Phase 6 Operation & Maintenance
When the biogas plant has started its operation, which daily, monthly or yearly measures are necessary for control, maintenance and for securing substrate and fuel provision? From now on the farmer has to do regular controls and maintenance to assure security, safety (especially concerning emission standards) and efficiency.
For a farm scale biogas plant, with an electrical power capacity (equivalent) of up to 75 kWel, the labour time (net) for operating and maintenance is usually approx. 1.8 hours per day [KTBL 2013].
Figure: Aspects for plant operation [Castillo et al 2012, modified]
Figure: Most frequently incidents documented on 31 German biogas plants over one year period [KTBL 2009]
The required measures/tests (either on a day to day basis or in case of problems) should have been listed and detailed in the plant instructions of the manufacturer who built and installed the facility.
An analysis of practice data from operations diaries of 31 German biogas plants showed that within a year a total number of 1,168 operational disturbances were documented by the operators [KTBL 2009]. It was found that the plant component CHP-Unit, solid substrate feeder, pumps and agitators were the most susceptible parts. The qualitative evaluation of this disturbances showed, that the biological process has been the fifth most frequent reason (see Figure 9) of malfunctions.
For this example, 4,282 working hours were necessary to solve all the malfunctions. This corresponds to an average of 138 working hours per biogas plant and year. On average for every 10 kW of installed electrical power 1.2 malfunctions occurred per biogas plant and year.
These results point out the importance of consequent process control. Most significant indication for a process disturbance is a noticeable decrease of biogas yield respectively methane concentration. In contrast to technical problems – which can regularly be solved quite fast – process disturbance are more difficult to correct and require a basic understanding of the farmer for the biological processes and inhibition of the anaerobic fermentation.
Safety of biogas plants
Construction and operation of a biogas plant is related to a number of important safety issues, potential risks and hazards for humans, animals and the environment. Taking proper precautions and safety aim at avoiding any risks and hazardous situations, and contribute to ensuring a safe operation of the plant.
Fulfilment of important safety issues and stipulating clear preventive and damage control measures is a condition for obtaining the building permit (this may differ depending on country):
- fire & explosion prevention
- mechanical dangers
- sound statically construction
- electrical safety
- lightning protection
- thermal safety
- noise emissions protection
- asphyxiation, poisoning prevention
- hygienic and veterinary safety
- avoidance of air polluting emissions
- prevention of ground and surface water leakages
- avoidance of pollutants release during waste disposal
- flooding safety
- collision and tear-off protection
Apart from poisoning and asphyxiation, there are other potential dangers related to the activity on a biogas production site. In order to avoid these types of accidents, clear warnings must be placed on the respective parts of the plant and the operating personnel must be trained [Seadi et al 2008]:
- Other potential sources of accidents include danger of falling from ladders or uncovered areas (e.g. feed-funnels, maintenance shafts) or to be injured by movable parts of the plant (e.g. agitators).
- Equipment like agitators, pumps, feeding equipment is operated with high electrical voltage. Improper operation or defects of the CHP unit can result in fatal electric shocks.
- Risks of skin burning through unprotected contact with the heating or cooling systems of the biogas plant (e.g. motor coolers, digester heating, and heat pumps) must be considered. This also applies to parts of the CHP unit and to the gas flare.
For these reasons, it is advisable that operators and staff of a biogas plant are trained on plant safety. This qualifying measure raises the awareness of the operator for potential hazards on site, it helps to safely operate the plant and to establish safe procedures if external companies need to work on the biogas plant.
Most of the controls may be done continuously by control and monitoring systems (like temperature of the reaction, the amount of substrates, the quantity of gas/electricity/heat produced, etc.) but others might require expert support or can be done by the farmer himself (e.g. resolve liquid leakage at pumps, oil changing at CHP, small repairs etc.).
The farmer has to ensure maintenance and observe the maintenance intervals (important for warranty of plant parts) of the biogas plant and the downstream equipment. Some maintenance can be done by the famers himself (e.g. scheduled replacement of wear parts as filters, seals and replacing or replenishing supplies or consumables like engine oil or water) or by certain service providers (e.g. general overhaul of CHP unit).
Measuring is the precondition for process control and optimisation. But the necessary measurement equipment creates costs, which often – especially on small scale installations – tried to be avoided during implementation.
Consequent documentation is the only way to get reliable information on status and efficiency of the biogas production process. The collection of data over a longer period is not only necessary for self-control, but also relevant in case of troubles diagnostics and consultancy by external experts.
As a very simple, effective and less laborious method is the so called „input or operations diary”. Furthermore in some countries (for example Germany) data on input of the plant must be available due to legal requirements.
Advantages of an input diary are [Gomez 2008]:
- traceability of quantity and quality of input substrates (also supplier where needed)
- control and optimisation of volume load
- economic efficiency calculations (supplier, gas demand, electricity demand, operation hours)
- process control
- gas leakage detection
The input diary should include the following information:
- amount of every substrate or substance given into the process
- process temperature
- gas quality (CH4, CO2, H2S)
- gas yield
- amount of gas utilized
- power production (net)
- operating hours
- fuel demand (in case of pilot injection gas engine)
- electricity injected to the grid
- services, disturbances