That's what I was going to suggest.
I would like to add to that then,
You can also controll a small pressure this way by how much water you are displacing and how high that water rises.
Water weighs about 8lb per gallon, a 30 gallon tank therefor will displace 240 lbs of water if you do it correctly. You do not want much pressure on your wood barrel though.
For what you want to do I really think you should give up the idea of the gasifier and just make charcoal instead. Gasifier might be great if you wanted to move around with a running engine, but you don't, so why waste the resources that way and have the extra problems like how to create the suction to keep it burning?
Here, look at this posting I did about my 20 minute quicky
http://www.fieldlines.com/story/2004/11/27/8238/3446
As I recall, and probably mentioned in the post, It had been raining around here and everything was wet or at least moist. Look at the flames in the picture where I lit thesmoke comming out the pipe. Aren't those nice pretty flames 
That's the gas you want to compress to use later!
You can make this thing as big or small as you want.
Make charcoal like this, use the heat however you want, like heat your house with it, while making the charcoal. You will get the gas you want and far better and easier than using a gasifier which I am also working on myself. Once you have your gas, you also have the best charcoal you can get. You have no worries how to keep a vacume, because the wood in the barrel produces a small pressure which forces out the gas. Your not contaminating the gas with carbon monoxide like you will burning wood in a gasifier.
If you want you can use the charcoal for a great many things, blacksmithing, casting metals, cooking food, or just sell it to someone who wants it! If nothing else, use the charcoal as the feul your burning to make the next batch of gas!
Charcoal itself is a great filter, usable in many ways. Set up a barrel to bake wood, pass that gas produced into the bottom of a barrel full of charcoal to filter the gas, pass the gas to a barrrel of water and let it bubble up into the inverted 30 gal barrel as mentioned. You should have really clean gas, now you can use it as it's made and stored here, or pump it into another type of tank.
As for a pump. Run a line/pipe from your 30gal tank, install a flash back arrester, connect to the in port of the aircompressor. Connect a line to the output of the compresser, install another flashback arrester in that line, connect to your storage tank. Now there should be no problems. I will not state for a fact nothing bad can happen, but the worst that "SHOULD EVER" happen is you might get a large pop like a carburator back firing. That should never happen either, but I don't know what you might actually do or how you would do it. But the flasback arresters should stop any fire from getting into either tank, no fire/no explosions. If you made everything correct it should work fine.
I have not tried this yet because I don't want to store the gas like that normally nor use and aircompressor that eats electric
Your gonna be working with methane gas, look for info on that. Think ahead and be creative. What is the tempature that Methane gas turns to a liquid, what presure, etc. You will need to check the capabilities of the tank you plan to use. Will it hold that pressure or burst? You actaully will be wanting to turn the methane gas into a liquid most likley if you can. Propane is liquid, it boils off to a gas as you release the pressure on it by using it. Kinda like soda pop fizzles and looses the carbonation when you release the pressure. Will Methane gas act in the same way? Find out.
a 100lb propane tank holds very few gallons of propane in a liquid state. I think a 20lb tank is about 2-4 gallons and 100lb tanks maybe 20gallons? I really don't remeber off hand though I just asked the other day when I was filling one of mine.
If you just equalize the pressure from one full propane tank to an empty tank, no you don't get 2 half full tanks! Alot of people think that. The pressure equalizes but the empty tank only gets a pressure, the propane that entered into that tank does not become a liquid at that pressure. So what does it take to turn it into a liquid? I am thinking I could simply freeze it and that would do it for me, but I have not tried and just geussing. So the same thing you need to think about for storing your methane gas in tanks or bottle like propane or old freons etc..
A tank full of gas is basically nothing, alittle pressure but no volume, so it will last only a few minutes maybe depending on what it's used for.
Now in my case of propane, I simply turn the full tank upside down when I connect the full and empty directly to each other and open the valve on both tanks, the liquid flows from one tank to the other, and in this case they do equalize or at least get close. I am not flowing the gas, but I am flowing the liquid. I also have a neat little adapter I bought that lets me re-fill those little 1 and 2 lb tanks directly from a 20lb tank this same way.
The trick again though, it's a liquid you have to store, not the actaull gas as fumes. You should look into the boiling and freezing points of Methane gas and see what they are if you want to store it in propane tanks. It could be something as simple as just sitting the tank in an old freezer and lowering the temp to around 20F? Maybe you won't even need the compresser, perhaps the low pressure while creating it and a cold bottle liquifing it is all you need besides filtering it very well and cooling as much as possible before it gets to the cold bottle. I don't know the temps, so I don't know if that will work well or not, look into it.
In any case, a gasifier I think would be good for your car or tractor, but not for creating gas to store for latter use.
As for me I keep getting new ideas to re-design the stuff I am building. For instance where I was going to build my charcoal maker below ground level I have decided to move it. I got to thinking, why burn feul to make the charcoal in the first place? Then I have to waste perfectly good heat (lots of it) when I am blacksmithing or melting aluminum for castings. Ok, now my charcoal maker is going to be ABOVE my work area! I have the side of a small hill to work with here. I put the shop for blacksmithing and casting lower than the charcoal oven and then flow the excess heat while working up into the oven. I should be able to keep that oven hot enough to bake charcoal with the heat created while melting aluminum alone. In this case I will not actaully be using any feul to make the charcoal and gas, just using waste heat from some other process instead. And while using that heat baking charcoal, I still have to remove the heat from the gas and charcoal after baking it, so it's still usable for yet another purpose like heating the house eventually.
Is there a limit to how far a stick of wood can actaully go??
