Simple Biogas Instructions

Biogas

How to make it work in rural Africa

2006

biogas_how_to_make_it_work

Introduction

Believe it or not, the biogas technique is something that could solve both your cooking fuel and your agricultural problems at once, using what you would normally call rubbish! The usage of the technique is spreading all over the world, but especially in the developing world, such as in China, India and all over Africa. The technique is very simple and adjustable, and could be done with very small costs, and it has great advantages for all from household economy and health to environment.

What you need to acquire this technique is in front of all time. Although the technique has been around for a great many years, there is still a lack of standardized equipment and user instructions around (even though this might soon change with a growing demand). This means that you will need to do a lot of trying, failing and finding out for yourself. Once the biogas system is up and running, it takes only a little time each day, and very little compared with for example the collection of firewood, but it might take you some months or years of struggling before you manage to make it work perfectly, depending on the circumstances.

On top of time, you also need a little bit of money, to buy some things like a plastic drum, some iron pieces, some pipes, tools, paste, pottery clay and so on, and you need some pairs of good working hands to put it all together. But once it is done, all it will cost you is a little time of work each day, and for that you will have a constant supply of cooking fuel and a crop fertilizer that will make your crops grow like never before.

For any questions, always feel free to contact me. My contact information is in the chapter About the author.

Biogas  1

Introduction   1

What is biogas?  3

The digester 4

The gasbag. 7

How to run the digester 8

To start it up. 8

Week 1 and 2. 8

Week 3. 9

Week 4, normal use. 9

Notice. 9

The insulation house  9

The straw.. 10

The pottery-clay. 10

Constructing the house. 10

Making the clay-straw blocks. 10

Building. 11

Covering. 13

Roof 14

The stove  15

About the author 15

What is biogas?

The production of biogas is a process that transforms organic material, such as animal or human excrements, kitchen waste or plants, like water hyacinth, into a combustible gas and crop fertilizer.

You use a digester, which is something like a mechanical cow stomach: A barrel with an inlet and an outlet, and with lots of bacteria in it. The bacteria digest the organic material mixed with water, while producing methane, CH4, and carbon dioxide, CO2 (CH4 being the combustible component). The gas is taken out through a hose-pipe in the top, and now and then you stir the content by turning an axis stirrer around.

For the process to work well, the most important thing is that the digester be kept completely free of oxygen. Therefore it must be very air tight. It should also keep a very stabile temperature, so the bacteria can adjust properly. They like around 37°C, but stabile is the most important. Furthermore you can experiment with how well chopped the feed should be, what feed material works the best, how much water to mix the feed with etc, but with air tight and temperature stabile, you will no doubt have gas.

Biogas can be used as for example cooking fuel, car fuel, heating and lighting, and it can also be turned into electricity, using a fuel-cell. It burns without any smoke or soot, which is a great health benefit for the user.

The residue, that is the material that has gone through the digester, can be spread on any crop, and works as an excellent fertilizer, also giving your crops some resistance to pest and helping your soil to maintain water. It is much better for the soil than chemical pesticides, because it serves the micro-organisms in the soil rather than killing them, and therefore keeps the soil in good condition also for the future. image010

The digester

There are different kinds of biogas digesters, and of course you do not at have to do it this way, if you have better ideas, but I will here give you one example. The digester I will present, which is the one I have worked with, is a so called plug-flow digester made from a lying down plastic water drum of a little over 200 l.

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The lid and bottom of the drum are reinforced with metal on the inside and plywood on the outside. Two pipes of about 20 cm across for in- and outlet, as well as a stirrer axes of 3 cm, go through these reinforced gables.

For the in- and outlet pipes a nozzle is welded on to the metal plate on the inside, and the hard plastic pipes are slipped on to the nozzles on the outside of these, and on the inside of the plastic pipes is a rubber collar. The joint is sealed at the rubber collar with some glue or silicon paste to make it completely air tight.

Both in- and outlet pipes are sloping upwards after the nozzle with a roughly 45° angle, so that the inner openings of them are near the bottom of the drum, whereas the outer openings are situated over the surface of the slurry. In fact the level of the slurry inside the digester is determined by the height of the outlet pipe, and of how much excess pressure there is inside the digester. That is, the digester is being filled up until it overflows at the outlet.

image012

The inlet pipe is made longer than the outlet so that the slurry will not overflow that way. The slurry is being pushed forwards in the digester only through this process of filling and overflowing. The inlet pipe should be supported from underneath, to take up the force applied when feeding the digester. Otherwise a small gap might appear in the joint and oxygen might leak into the digester.

Also the stirrer has a welded nozzle in the gable, and goes through this. The joint between the nozzle and the stirrer axes is sealed with a rubber gasket on the inside of the nozzle. The axes itself should be jointed, and the joints slightly flexible. This will be of great help to ease the stirring, should the gables not be completely fixed and parallell. From the axes, four straight stirrer hands sprout, with 90° angle from one another, as to create an even load when turning the stirrer. The purpose of the stirring is only to move material that floats to the surface back down into the slurry. It is not meant to also move the slurry forwards, as mentioned before.

To give the stirrer room to point out rather far from the digester, to be connected to a crank handle which also needs room to be turned around, the in- and outlets must not be placed straight underneath the stirrer axes (which will be in the very centre of the gable) but a little bit to the left or right, so that the pipes can come up beside the stirrer crank.

If you are using only cow-dung for your biogas production, it might work even without a stirrer.

In the ceiling of the digester the gas is taken out through a hole with a pipe nipple, via a hose-pipe to a gasbag. A regular hose-pipe is not gas proof and should therefore be as short as possible to limit leakage. Alternatively a gas proof pipe could be bought, but they are quite expensive.

Between the digester and the bag it is a good idea to have a water-lock, which could for example be welded out of some pipes and iron plates, to prevent oxygen to flow backwards into the digester when the gasbag is removed to use the gas.

image014

In the pipe now connected to the gasbag, it is nice to have a tap that could be closed for the transport of the gasbag between the production site and the stove. These could be bought in hardware shops.

Into the side of the drum a construction hatch must be made to construct the insides of the gables and the stirrer. This hatch should be placed so that it will end up below the slurry surface when the digester is in use, to prevent gas leakage.

The gasbag

This problem needs yet to be solved. The best is to use a rather thick rubber bag, with some sort of coating to protect it from damage, but they are quite difficult to find. Some schools or laboratories where gas is being used might have them, but otherwise try to use a thick rubbish bag, and maybe put one inside the other to minimize leakage through them. Make a hole in the bag, big enough to run a nipple through it, and attach the nipple with two plates, one on the inside and one on the outside of the hole, and then press the plates together with a bult and mutter construction, and maybe with some glue as well.

image015

Then seal the opening of the bag completely with glue. This type of bag will not hold the gas for very long, and you will find that the methane escapes quicker than the carbon dioxide, so that even though there might be gas left in the bag, it will not be combustible after some time.

How to run the digester

To start it up

Fill the digester until it just overflows, with fresh cow-dung and water. For the type of digester described before, that means around 200 l. The final concentration of dry substance (DS) should be around 10 %. Since fresh cow-dung, as well as most plants, has a DS concentration of about 20 %, if you mix it half and half with water, it will be good. Try to empty the gasbag as much as possible before attaching it, and avoid unnecessary air all through the system.

Now wait until the micro-organisms in the digester start producing gas. This will take half a week to a week. Then move on to following steps.

If you use only fresh cow-dung you can go directly to Week 3, just reading “cow-dung” in stead of “plants”.

Week 1 and 2

Start feeding the digester each day with plants, 0.1 kg DS per m3 active digester volume. For a digester that contains around 200 l = 0.2 m3 active volume, that means 0.02 kg DS per day. That is around 0.1 kg fresh plants, mixed with as much water, to maintain the DS concentration of 10 %.

The gas will not be combustible at first, due to too large a content of CO2. After some days though, when the COcontent becomes lower then 55 %, the freshly produced gas will be combustible.

Week 3

Each day 2 kg DS per m3 active digester volume, meaning 2 kg plants and as much water.

Week 4, normal use

Each day 4–6 kg DS per m3 active digester volume, meaning 4–6 kg plants and as much water. Try it out, and see how much it can take.

Notice

The digester can be fed with a variety of plants and other organic material, eg. Tethonia, Water Hyacinth or kitchen waste. If you are using only cow-dung it could work without a stirrer. With other material you need a stirrer that you turn maybe once a day. Chop up your material in pieces of at most 2 cm, and mix it with water to a DS content of 10 %. Different types of feed give a different yield of gas.

To find out the correct DS content of something, you can use a scale. Weigh a piece of material, then dry it completely and weigh it again. The second value divided by the first, times 100, is the DS content in percent.

The insulation house

To build this insulating little house, you need a wooden frame, some pottery clay, and lots and lots of straw.

image016

Again, feel free to experiment with the instructions here given.

The straw

The straw needs to be long, strong and able to bend without breaking. The stem should be slightly hollow to keep some air in the construction. In the Orongo project we used rice straw from Kachok rice-plantation, which worked very well.

Before using, the straw should be thoroughly dried, and then also stored very dry, so that it has no chance of starting to mould.

The pottery-clay

The clay will bind the straw together, so it must be very sticky, like good pottery clay. It must not contain any humus, and therefore it should be taken at least 0.5 m below the surface of the ground. Clay particles are less than 0.1 mm, so the clay should feel completely smooth when you touch it wet.

The purer (smoother) the clay is, the better it works to bind the straw, but the harder it is to dissolve it in water. Therefore, pottery-clay that is mixed up with some sand might be the best to use. We used clay from Nyalunya village, taken 0.5–1 m underneath the ground.

Constructing the house

First you start making so called clay-straw blocks out of the clay and the straw. Then, if you like, you can build a little wooden cage construction to attach the grass-roof to, and to give the walls some stability. You build the walls from the blocks, fixing them with some mortar made of pottery-clay and sand. Finally cover the walls with three layers of plastering for stability and resistance to rain and insects.

Making the clay-straw blocks

Spread the straws on something that does not let water pass, such as a plastic sheet. Dissolve the clay in water and dilute it to a concentration roughly like thick tea. Sprinkle the clay-water over the straws and turn them around until every bit of every straw is thoroughly wetted. Then cover it up with a plastic sheet and leave it to soak over night.

In the morning, the straw should be nice and soft, slightly moist and just a little bit sticky when you squeeze it in your hand. There should be only just enough clay on them to make the blocks stick together. If there is more, the construction will not insulate well.

Place a wooden frame, 1 by 2 feet, on a waterproof surface. Fill it with the moist clay-straw, then use some sort of lid to press the clay-straw into the frame by stepping on it. The lid must go inside the frame, and it should leave some half inch on each side, so that it will not get stuck if some straw is trapped along the sides.

Step on the straw gently until it nearly does not bounce anymore, but not until it is completely hard, because you want to keep some air in it. Then remove the lid, roughen up the top straws a bit before adding more straw, so that the first layer and the second do not become separate from each other, in which case the block will fall apart later on. Repeat the procedure until your block has the desired height.

Remove the frame immediately, and leave the block out in the sun to dry. Turn the blocks around every day, and protect them from rain. Let all the blocks dry for at least some three days before starting constructing the house.

Building

Make a mortar out of one part clay, and 1.5 – 2 parts sand, depending on how pure the clay is. (More pure means more sand.) Cover the ground with the mortar where you want to place the blocks, and let it dry. Then start the building.

Before placing the blocks, one by one, you always cover both surfaces that are to be joined together, with a layer of mortar. Once you join the surfaces, they should immediately stick rather firmly. Therefore you must take care so that you place the blocks accurately at once, because if you try to move it afterwards, the blocks will start to break, and also the joint between them will become weaker, due to the characters within the clay.

Place the blocks like this:

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Make a lid for the box-like house. Here are two suggestions:

image019

Use some sort of thin timber or other armouring, that you just push through the blocks, to prevent them from breaking apart and falling into the house.

Covering

1st layer plastering, ½ – 1 inch

1              part pottery-clay

2–3          parts sand

½             part cow or horse dung

1              part chopped straw (for fibre)

                 water, in whatever quantity you find makes it stick the best

2nd layer plastering, ½ – 4 inches

1              part pottery-clay

2.5 – 4     parts sand

½             part cow or horse dung

2              parts chopped straw

1              part sawdust or other short fibre

                 water

3rd layer plastering, ½ – 1 inch

1              part pottery-clay

2.5 – 4     parts sand

1              part fine fibre

some       cow hair (just a little bit)

As soon as the wall is built you can start covering with the first layer, the main purpose of which is to make the second and third layers stick better. Therefore it should not be very smooth. Start applying at the ground working your way upwards. Take a handful of plastering in one hand, hit it firmly onto the wall, and then smear it out in one, smooth go. Do not touch it again! The more you disturb the clay once applied, the less good it will stick. The first layer needs to dry only for a few hours before you can apply the second layer, and if the first layer has gone completely dry, wet it with some water just before applying the second one.

The second layer needs to dry completely before applying the third one. The main purpose is stability and strength. The best way to apply it is to throw it hard on the wall from a close distance, and then just leave it there. If you try to smear it, it will just fall off again. Do take extra care about all the corners, joints and ground parts, so that you make the layer extra think in these areas.

The third layer is for protection. Its surface should be quite smooth. Again, extra care to corners, joints and ground.

Roof

This house will melt in the rain, so over it you need some sort of roof. Also, to maintain a stabile temperature, the house should be protected from direct sunlight, at least during midday hours. For the same reason, an iron sheet roof is not so suitable, since it gets quite hot under one of these when the sun is up. If you can keep the digester and its insulation house inside a proper house that might be good, but otherwise, just a grass roof over it is a good idea.

The stove

A regular LPG-burner can be used also with biogas. The problem with the LPG-burners you can buy for cooking is that they are made for very high pressures. To be possible to use with the low pressure you will get in the gasbag, they need to be drilled open further, so the gas can pass more easily. But do ask around and see what you can find!

It is also a good idea to build something like an oven around the cooking pot, so that the heat will be used only to warm up the pot, without escaping to the room. This will mean that you can use a much smaller flame, and therefore much less gas for the same result. The oven could for example be made from clay or bricks. Of course you must make sure that the flame can still get enough oxygen to burn.

When cooking, you attach the gasbag to the inlet of the stove, and then you just put some stones of different weight on top of the gasbag, to achieve the desired size of the flame.

About the author

My name is Karolina Hagegård, and I am a master student of Biotechnology at Chalmers University of Technology, in Gothenburg, Sweden. I am doing my masters thesis on Biogas in East Africa.

For any questions or inputs, please contact me at karolinahagegard@gmail.com and we’ll see what we can do!


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