This document covers technical details and information gained during experimental work on running a small diesel engine using biogas produced from a cow manure biogas digester.
Background:
In May 2008, a 10 m³ fixed volume digester was installed at the RTC (Rural treatment centre - Hospital) located at 45 Km of the Laos Capital, Vientiane, 5 km from Road 13 North(1,2). The biogas is primarily used for cooking in both the doctors’ house and the hospital. However the remoteness of the site meant that it is “off grid” and they relied on a 10.5 Hp diesel engine driving an old generator for power, it was speculated that the biogas could be used to power the generator, thus relieving the hospital of some of the burden of the cost of diesel fuel.
Technical data:
The bio digester is a 10 m³ fixed volume construction (3) therefore relies on the anaerobic digestion and the domed shape of the roof to create the pressure. The Digester is fed with 100kg cow manure and 100 Litres water every morning. It could be said that, since the installation, the RTC have had some issues with the digester and it is not running at its optimum capacity (estimated about 40% - 50%) and the volume of gas (and hence the pressure).Also the quality of the biogas does vary depending on conditions, IE: heavy rain reduces the capacity significantly. On a good day 17 Kpa of pressure can be achieved.
Questions have been raised over the chemical make up of the biogas and its relative quality. At this stage without any suitable test equipment, is very difficult to assess, but it does seem to vary again depending on atmospheric conditions. A guess would put the methane component at about 50 %, the others being made up of water vapour, sulphur, Co2 and others.
Tests:
Test 1
January 2009
The initial tests were undertaken on the RTC’s diesel engine where an adapted air filter housing with a hose connection for the biogas an a ball valve( see picture 5). The generator was disconnected thus having no load. The condition of the engine is poor, they do no charge the air filter with oil and the fuel filter does not work. It is impossible to say what the compression ratio is, at a guess it could be as low as 13:1 or even 12:1, the result of this is the stoichiometric ratio is achieved a lot later in the stroke.
As mentioned before the quality of the biogas is undetermined, however there was a lot of water in the feed pipe which was resultant of condensed water vapour. The pipe needed to be drained for several minutes before attempting the test. The gas pressure was indicated at 12 Kpa. (1.7 PSI/ 50 “H2O)
The Engine was started on 100% diesel and ran for a few minutes and the valve on the air filter slowly opened to allow the ingress of the biogas.
The engine immediately spluttered and died. This was repeated several times the engine did not pick up on the biogas at all.
Test2:
February 2009
(See picture 10)
After some deliberation it was decided that the introduction of a mixing chamber could possibly help the combustion process. Methane particles and much smaller than air particles and hence would be required to be mixed prior to draw into the cylinder. (See pictures 6 to 10 ). The mixing unit consisted of a of a sealed chamber of approx 4 litre capacity split by a ‘nozzle’ plate ( a stationary impellor) with the gas inlet and valve in the upper chamber.
The gas line again had to be drained of water prior to the tests. The indicated pressure was 10 Kpa.
After initially starting on diesel the valve the biogas was introduced. It was noted that the spluttering and engine fade occurred at a specific point; the engine accepted the gas more with the mixer.
Another interesting observation was the change in the colour of the smoke when the valve is opened to let more biogas in (grey), then closed suddenly (black).
It is understood that the likely reason for this is the oxygen starvation caused by too much unignited biogas. The slowing of the engine causes the rack to be pushed to the end of travel increasing the diesel fuel input. The rapid closing of the biogas valve causing the increase in speed and the burning of excess diesel fuel, hence the black smoke. Ref: www.erawan-biogas.blogspot.com)
At a stable position of the biogas inlet (IE: just before spluttering and engine fade) we conducted a test to ascertain the ratio of biogas/diesel being used. This was achieved by rigging up a 100 cc plastic syringe which was filled with diesel fuel; the injector return line was also fed back into the top of the syringe. A time was taken for a known quantity of fuel to be consumed. This was repeated without the biogas.
Result: Ratio 15-20% biogas / 80% - 85% diesel.
Test 3.
November 2009
(see picture 14)
For the next test it was decided the only logical next step would be to try to improve the quality of the biogas. For this a crude filter was constructed using a 20 litre plastic bin filled with soaked wood chips. The bin was fitted with an inlet and an outlet connection which consisted of holed pipes which protruded though the bin and wrapped in sponge to keep the woodchips out of the pipes.
(See pictures 11 to 13)
Unfortunately this visit ,the Doctor sad he was unwilling for us to use his engine in tests, unbelievable the engine is now in a worse condition than in February.
An alternative engine was sourced this time an 11 hp Kubota diesel single cylinder. In good condition which had been used previously to run on bio diesel from Soya.(4) Because of this the engine was already equipped with a set of four off 100 cc syringes mounted in a frame above the engine. The diesel engine is on a bed integrated with an Onan 3 KV power plant driven through a duplex pulley with a 130:90 ratio. The generator remained in place for the duration of the test. The output power was not connected.
Because of the mass of the bed in total about 170 Kg. The unit was transported to the RTC on the back of a flat bed truck (Ute) where it remained for the duration of the test. The mixing unit utilised in test 2 was connected to the air intake of the engine.
The biogas was initially at 7.5 Kpa, the gas was dry (no condensed water vapour)
Test 3a. 100 % diesel running the diesel engine on 100% diesel , 46 seconds to consume 10 cc of fuel
Test 3b. Introducing biogas.
The most immediate response was an increase in engine speed. The throttle was to reduced to that approaching test 3 a to try to reflect a true test however this was only estimated by ear.
3 minutes 36 seconds to consume 10 cc of fuel: ration 80% biogas: 20% diesel
Test 3c. As 3b introducing the gas filter, unfortunately a reaction between the hose connections and the cement caused the threads to disintegrate, thus making a seal impossible. We could not conduct any further tests with the gas filter.
Gas pressure at end of Test 2; 2Kpa. Note the valve on the top f the tank had been turned off therefore the gas consumed in the test was the gas present only in the system. Also the gas had been utilised for cooking during this time.
Notes and conclusions:
1. From test 3 b it is concluded that a viable power source is capable using biogas however diesel fuel will always be needed for a. ignition and b. lubrication of diesel injectors. A literary search concludes that the optimum ratio for biogas/diesel fuel is 80:20.
2. The longer term issues of using biogas as source fuel such as the harmful effects of sulphur on vitreous products within the engine. This was one of the elements that the gas filter is expected to remove. Commercial desulpherisers are available at low cost and could be sourced if required.
3. Biogas input control: As it is envisaged that the appropriate use of the biogas/diesel engine configuration is thought to be for power generation. In this instance control of output of the engine is critical as the biogas pressure will drop over a period of time. There are numerous preliminary designs in progress for a cheap workable system; this would form stage 2 of the development.
4. Field testing of biogas: It was recognised that the success of test 3 could be attributable to two variables. Firstly the superior condition of the replacement engine (higher compression) the second being the quality of the biogas. At present we have no practical way of testing the biogas .
Gordon Hirst
Engineers Without Borders ( Victorian Chapter )
November 2009
References and Acknowledgments:
1. Many thanks Master Kampuu & RTC for there contribution to these experiments
2. The digester was constructed with the aid of the Australian Embassy - Laos (Direct aid program (DAP) through the offices of SEDA-Laos (www.seda-laos.org)
3. Bio digester constructed as part of the Laos Biogas Pilot Program.The program is being implemented by the Department of Livestock and Fisheries, within the Ministry of Agriculture and Forestry, with support from SNV Netherlands Development Organisation, and with funding from the Government of The Netherlands. www.biogaslao.org
4. Many thanks to LTE for the use of Kubota diesel/ generator set and the use of the Ute.
1. The 10 m³ biogas digester
2. Building the Digester
3. Digester location @ RTC
4. The RTC diesel generator set
5. Test 1; biogas direct into air filter
6. Test 2; Biogas air/gas mixer
7. The nozzle/impellor for mixer
8. Making Mixer (1)
9. Making Mixer (2)
10. Test 2
11. Making gas filter (1)
12. Making gas filter (2)
13. Making gas filter (3)
14.Test 3
