Welcome to Barrett's Mill in Shropshire. 
This is the site of the Evergreen Gas pilot-prototype anaerobic 
digester, small-scale, which we're using to test a new concept of 
anaerobic digestion which is below-ground plug-flow digestion 
compared with continuous stirred-tank reactor and here we're able 
to use this site both for experimenting with different types of 
feedstock and also with different types of energy utilization.
So, starting with the site, this is our reception 
hall where we receive feedstocks for the digester. 
The feedstocks are principally farm feedstocks, manures and grass 
silage and maize silage as well as poultry manure and we also put 
all the grass cuttings from this site through the biogas plant.
The first stage of the process is an 
auger feeder with a hopper on top. 
The forklift picks up the plastic box, spins it 280 degrees 
and tips the contents into the auger feeder hopper. 
From this point there is a screw that goes gently down 
into the digester so the semi-solid feed stock is not 
pre-treated but simply fed straight into the digester. 
So moving outside, we're able to have this feed mechanism 
because what we have here is two novel concepts. 
The first is the fact that its below ground which is unusual in 
the AD world at the moment, and the second one is it's a plug flow,
rather than being a cylindrical tank where everything is mixed up 
and you're feeding it in and taking it out this plant there is a 
plug flow so the feedstock comes in at that end and it flows 
gently down through the digester and it comes out the other end. 
What you see there is a membrane roof to collect the biogas with a 
larger biogas storage at the back so we've got more flexibility 
for experimenting with different sorts of end utilization. 
The digester is heated up to a temperature of forty degrees by 
internal heat exchanges and it's mixed by gas recirculation so 
there are no mechanical mixers or moving points within the digester.
Within the digester, because the digestion process actually liquefies
the material because solids are being digested we're only able to 
take the material from the digester through a pump so that the 
macerator there is connected to the digester and it simply macerates 
the material through a pump and then into a screw press so we then 
have a screw press that separates the digestate into a nice 
compost-like nutrient-rich soil fertilizer, soil conditioner and 
also a liquid digester; that is the flow of the material. 
There are three other forms of energy utilization here with 
our experiments, the first is that there's a pipeline here 
that just fuels the AGA cooker in the house. 
Number 2 is that we have a small engine which is a little 
Suzuki CHP unit for producing electricity from the biogas. 
At seven kilowatts it is tiny. 
Thirdly, we have a biogas boiler which is 
used to raise hot water from the biogas.
Thanks Michael, so now that we've looked at BMAD and 
where the biogas comes from we're going to talk a little 
bit about how the biogas is turned into vehicle fuel. 
So the biogas is compressed and it comes into this column here.
Then there's a flow of water that comes from the top of the column 
and the carbon dioxide and hydrogen sulphide dissolve into the water. 
Next, the cleaned-up gas goes off to be compressed. 
From the compressor it either goes into storage bottles 
or directly into a vehicle for using at a later date. 
The water which contains the carbon dioxide and the hydrogen 
sulphide passes into the top of the flash column where its 
partially depressurized and any methane that has dissolved into 
the water is recycled to the beginning of the process, the water 
then goes to the desorption column where it's desorbed from the 
water and then is released to atmosphere leaving a flow of water 
which is recycled back to the beginning of the upgrading process.
The important thing about this biogas upgrader is the scale; it has 
an inlet flow rate of ten cubic metres per hour of biogas and that 
is enough gas to produce vehicle fuels to take one of our caddies 
approximately nine hundred miles in a twenty-four-hour period.
