Operation of a Biogas Plant
The economic efficiency of a properly planned biogas plant is determined by the availability and capacity utilisation of the process as a whole. Key factors are the functionality and operational reliability of the technology employed, and consistently high degradation performance within the biological process. As the operation of technical facilities is subject to inevitable malfunctions, appropriate tools must be on hand in order to detect such malfunctions and identify and rectify the fault. Process control is always performed in interaction with the personnel, although the degree of automation can vary extremely widely. If monitoring and control algorithms are automated, the benefits are that the system is constantly available and a degree of independence from expert personnel is achieved. The remote transmission of data also decouples the need for staff presence at the plant from process monitoring. The following sections first examine the measured variables that can be used to observe the biological process. The descriptions relate to wet fermentation plants. Biogas production rate.
The primary purpose of the operating parameters of a biogas plant is to describe the load situation.
It is only during the start-up process that the parameters can help with plant control in terms of achieving a slow, steady rise. Normally, most attention is paid to the organic loading rate. In the case of plants with large volumes of liquid on the input side and a low content of degradable organic material (slurry plants), the retention time is more important.
Organic Loading Rate (OLR)
The aim is to obtain optimum degradation performance at acceptable economic cost (rentention time, sizing of the digester).
In this regard the organic loading rate (OLR) is a crucial operating parameter. It indicates how many kilograms of volatile solids (VS, or organic dry matter) can be fed into the digester per m3 of working volume per unit of time. The organic loading rate is expressed as kg VS/(m3 · d). The organic loading rate can be specified for each stage (gas-tight, insulated and heated vessel), for the system as a whole (total working volumes of all stages) and with or without the inclusion of material recirculation. Changing the reference variables can lead to sometimes widely differing results for the organic loading rate of a plant.
⇒ Organic loading rate (OLR) [kgVSm-3d-1] m = amount of substrate added per unit of time [kg/d];
c = concentration of organic matter (volatile solids) [%VS]; VR = reactor volume [m3])
Hydraulig Retention Time (HRT)
Another relevant parameter for deciding on the size of vessel is the hydraulic retention time (HRT). This is the length of time for which a substrate is calculated to remain on average in the digester until it is discharged. Calculation involves determining the ratio of the reactor volume (VR) to the volume of substrate added daily. The hydraulic retention time is expressed in days.
⇒Hydraulic Retention Time [d] VR= reactor volume [m3]; V= volume of substrate added daily [m3/d])
The actual retention time may differ from this. There is a close correlation between the organic loading rate and the hydraulic retention time. If the composition of the substrate is assumed to remain the same, as the organic loading rate rises more input is added to the digester, and the retention time is consequently shortened. In order to be able to maintain the digestion process, the hydraulic retention time must be chosen such that constant replacement of the reactor contents does not flush out more microorganisms than can be replenished by new growth during that time (the doubling rate of some methanogenic archaea, for example, is 10 days or more). It should also be borne in mind that with a short retention time the microorganisms will have little time to degrade the substrate and consequently the gas yield will be inadequate. It is important to adapt the retention time to the specific decomposition rate of the substrates.
Monitoring the Biological Process (Parameters)
Disturbance Management of Biogas Plants
The Problem of Scum
If there is heavy gas release from the inlet but not enough gas available for use, a thick scum layer is most likely the reason. Often the gas pressure does not build up because of the continuous gas release through the inlet for weeks. There is a danger of blocking the gas pipe by rising scum because of daily feeding without equivalent discharge. The lid (or man-hole) must be opened or the floating drum removed and scum is to be taken out by hand.
Occupational Safety and Plant Safety
Biogas is a gas mixture consisting of methane (50-75 vol. %), carbon dioxide (20-50 vol. %), hydrogen sulphide (0.01-0.4 vol. %) and other trace gases. In certain concentrations, biogas in combination with atmospheric oxygen can form an explosive atmosphere, which is why special plant safety regulations have to be observed in the construction and operation of a biogas plant. There are also other hazards, such as the risk of asphyxiation or poisoning, as well as mechanical dangers (e.g. risk of crushing by drives). The employer or biogas plant operator is obliged to identify and evaluate the hazards associated with the biogas plant, and if necessary to take appropriate measures. The 'Sicherheitsregeln für Biogasanlagen' (Safety Rules for Biogas Systems) issued by the Bundesverband der landwirtschaftlichen Berufsgenossenschaften (German Agricultural Occupational Health and Safety Agency) provide a concise summary of the key aspects of safety relevant to biogas plants. The safety rules explain and substantiate the safety requirements in terms of the operating procedures relevant to § 1 of the accident prevention regulations 'Arbeitsstätten, bauliche Anlagen und Einrichtungen' (Workplaces, Buildings and Facilities) (VSG 2.1). issued by the Agricultural Occupational Health and Safety Agency. They also draw attention to other applicable codes of practice.
This section is intended to provide an overview of the potential hazards during operation of a biogas plant and raise awareness of them accordingly. The latest versions of the respective regulations constitute the basis for the hazard assessments and the associated safety-related aspects of plant operation.
Fire and Explosion Hazard
As mentioned in the previous section, under certain conditions biogas in combination with air can form an explosive gas mixture. It should be borne in mind that although there is no risk of explosion above these limits it is still possible for fires to be started by naked flames, sparks from switching electrical equipment or lightning strikes. During the operation of biogas plants, therefore, it must be expected that potentially explosive gas-air mixtures are liable to form and that there is an increased risk of fire, especially in the immediate vicinity of digesters and gas tanks. Depending on the probability of the presence of an explosive atmosphere, according to BGR 104 – Explosion Protection Rules the various parts of the plant are divided into categories of hazardous areas ('Ex zones'), within which the relevant signs must be prominently displayed and appropriate precautionary and safety measures taken.
Zone 0 In areas classified as zone 0, an explosive atmosphere is present constantly, over long periods, or most of the time. Normally, however, no such zones are found in biogas plants. Not even a fermentation tank/digester is classified in this category.
Zone 1 Zone 1 describes areas in which an explosive atmosphere can occasionally form during normal operation. These are areas in the immediate vicinity of manholes accessing the gas storage tank or on the gas-retaining side of the fermentation tank, and in the vicinity of blow-off systems, pressure relief valves or gas flares. The safety precautions for zone 1 must be put in place within a radius of 1 m (with natural ventilation) around these areas. This means that only resources and explosion-protected equipment with zone 0 and zone 1 ratings may be used in this area. As a general rule, the operations-related release of biogas in enclosed spaces should be avoided. If it is possible that gas will be released, however, zone 1 is extended to include the entire space . Zone 2 In these areas it is not expected that explosive gas-air mixtures will occur under normal circumstances. If this does in fact happen, it can be assumed that it will do so only rarely and not for a lengthy period of time (for example during servicing or in the event of a fault).
This applies to manholes, for example, and the interior of the digester, and in the case of gas storage tanks the immediate vicinity of aeration and ventilation openings. The measures applicable to zone 2 must be implemented in these areas in a radius of 1 to 3 m. In the areas subject to explosion hazard (zones 0-2), steps must be taken to avoid ignition sources in accordance with BGR 104, section E2. Examples of ignition sources include hot surfaces (turbochargers), naked flames or sparks generated by mechanical or electrical means. In addition, such areas must be identified by appropriate warning signs and notices.
Danger of Poisoning and Asphyxiation
The release of biogases is a natural process, as is well known, so it is not exclusively restricted to biogas plants. In animal husbandry, in particular, time and again in the past there have been accidents, some of them fatal, in connection with biogenic gases (for example in slurry pits and fodder silos etc.). If biogas is present in sufficiently high concentrations, inhalation can produce symptoms of poisoning or asphyxiation, and can even prove fatal. It is particularly the hydrogen sulphide (H2S) content of non-desulphurised biogas that is highly toxic, even in low concentrations. In addition, especially in enclosed or low-level spaces, asphyxiation can occur as a result of the displacement of oxygen by biogas. Although biogas is lighter than air, with a relative density (D) of roughly 1.2 kg per m3, it tends to segregate. In this process the heavier carbon dioxide (D = 1.98 kg/m3) collects close to floor level, while the lighter methane (D =0.72 kg/m3) rises.
For these reasons it is essential that adequate ventilation is provided at all times in enclosed spaces, for example enclosed gas storage tanks. Furthermore, personal protective equipment (e.g. gas alarms, respiratory protection etc.) must be worn in potentially hazardous areas (digesters, maintenance shafts, gas storage areas etc.).
Other Potential Accident Risks
In addition to the sources of danger described above there are also other potential accident sources, such as the risk of falling from ladders or falling into charging holes (solids metering equipment, feed funnels, maintenance shafts etc.). In these cases it must be ensured that falling into such openings is prevented by covers (hatches, grids etc.) or by installing them at a sufficient height (> 1.8 m) [5-6]. Moving plant parts (agitator shafts, worms etc.) are also potential danger points, which must be clearly identified by appropriatesignage. Fatal electric shocks can occur in and around combined heat and power units as a result of incorrect operation or faults, because the units generate electrical power at voltages of several hundred volts and with currents measured in hundreds of amperes. The same danger also applies to agitators, pumps, feed equipment etc. because these also operate with high levels of electrical power. The heating and cooling systems of a biogas plant (radiator, digester heater, heat exchanger etc.) also present a risk of scalding in the event of malfunctions. This also applies to parts of the CHP unit and any emergency systems that may be installed (e.g. gas flares).
In order to prevent accidents of this type, clearly visible warning signs must be displayed at the appropriate parts of the plant and the operating personnel must be instructed accordingly.
7 Rules to Run a Digester
- Maintain constant feed rate and composition
- Avoid overfeeding and abrupt changes
- Avoid foaming, don´t: add a substrate with a low pH value, add substrate with a considerably high protein content, overfeed the digester, mix improper and/or inconsistent
- Mix as much as necessary and as little as possible: Keep in mind: all in = all out
- Maintain continuous mixing
- Choose an appropriate temperature
- Keep the temperature constant