[Gasification] Wet wood/green wood debate...

[Zietsman, Rex]

Jeff, Steve and Listers,
Firstly, my wish is for everyone to have a fantastic 2007. May it be the best year ever!
I have just come back from leave and have waded my way through three weeks of gasification debate. The wet/green/dry wood debate really got me thinking. Let me paint a picture and see what you guys think...
1. The whole idea of maximizing performance of a gasifier is to keep the temperature of the gasification zone high enough to maintain the gasification reactions. If you cool below this, you get reversion of CO to CO2 among others.
2. Heat for gasification is normally provided by burning (oxidising) part of the fuel to provide heat for the endothermic gasification reactions. Air is generally the source of oxygen. If a lot of energy is required, a lot of oxygen will be consumed. This adds nitrogen to the outlet fuel gas reducing its LHV.
3. I found a paper that deals with steam vs oxygen (CO2 actually) gasification [Paper presented at the ECOS 2000 Conference July 5-7, 2000, University of Twente, Enschede, The Netherlands "International Conference on Efficiency, Cost, Optimisation, Simulation and Environmental Aspect of Energy and Process Systems" Low Tar and High Efficient Gasification Concept by Jens Dall Bentzen, Reto M. Hummelshøj, Brian Elmegaard and Ulrik Henriksen]. This paper states that using steam as the gasification agent results in higher reaction rates than gasification with CO2 [Walker. P. L, Jr., et al., Gas reactions of carbon, Advances in Catalysis, D. D. Eley, P. W. Selwood and P. B. Weisz (Eds), XI, 133 (1959)]. If steam is the primary gasification agent instead of CO2 the temperature in the gasifier can therefore be lowered. The authors of the paper found several other advantages related to steam gasification: 
- Lower temperatures: Experiments with a two-stage gasifier, with fuels with different moisture content, showed that the temperature in the partial oxidation zone decreases about 200°C when gasifying with steam compared to gasification of dry fuels. These observations are confirmed by mathematical modelling.
- Lower soot production: The amount of soot particles are reduced when gasifying with steam. This is partly due to lower maximum temperatures and partly due to higher concentration of OH radicals which reduce the soot production. 
- Lower emissions: The gas composition is different when steam is added as a gasification agent. The H2 content is increased and the CO content is lowered. The results in a faster combustion and lower emissions of CO and NOx.
My conclusion: If you use steam for gasification, you can run at lower temperatures which means less energy required from oxidation hence less air and higher LHV of fuel gas. 
So how do we use this information? In order for a steam gasification system to work, you will have to firstly generate the steam and then heat it up so as not to use too much oxidation energy. To do this, you heat up the wet/green wood with say 50% moisture indirectly using the exhaust gas from the engine (I am assuming we are trying to generate power here). I have a jacketed screw conveyor in mind for this section of the operation. As the amount of exhaust gas is roughly 6x the fuel gas, there is a lot of energy available in the exhaust to heat the wood up to above 100degC. This will boil off the water to steam while heating the wood and steam up. 
This hot mixture then enters the pyrolysis zone of the screw conveyor where you use gasifier off gas (850degC?) in a separate jacket to heat the wood/steam up to about 600degC. During this phase, the wood pyrolyses to a great extent. The hot mixture of char, pyrolysis gas, tar and steam is then fed to the gasifier. Preheated air is introduced to the gasifier where it burns off most of the pyrolysis gas and tar and takes the mixture up to say 1100degC.
The char lands on the grate where the steam, CO2 and CO pass through it. As the gases are hot, the carbon gasifies and produces fuel gas with a much higher H2 content (about 30%) and a relatively low (about 30%) N2 content. Tars leaving the gasifier are in the region of 25mg/Nm3 which is acceptable to most engines "as is". If you use a downstream bagfilter for soot removal, the soot will capture tar and reduce the tar concentration to <5mg/Nm3 - see DTU paper on this.

The attraction of this system is that the quality of the gas is very good both from a tar/solids content and an LHV point of view. At the same time, the money spent on getting wood dry from 50% down to <25% or 15% can be used in a slightly different way to achieve, in my opinion, a better result. 
Makes you think doesn't it? Comments and criticism are most welcome! It is the only way you make progress.
Rex

PS: I have a simpler design than proposed above (that I want to build before the end of the year) that will combine a hot water boiler with the gasifier. This simplifies the equipment and reduces the cost but is only feasible because I need additional hot water for reticulation. In this design, the tube conveyor runs through the firebox of the boiler getting heat from the firebox. The air preheater is a serpentine pipe mounted at the firebox outlet where the hot outlet gases heat the air up, I think to about 800degC or so. 
I will cool the gasifier fuel gas down to about 100degC with a double pipe heat exchanger to heat up water. The gas will be filtered for soot/tar removal in the gas phase prior to further sub-cooling to condense out excess moisture. The engine exhaust and the jacket cooling system will also be set up to generate hot water at 60degC. The hot water will be reticulated to warm poly tunnels, stables, piggery and a cow barn. 
For simplicity, I will run the wood screw at a constant speed and generate gas continuously. The engine will take what it needs with the excess gas going to the boiler firebox. That way I do not have to worry about trying to control fuel supply with electrical load. The engine throttle will be controlled to maintain 50Hz on the generator.
Comments on this design are also most welcome.
I see the biggest challenges here on getting a gas tight seal between the screw and the gasifier and how I will activate the grate...