Renewable Energy Technology 3(2+1)

Lesson 14. Problem identification and rectification of biogas plants

 

S. No.	Defect	Reason	Remedy
1.	Crack in the digester wall which normally occurs in floating drum (KVIC) and fixed dome Janata models.	(a)Improper soil settlement (b)  Defect in the foundation-concrete mixture (c) defect in the construction of digester wall, e.g., improper laying of bricks, mortar proportion and application (d)  Use of inferior quality materials (e)  sliding of earth (f)  improper back filling.	By adding water 2 to 3 times a day, the leakage gets stopped in 2 to 3 days.  If there is no improvement after 3 days or so, it indicates that the crack is wide.  In case of suspected wide crack(s) the entire slurry in the digester is required to be pumped out and after cleaning the inside of the wall the cracked portion(s) are first marked. From all around the crack about 12" of plaster must be chipped out carefully from top to bottom and the gap of the crack should be filled up with cement mortar of 1: 4 proportion.  Then fix the chicken mesh to cover the entire area chipped around the crack and plaster with 1:4 cement mortar or rebuilding of the digester wall will be necessary.
2.	Gas leakage in floating drum of KVIC model or fixed dome of Deenbandhu model	(a)  In a floating drum of KVIC model gas leakage would be due to the hole in the gas holder. This hole would be generally at the welded joints of sheets if the gas holder is new and in any part of the M.S. gas holder due to rusting or corrosion if the gas holder is old Leakage in FRP gas holder can occur due to small holes or cracks. (b) In the fixed dome the leakage could be due to improper construction, improper mortar, improper curing etc.	(a) In case of floating drum, M.S. drum would be welding, sonic durable adhesive can also be applied for quick rectification.  If the problem is with FRP gasholder rectification is done by putting fibre glass mat a the points of leakage and then applying resin. (b)In case of fixed dome, If the leakage is only in one or two points, the plaster on the outer surface of the dome at those points of leakage should be removed with care.  A thin paste of a mixture of sodium silicate and cattle dung in equal proportion should be prepared by adding water. This paste should first be applied on the points of leakage and then a plaster of cement mortar of 1: 3 is provided.  If the leakage is in several points, the entire plaster on the outer surface of the dome should be done with a fresh 1:3 cement mortar.
3.	Large gap between the digester wall and gas holder (KVIC Model)	(a) Defective construction without checking the diameter after completing the layer of each course of brick construction. (b)  The gas holder was made smaller than the required size.	(a) If the gap is not very wide (less than 3') no correction need to be attempted. However, if the gap is very wide, then a cement concrete ring may be inserted as a collar around the gasholder leaving enough space for the gasholder to move up and down. (b)  Wherever a strong ledge has been constructed the gap can be corrected by building additional brick line over the ledge to cover the gap.  This must be done after removing the plaster and making sure that the additional brick line is jointed to the digester wall properly. The gasholder of wider diameter can be exchanged for the smaller one.
4.	No gas is formed after first filling	If it is floating drum model inflation of gasholder indicates the collection of gas. In fixed dome rise in slurry level in the pressure chamber indicates collection of gas.  If there is no such indication as stated above, it means that either the gas is not generated or the gas is leaking.	1.If the  weather is cold, (winter season) it takes more time for generation of gas. 2.   If the gas is leaking, check the gas holder/dome with soap water, then Stop the leakage. 3.   If-there is leakage in the pipe joints,  tighten the joints. 4.  It is possible that the gas tap/ cock are kept open and hence gas escapes.  Close the tap/cock and check.
5.	First collection of gas does not burn	More of air or more of CO2 in the first collection	First collection of gas generally does not burn. Second collection also may or may not burn well. Allow the gas to escape till there are clear indications of burning.  Third collection would burn normally.
6.	Gas exhausts very quickly after opening of main gas valve	Gas tap fitted to burner/lamp is open. 2.There may be a considerable leakage in pipe line. 3.  Higher capacity burner is used. 4. Gas pressure is high	1.Close the gas tap. 2.   Mainly check T-joints in the pipeline including the one provided for removal of condensed water. Stop the leakage by putting plug or by whatever other method that would be appropriate. 3. Replace with suitable burner. 4.  Adjust the burner cock suitably.
7.	Generation of gas is very slow	1. Low temperature and pH 2.  Quantity of animal waste daily feeding is less 3.  More water is added 4. Formation of scum 5. Gas leakage	1. In winter, particularly when the ambient temperature is very low, the gas production will slow down since methanogens are very sensitive to temperature and pH.  The remedies are, (a) create green house effect by covering the biogas plant by solar canopy (b) use warm water for daily feeding, preferably by putting the slurry in the mixing tank and       allowing it to warm by sun rays. (Direct solar heating)  (c) adjust pH by adding lime water, 2. Add correct quantity, 3. Don't use more water than specified either in daily feeding or otherwise, 4. In case of floating drum model gas holder is to be removed and scum is taken out and gas holder fixed again.  In case of fixed drum model some stirring by any method may be of some help. If the problem is severe, the entire slurry may have to be taken out and recharged, 5.  Locate the points and stop.
8.	Gas generation has stopped after the unit has worked  for some months/years.	1. The slurry has become acidic (i.e. lowering of pH). This would be due to overfeeding of digester. 2. Toxic materials like soap, detergent, pesticide etc. might have entered the digester	1. Below pH 6.2 biogas generation stops practically. Addition of water from inlet will help in bringing down the solid content and slight increase in pH level. Since there is buffering effect of cattle dung, this simple process may make the unit to become normal. If there is no improvement, then add in small dose mild alkali (CaOH, NaOH) through the inlet and some stirring is done. At regular intervals measure pH, till it goes slightly above 6.2 and then leave the system to get normalized with time. 2. If little quantity of toxic material has entered, the digester continues daily feeding as usual and wait for normalisation in generation of gas. If the quantity of toxic material that has entered the digester is high, gas production stops completely.  Best remedy in such a case would be to empty the digester and recharge. Care should be therefore taken not to allow toxic substances to enter the biogas digester.
9.	Gas does not burn	1. If it is in the initial stage of charging, it is due to presence of more air and/or CO2, 2.   There could be obstacle in the pipe line at any point upto the burner nozzles. The gas is, therefore, not reaching the burner ports, Too much of primary air getting mixed with gas	1. Allow the gas to escape. Try next collection, 2. Start checking and cleaning from the burner. Clean the nozzle and injector. Then check pipe line. One common obstacle for flow of gas is condensed water. Drain it out through the water trap.  Even after this, if the gas does not flow, pipe line has to be checked for any solid material obstructing the flow of gas, 3. Adjust the air shutter.
10.	Flames are long and yellow	1. If it is floating drum model, insufficient primary air is mixed with gas. 2. If it is fixed dome model, there is high pressure.	1. Adjust the air shutter, 2. Operate the gas valve to reduce the height of the flame and adjust the air shutter.
11.	Flames are not steady	Generally due to condensed water in the pipe line.	Remove the condensed water in the pipeline.
12.	Poor illumination of lamp	1. Low gas pressure 2. Obstruction between gas regulator and ventury or the pin is not operating properly.	1.   If it is floating drum model put additional weight on gas holder. If  it is fixed dome model wait for pressure build up. 2. Clean the passage and make the pin operate properly
13.	Slurry not passing through inlet pipe	Chocking in inlet pipe.	Insert a bamboo inside the inlet it pipe and move it up and down. Take care not to damage the pipe.
14.	Slurry in the digester of floating drum model  overflows from the top.	1. Chocking of outlet, 2. Outlet is above the level of digester.	1. Remove the blockage using   bamboo, 2. Lower the outlet level.
15.	Slurry water comes through the gas outlet pipe.	Either slurry level inside the digester is high or gas pipe inserted inside the digester or too much frothing is there	Main remedy will be to reduce the slurry level inside the digester by  adjusting the slurry outlet passage, This kind of problem is encountered in case of fixed dome models and not with KVIC model

(Source : TNAU-Bioenergy-BDTC manual)