Biogas plant KPIs: the indicators that show whether the plant is running well
CHP availability, methane yield from VS, OLR, HRT, FOS/TAC, own consumption and heat utilisation: a set of indicators with formulas, response thresholds and an example of a monthly report.
Other language versions: polski.
A biogas plant runs well when three groups of indicators — process, energy and economic — remain within a known, stable range. The most important KPIs are: OLR and HRT, methane yield per unit of VS, the stability of FOS/TAC and pH, CHP availability and efficiency, own energy consumption and the degree of heat utilisation. A single number says little; what matters are trends and reference to the plan and to your own history.
Process indicators
| KPI | Formula | What to look at |
|---|---|---|
| OLR | kg VS/day / m³ of working volume | the trend and the stability limits |
| HRT | volume / daily flow | a fall below the safe value |
| CH₄ yield from VS | m³ CH₄ / kg VS fed | a fall means a disturbance or a poorer substrate |
| CH₄ content of the gas | % vol. | a fall together with FOS/TAC is a signal of acidification |
| FOS/TAC, pH, VFA (LKT) | from analyses | keeping within the working range |
| H₂S content | ppm before and after desulphurisation | the effectiveness of desulphurisation |
Energy indicators
| KPI | Formula | Notes |
|---|---|---|
| CHP operating hours | h/year | as close as possible to availability |
| CHP availability | ready hours / calendar hours | in other definitions, taking planned inspections into account |
| Capacity utilisation | energy produced / (capacity × hours) | takes part-load operation into account |
| Electrical efficiency | electricity / fuel energy | measurement of gas flow and its calorific value |
| Own consumption | energy used in the installation / energy produced | makes it possible to detect inefficient mixing and pumps |
| Heat utilisation | useful heat / available heat | seasonality |
| Flaring | gas flared / gas produced | target: as close to zero as possible |
Economic indicators
- operating cost per kWh or m³ CH₄ (OPEX/production),
- substrate cost per kWh,
- revenue per tonne of substrate,
- substrate consumption relative to the plan,
- servicing cost per CHP operating hour.
Calculation example
Assumptions (illustrative): in a month 1,650 t of fresh matter was fed, of which VS was 270 t (270,000 kg). 1.02 million m³ of biogas with an average CH₄ content of 53% was produced, i.e. 540,600 m³ CH₄. Yield: 540,600 / 270,000 ≈ 2.0 m³ CH₄/kg VS — an unrealistic result, because yields from typical agricultural substrates do not usually exceed about 0.35–0.5 m³ CH₄/kg VS. Such a discrepancy means an error in the data: an understated VS mass, an overstated gas measurement or a mistake in the units. In this example the cause turns out to be the gas meter reading: the cumulative reading was entered instead of the monthly increment. Actual production was 173,000 m³ of biogas, i.e. about 91,700 m³ CH₄, and the yield 91,700 / 270,000 ≈ 0.34 m³ CH₄/kg VS — a typical value for a mix of silage and slurry. Checking the yield as a KPI therefore also serves to catch measurement errors: a result that is too good is just as much a signal to check the data as one that is too poor.
Response thresholds
| Indicator | Threshold triggering action (example) |
|---|---|
| FOS/TAC | a rise of more than 0.1 per week |
| CH₄ in the gas | a fall of more than 2 pp relative to the norm |
| CHP availability | below the set monthly level |
| Own consumption | a rise of several percentage points without a cause |
| CH₄/VS yield | a fall of more than 5–10% relative to the average |
Set the threshold values on the basis of your own historical data.
A one-page monthly report
Elements: the substrate mix and masses, OLR/HRT, gas and energy production, CH₄ yield, gas composition, FOS/TAC and pH (min–max), failures and downtime, own energy consumption, the quantity of heat sold, servicing costs, conclusions and actions. Add 12-month trend charts. Read the report with the team; the very fact of discussing it improves discipline.
Data quality
- calibration of gas meters and analysers according to schedule,
- consistency of units (Nm³ under normal conditions),
- weighing substrates (a weighbridge) instead of estimating,
- regular TS and VS analyses,
- recording events that affect the data (e.g. a sensor failure).
KPIs are not meant to be used to hold people to account, but for quickly detecting deviations and supporting decisions, e.g. whether an investment in desulphurisation or in replacing the agitators pays off.
The improvement cycle
KPIs make sense when they fit into a cycle: measurement, analysis, decision, implementation, assessment of the effect. Example: a rise in own consumption of several percentage points leads to a check of the agitators and pumps, a change in the operating mode, and a repeat measurement after a month. Record decisions and their effects — after a year you will have a knowledge base on which actions actually improve results.
Choosing indicators to suit the scale
A small installation does not need several dozen indicators. Start with 8–10: gas and energy production, CH₄ yield from VS, CH₄ in the gas, FOS/TAC, CHP availability, own consumption, heat utilisation, servicing cost per hour. Only when the team uses them regularly should you add more.
Sources
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