Technical Report

Process monitoring in biogas plants

December 2013

1.1 Why is process monitoring necessary?

Biogas plants are biological systems involving various interacting microorganisms that anaerobically degrade organic matter. The main product is biogas, a gas rich in methane (CH4 ) that can be used as a renewable fuel for vehicles or to generate heat or electricity for local use or for use via energy distribution grids. The degradation involves four consecutive biological processes: hydrolysis, acidogenesis, acetogenesis and methanogenesis (see Figure 1). If one of these processes is negatively affected in any way there is an immediate influence on the other processes and the biogas plant can become unstable. Typical process failures include, among others, organic overload, hydraulic overload and ammonia inhibition (see section 2 for details).

Process monitoring can help to understand what happens in a biogas plant and help to maintain a stable process. In many cases, a strongly inhibited microorganism population or a total crash of the whole plant can have severe financial consequences for the biogas plant operator.

• In general, process monitoring can help to:
• give an overall picture of the biogas process
• identify upcoming instabilities in anaerobic digesters before a crash happens
• accompany a successful start-up or re-start of a plant

The costs of basic monitoring are often much lower than the costs and lost revenue associated with re-establishing a biologically destabilised plant. For example, if a biogas plant has totally crashed it may have to be emptied and filled again with new inoculum. This, together with the necessary start-up period, means that several months can be lost during which the plant could have operated at full load (Henkelmann et al., 2010). The financial consequences can be devastating for the plant operator.

1.2 What is meant by process monitoring in this brochure?

This brochure focuses on the monitoring of parameters that are concerned with stability of the anaerobic degradation process. These parameters are mainly driven by biological interactions and as a result the monitoring of a biogas plant is very different from many other industrial processes. This brochure describes the different monitoring methods, the way they are applied and how monitoring data are obtained.

In addition, advice regarding the amount and frequency of monitoring is given. In addition to the biological parameters, there are also technical parameters that need to be monitored in a biogas installation. This means a regular check of the functionality of equipment (e.g. pumps, valves, CHP – combined heat and power plant, etc.). Another important point to take into account is the monitoring of plant safety, for example emissions of explosive gas mixtures and toxic gases (e.g. hydrogen sulphide). Whilst important, monitoring of technical equipment and safety is not described in this brochure. Another aspect not covered in this brochure is detailed process optimisation of, for example, gas production and economic performance.

1.3 How can process monitoring be established?

Every biogas plant develops its own unique process conditions and as a result there is no single value for each process parameter that can be referenced to all plants. For each plant it is therefore important that values of relevant process parameters, such as temperature and pH, are established during stable operation. By recording these process parameters over the life of the plant, any change from “normal” can be identified quickly. Apart from recording these parameters, general process information such as mass of input, organic loading rate and operational problems should be documented (Schriewer, 2011). Whilst much of this information is recorded automatically in automated plants, it is recommended to keep a manual operational logbook.

Apart from the off-line analysis of parameters, which means analysis of samples in a laboratory, a minimum of on-line process monitoring equipment will have to be installed in every biogas plant. In general, the level of investment in on-line equipment should always be made in relation with the economic risks in the biogas plant (Henkelmann et al., 2010).