pumps to pressurize bioslurries with particle size up to 7,5 mm and DM-content of 20 %. The process is tested on a pilot scale and, depending on the biomass, an energetically and financially interesting process route is developed to wash and to reduce the particle size of the biomass. The most interesting way to produce a pumpable slurry is reduction of the particles size by grinding. The maximum costs for the feedstock and the pretreatment are 3 euro/GJth of hydrogen. The cost of the slurry production fluctuates from 1.8 to 11.5 euro/GJth of hydrogen. Only a selected part of the available biomasses will be suitable as feedstock for supercritical gasification.
The conversion of wet biomass and waste streams in supercritical water is a rather novel approach. The main products aimed for are hydrogen, methane (SNG), or mixtures thereof. Market conditions will determine the most likely product (at different times). A continuous pilot-plant - one of the few world-wide is operated successfully with model components, and some real waste streams like rme-waste glycerine. For more complicated feedstocks (high lignin and/or high ash) a smaller bench scale unit is operated. The first demonstration unit is expected for relative easy feedstocks like RME glycerin or possibly vinasse ethanol waste. RME glycerin is the preferred feedstock as it seems to become an increasing problem in Europe.

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