Results:

Huge Digester Size and long Retention Time (RT) do not necessarily mean MORE biogas. An RT=40 days, DR=3 and a small 1,451 m3 PLH digester can produce a lot of biogas and HUGE SAVINGS compared to giant digesters. Optimum design specs are the KEY.

FAQ: How can I transport biogas?

Biogas is a mixture of CO2 and CH4. It also carries some impurities like H2S which may react with pipe work. There are techniques for removing C02 by using the fact it is more soluble than methane and H2S by reacting it with scrap iron which is then regenerated in air. Moreover, Methane is hard to liquify.

Although compressing biogas would be a major cost, it would cost little MORE to clean it. Both CO2 and H2S are much more soluble in water than CH4, so simply contacting the compressed gas with water (eg in a spray tower) would greatly reduce the concentrations of both impurities.

Small amounts of CO2 are no problem at the point of use but even quite small concns of H2S are toxic and would also produce corrosive SO2 when burned. So you'd need to check that concentrations were acceptable. A lot depends on how it is used. If cooking is done in an open structure adjoining the house, there's far less risk than if it is used indoors.

It is possible to liquify biogas to provide a fuel for vehicles. But liquifying methane for sea transport or piping implies that about 40% of the energy could be lost in the liquefaction.

One way to make biogas transportable and therefore usable for decentralised cooking in individual rural households without having to invest in kilometers of piping form the digester to the end-user is to use variable volume storage. Flexible low-pressure bags would be suitable for transport. We are now studying a way for filling the biogas at high pressure into 20, 40 and 100 lbs cylinders. 0.4m3 of biogas could be enough for a family meal so flexible agricultural bags may be viable.

There are hand operated pesticide pumps which can spray pesticides from a bucket or a barrel. By using such a pump we have been able to fill biogas from a the gas tank into an old inner tube of a truck tyre. It is then transported and connected to a domestic stove. The tube has to be kept on a smooth surface with a plank on top of it. A few bricks or stones kept on the plank provide the necessary pressure. There is no need to remove CO2 or H2S from the gas if it is to be used as cooking fuel. If starch/sugar is used as feedstock, the CO2 percentage would be very low.

FAQ: LPG vs CNG vs biogas ...

LPG-type products are quite different chemically: mostly butane and propane (C4H10 & C3H8, not CH4), which liquefy at modest pressure. So the widely-sold bottled gas cylinders mostly contain liquid LNG, which vapourises as you draw it off for use. This means you can get far more gas into the same bottle - which greatly reduces the costs of containment and transport.

Moreover, LPG may well be produced under some pressure, so the bottler has less compression work to do, whereas biogas is always produced at low pressure. LPG is a fairly mature technology and is used for large delivery vehicles in northern europe.

Compressed Natural Gas: examples in the world. The North Island of New Zealand has an extensive network of CNG filling stations with the fuel well established there. So too is there an extensive network of CNG filling stations in Italy. With India about to connect to Iran's extensive natural gas reserves and also to Burma's reserves it appears that India will also be using natural gas as a prime fuel.

FAQ: How can we treat the effluent?

The slurry could be spread on land with special tools with similar to large rakes, with hoses draging on the ground instead of teeth, and this should preferably be done in the groving crop. In this way the loss of nitrogen will be minimised. However this is not that easy because of the logistics. All of the slurry that is produced during the year outside the growing season must be stored.

The slurry has a very high water content 90 - 95 % or so or more and it is costly to transport. The spreading equipment is heavy and may compact certain soils. If you spread early the crop will grow in spite of wheel tracks but is is doubtfull if you spread late. You have to compete with the spreading of manure from animal husbandry. So one method that is used instead is that the slurry is dewatered and the residue is composted. Some of the the water from the dewatering process is put back into the process and some is used for sprayng the compost to keep it reasonably moist. In this process quite a lot of nitrogene is lost but the political focus is on the phosphorous although the original nitrogene content in the slurry represents a higher commercial value. To keep the nitrogen one would have to oxidise the ammonia and probably some of that happens in composting process but much is lost. The only idea that I have heard about for dewatering without loosing nitrogen is reversed osmosis but I see practical problems and very high cost.

So much loss of nitrogen and possibly potassium seems difficult to avoid and one may have to worry about environmental effects. Ammonium is mainly lost to air, some is converted by bacteria to nitrate (the process has an acidifying effect) and nitrate can be leached out but also converted to N2 anad N2O. All this depends on local conditions like soil, type of crop and local climate.

FAQ: Can I burn the biogas directly inside a greenhouse?

Ii is not necessary to remove the CO2 from the biogas to burn it, but one should obtain a burner that burns biogas. The CO2 released will encourage plants in the greenhouse to grow faster, so it should be possible to release the exhaust gases into the greenhouse. With pig dung as the feed, there will be trace amounts of H2S in the biogas. This burns to SO2, but there is not enough to cause toxic problems. The main problem is that it can be absorbed into water condensed from the flue gases and cause corrosion.

The main problem with burning the gases inside the greenhouse is getting the heat balanced within the building. A burner will release large amounts of heat in one place, so you need a system to circulate this heat to get the whole greenhouse at a uniform warm temperature. A boiler can be used to heat water, which can be pumped around heating pipes. Alternatively, the flue gases can be mixed with a larger quantity of cold air, to get the correct temperature, before it is vented into the greenhouse via air ducts.

FAQ: Are there any risks of asphyxiation or explosion?

CO2 becomes toxic to humans at very low concentrations - this can limit even modest elevations meant to boost plant growth in glasshouses. The limit is at least a ten factor below the lower explosive limit for CH4. Assuming any slow leak is of roughly 50:50 CO2:CH4, you will reach the CO2 toxicity limit long before the CH4 explosive limit. A sudden rupture could of course exceed both in seconds.

The risk of explosions is linked to the presence of both fuel and O2, because sooner or later there will be a source of ignition. In fact often the source may be either something you've overlooked - or something you thought you had controlled. Often that's because years of safe operation has bred complacency and a casual attitude to risk - or an assumption that the controls will handle it.

FAQ: What methods are used to prevent the risk of explotions?

One rough & ready way of reducing the risk of an explosion might be to keep a biogas pilot light burning in the air space, since a gradually rising CH4 concn will burn in a flash flame before it reaches the explosive limit.

Another way is keeping a canary in a cage. It's an excellent idea and worth doing anyway.

Controlling the unintended release of gas would be the best thing to do. We recommend that all piping be tested with compressed air to locate and rectify any leaks. Leaks can be detected by applying soapy water to joints. Also if the air pressure holds constant then you have no leaks. (Expect an initial drop in air pressure due to cooling of the air.) Also extend the relief valve to the outside.

As for the ignition sources, eliminate any electrical switches, receptacles, motors, etc, in the greenhouse. Open flames from heaters or burners should be eliminated. Since it may be impractical to eliminate all sources of ignition, the best option may be to ventilate.

FAQ: What happens to chlorine during digestion?

It remains in the slurry. Many manures can contain up to 1% chlorine (dry basis). A good anaerobic digester is well-buffered, which means that it contains carbonate ions that can absorb carbon dioxide to become bicarbonate, which means the pH is kept at around neutral. If you have carbonate ions, there must also be metal ions, such as sodium, potassium and calcium. If there is free chlorine, or chlorine bound fairly loosely to organic compounds, it will react with the metal ions fairly quickly to form chlorides.

As far as engines are concerned, sulphur is more of a problem, as it emerges as hydrogen sulfide. If the biogas is used in an engine, this burns to make sulphur dioxide and does corrode the exhaust pipes. Any free chlorine in the gas will also form acid in the exhaust.

Most biogas engines are either dual-fuel (i.e. diesel) or are based on a diesel engine, but with a spark plug, so they are usually strong enough to cope with a bit of corrosion. Where gasoline engines have been used with biogas, they have had a fairly short life.

FAQ: What are the challenges in combining cow/chicken manure?

Anaerobic digesters seem to work better on mixed feeds than on single feedstocks. A mix of chicken and cattle has more advantages than problems. The cattle dung provides all the right bacteria, while chicken dung provides extra nitrogen and a richer feed (cattle are just too efficient at using all the foodstuffs in the grass they eat, so cattle dung is not the most productive of feedstocks). If other materials, such as waste food, are added, the gas production per kg of feed can be much higher. Mixed feeds also encourage a wider range of bacteria to develop within the digester, which gives greater stability for the system.

A danger of mixed feeds is that some materials need to be ground up, otherwise they can block the pipework. Chicken feathers and carcasses often end up in the dung, and they need to be chopped up. This is also true of food wastes. Even cattle dung can cause problems, as other materials can end up in the digester. If the dung is scraped up from the ground, it can contain stones and earth which can block pipes and fill up the digester. Cattle often eat stones to help in their digestion and these end up in the bottom of the digester and can cause problems in feed grinders.

FAQ: May the biogas bacteria provoke botulism?

There is a growing attention on this problem, which many farms are facing especially in Germany.

They are experiencing a sharp increase of botulism diagnosis in cattle and pigs. These animals are excreting bacterial forms with their faeces, and they harbour these pathogens in their guts. When faeces or gut contents from healthy slaughtered animals are used for biogas production it could happen that the digestate may contain these pathogens. More than 1000 farms are affected in Germany. Clinical cases have also been found in Austria, Switzerland, Holland and Belgium.

We test all faeces samples for botulism and all samples so far are negative. This is probably due to our production process (temperature and HRT in the digester).

FAQ: How can I calculate the carbon credits of waste reduction projects?

The input material would be naturally transformed in carbon dioxide and methane, plus H2S in percentages linked to different ambient conditions. If you know the organic matter of the waste, you can calculate the theoretical methane production, considering that :

1 kg of organic matter is 1.23 kg COD and 1 kg COD produces 350 NL of methane.

FAQ: Do I need to heat the process?

Methnogenesis in a biogas plant is an exenergonic reaction. So heat is generated but so slowly that it is a very minor factor in most digesters.

The dominant factors in the heat balance tend to be:

FAQ: Can we use wood to make biogas?

The problem of wood, as far as anaerobic > digestion is concerned, is that the bacteria do not degrade lignin. > However, the cellulose within the lignin structure can be degraded, given > the right conditions.

I have seen plants in Sri Lanka that generate biogas from straw, which gives a similar problem. The straw is packed tightly into a tank and sprayed with liquor from a working digester. The straw is only wetted, not flooded. The tank is sealed and produces gas after a week or so (Sri Lanka has tropical temperatures). The biogas is generated for a month or two. Once generation drops off, the tank is opened and emptied and refilled.

The first stage of digestion is hydrolysis. The hydrolysing bacteria are facultative, i.e. they can work in oxygen or without it. A water saturated compost heap will have an anaerobic center, with aerobic activity around the edges. It is possible that the aerobic bacteria (which are more aggressive) will break down the cellulose in the wood into soluble acids that can be used by the methanogens in the center to generate biogas.

The Sri Lankan approach would probably work with wood shavings as well as it does with straw. It might be worth a try. The wood could be aerobically composted for a while first. I am not sure when the cellulose gets to the point when the anaerobic bacteria can use the fatty acids. If you leave it too long, the acids degrade to carbon dioxide and water. If you do not leave it long enough, the cellulose has not broken down enough.

home >> clean >> waste >> manure