Free heat for my greenhouse

Hi all been looking for some time on ways to heat my greenhouse for free. I’ve looked at various ways from rocket mass heaters compost even hydrogen generators but these either don’t produce enough heat take to much power (only have a small solar set up) or require to much attention (work commitments).
So I’ve been looking at bio gas generators and hopefully the brilliant knowledge here will be able to help me. I’m looking at making one from a ibc to run my current propane heater so my questions are
1, can I run my propane heater on the gas produced.
2, will a ibc produce enough gas to run it
3, if no to no2 how much storage would be needed
4, would I need to scrub the gas if it’s only to be used in a greenhouse.

I’ve provided pictures of the heater and it’s specs and it’s only used for 3 months (feb-April) I’m based in the uk and if it helps last year I went through 2x19kg bottles of propane with the heater set at the temp and automatically fires up when needed
Thanks all and any advice and comments would be great

hi Mike,

here's my beginner's take on your proposal:

-a biogas generator is feed stock volume determined. perhaps the research I did produced bad information but the literature I found suggested strongly you should know how much volume of feedstock you have available for your biogas generator. The reason is the bacteria colonies' populations are surface area dependant....your biogas generator's internal surface area is directly related to the size and volume of the feed stock that goes into the biogas generator. Therefore, if you underfeed, there will be a die off of bacteria and a drop in your gas output. If you overfeed, there will be new colony growth time in which your gas production will also drop. So find what it is you're going to feed your generator every single day and how much you're going to feed it every single day.

Once you know how much you can dependably feed your biodigester every single day, then you can size your biodigester (how many ibc's), from this size, you can find general feedstock brown gas production tables.  These tables will tell you how long your propane heater will work.

I realize this sounds backwards but it isn't: your location only has so much feedstock available and you only have so much time to prepare feedstock and feed it each day.

-the biogas generator is going to need to stay around 75 to 80 degrees F....can you do this???  True, biogas can be generated at 60F and at 95F but not at predictable rates and not very much compared to a nice 75-80F range.

-yes, you can run a propane heater but you will need to modify the gas port which the gas exits just before it ignites ....the port will need to be larger.  Does your propane heater require the propane is pressurized?  If yes, then you'll need to pressurize the brown's gas from your digester....this is usually done via a mass on top of a gas bag.....but you need to consider how to force the brown's gas to where you want it.

-scrub the gas?  which gas?  the brown's gas going into the propane burner or the gas coming out of your propane heater?  

Scrubbing the gas coming from the digester and thank you for the response

It seems like you use around 40kg of propane, last season.
Propane and methane have about the same energy density by weight, so you will need about 40kg of methane too.
(by volume propane has a 2.5:1 higher energy density)

Based on what I have read, to make 1kg of methane it takes 10kg of input by dry weight.
So to make 40kg of methane it will take 400kg of input by dry weight. So 500kg of dry haybale/straw or 4,000kg of lettuce.
I think it might be easier to just directly burn the 500kg of dry hay/straw in a rocket stove over the 90days/3 month

You did mentioned that you used this 40lbs of propane/methane over 90days. So your daily avg comes out to being only 0.44kg/day.
or 4.44kg of input by dry weight per day. That said the usage in Feb is probably 4 times the usage in April.

Digester have a startup time of about 30days. So you are going to have to start up your system on New Years to get a nice output come Feb 1st.
They have a retention time of 60days

By any chance do you have a cow?
1 cow will give us 10kg of dung per day.
This will produce 1m3 of biogas/day
this will be purified to 0.6m3 of methane
0.6m3 * 0.74kg/m3 = 0.45kg of methane per day.

If you are trying to extract 1m3 of biogas per day. you are going to have to put in 5m3 of input per day give or take. You are most certainly going to need something bigger than just a IBC with is only 1m3.

I don’t have a cow unfortunately and was hoping to make the gas year round and possibly store it I worked out that a ibc wouldn’t be big enough to run it directly I have seen on some sites that they compress it into old propane bottles is this possible and practical?

Mike Nott wrote:Scrubbing the gas coming from the digester and thank you for the response

Scrubbng the gas from the digester isn't necessary to run your propane heater.  Scrubbing usually decreases the moisture content and raises the purity of the brown's gas more so that it can almost be called methane.  The more effort you put into scrubbing, the closer you'll get to pure methane. I'd just use the water trap methods you'll find on people's systems drawings and call it good enough to run the heater.

You didn't ask my opinion on this but I feel that running a biodigestor to harvest brown's gas makes sense when you have more than three people eating communal meals and or animals. To create a source of feedstock just so you can run a biodigestor seems to me to be unnecessary work and investment: but I'm lazy like that.

You didn't ask my opinion on this but I feel that running a biodigestor to harvest brown's gas makes sense when you have more than three people eating communal meals and or animals. To create a source of feedstock just so you can run a biodigestor seems to me to be unnecessary work and investment: but I'm lazy like that.

It’s just an idea to save money on buying propane every year and I don’t mind working at it

Unscrubbed biogas can have high levels of corrosive gasses that may corrode and damage your propane burner. Scrubbing for this is pretty simple though -- I've read that you include a chamber before or in the middle of the gas pipe, packed loosely with iron filings or steel wool or cheap steel scrubbies that rust quickly. Much of the corrosive gas will do its work by rusting the iron there and turn into less corrosive substances. To reduce moisture in the gas, you can make a condensing chamber before or in the middle of the pipe, and that will happen to work best in winter when you are using the biogas for your greenhouse.

One drawback for biogas for your purpose is that the digesting process as pointed out above needs to be very warm, ideally the temperature of a ruminant's gut, so in the coldest weather when you need biogas the most, the digestion process will be at its slowest. If you want to store the gas for months, you'll need some container that is very airtight and not vulnerable to corroding.

What is your outside temperature and climate? What are the current problem temperatures in your greenhouse and for how many nights? And what is your target, ie how much area do you have in tender plants that need additional heat? Because there may be simpler ways to protect your tender plants or improve the nighttime insulation, etc.

The winter part of my greenhouse is totally double glazed I need to maintain a temp of 15-20c as for keeping it warm I also have a large solar set up not enough to heat the greenhouse but enough to use a 100w water heater in the container that would be insulated

Rebecca Norman wrote:Unscrubbed biogas can have high levels of corrosive gasses that may corrode and damage your propane burner......

I'm aware of hydrogen sulfide (H2S) ...is there another corrosive gas you know of in homemade biogas?

I know you run a biodigestor in the Himalayans; have you had a biodigestor system without a scrubber crode the gas fittings?  

I like biogas and I'm glad you're on this forum Rebecca: could you post a pic of the business ends of your biodigestor(s)  (feed mouth, gas exit, scrubber, and tank)?   ...or the link on this site for your post. The Himalayans have many material problems to overcome when a person or organization doesn't have funds to transport the materials to the location

- I'm really looking forward to seeing your solution!