Supercritical Water Gasification
Meera - Simply raising the temperature and pressure of water can cause drastic changes to its reactivity as Gary Leeke showed me when I met him in his lab.
Gary - 'Super critical water gasification' is a fancy word for superheated water really. Water, as you know, exists as a liquid in ambient conditions and a vapour. We don't really see the vapour unless you get steam. If we were to heat water above 374 degrees C and apply sufficient pressure, we end up with a supercritical state for water. So the pressure we need to apply is 221 bar. So, it's quite large pressures and reasonably high temperatures. But at that condition, water changes its properties totally. So you know water, being a polar solvent, it can't really dissolve organics, but at that condition in the super critical state, we can start to dissolve organics. It totally changes from being a polar solvent to a non-polar solvent.
Meera - So, by heating water to these temperatures and raising it to such high pressures, you're changing its properties in a way that it becomes a solvent and it's reactive.
Gary - Yeah. We actually use it as a solvent and it also as you rightly say, it becomes reactive to attack bonds. And we can also accelerate this process further. As well as having the temperature and pressure, we can also add a catalyst into the system as well to speed up the actual gasification process because we want this to occur in probably about 2 or 3 seconds maximum.
Meera - So the fact that it can break bonds in this way is where it comes in use with the biomass that you're working with because that's quite a complex structure and the super critical water will break that apart and break the bonds within.
Gary - Yes, that's correct. So, water as we know now is in a super critical state is super reactive. We have a lot of these OH- and H+ floating around that we can use to break the bonds and its very complex structure.
Meera - The main selling points of a hydrogen economy are that it's an environmentally friendly way of delivering energy. So when high temperatures and pressures are required for its production, it's easy to question whether we should be promoting it. But the efficient rates at which hydrogen is released and the many ways in which this process can be integrated into current waste management systems keeps hydrogen at the forefront.
Gary - The majority of plants contain a large amount of water within their structure. So, if you were to burn that, a lot of the energy is actually used to drive off the water in the first place. We actually relish the water that is there and actually use it in the process. So, for wet biomass feedstock, this is very viable option to create new chemicals.
Meera - So, this is a pilot scale that you've got here in the lab. What would be the next steps to get this going on perhaps a more industrial scale?
Gary - We're working with industry on this. I can't mention the company's name. They have a feedstock which due to legislation can no longer be put in landfills or spread on lands, so they're looking for a viable option to actually get rid of this feedstock and raise other chemicals from it, i.e. gaseous feedstocks that they can then use to raise energy themselves. Basically, they get a double bonus. They don't have to pay the actual disposal cost to give to landfills and they create a feedstock that they can actually sell.