Optimization of the Biogas Production Process

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Overview

The aim of optimisation is to adjust the actual state of a process with regard to a certain property through selective variation of influencing factors in such a way as to achieve a defined target state (the optimum). In general terms, operation of a biogas plant can be optimised in three areas: technical, economic and environmental (see picture). These areas cannot be optimised independently of each other; on the contrary, they mutually influence each other. Furthermore, when it comes to solving an optimisation problem it should not be assumed that there will be a single solution, but rather it should be expected that there will be host of different solutions. The various possible solutions can then be compared with each other on the basis of evaluation criteria.

The criteria used for evaluation can include costs, for example, or gas yield, or minimisation of environmental impacts. Depending on the overriding objective the evaluation criteria then need to be weighted, so that a final assessment can be made and a decision
taken on which course to follow. In practice, every responsible operator of a biogas plant should aim to achieve the overall optimum that is attainable under the given general conditions, including those applying specifically to the particular plant. If the conditions change, the operator must assess whether the previous targets can be retained or need to be adapted. A precondition for optimisation is that the actual state and target state must be defined. Definition of the actual state is achieved by collecting appropriate data in the course of operation of the plant. If it is intended that the plant's own power consumption should be reduced, for example, the operator needs to find out which components contribute to power consumption and what quantities are consumed. The target state can be defined on the basis of planning data, comparable performance data for the technologies used in the plant, publications on the state of the art, information from other operators (e.g. forums, expert discussions etc.) or reports drawn up by independent experts.

Once the actual and target states have been defined, the next steps are to define specific target values, put measures into practice to achieve those targets and subsequently validate the measures to ensure that the targets are achieved and determine possible consequences for other areas of the plant. In many plants there are shortcomings in relation to the acquisition and documentation of relevant process data in particular, so proper analysis of the actual situation is often not possible. It follows, therefore, too, that only limited data is available for the generation of comparative values. A comprehensive collection of process-relevant data has been assembled as part of the German biogas measuring programmesand the KTBL (Association for Technology and Structures in Agriculture) also publishes key performance indicator data pertaining to the operation of biogas plants.

VDI Guideline 4631, Quality criteria for biogas plants, lists the KPIs for process evaluation. It also includes extensive checklists that are useful for data acquisition. A selection of the parameters that can be used for assessing and subsequently optimising a biogas plant are explained in the following.A general rule when running the plant is that the operating conditions should be kept constant if at all possible. This is the only way that a meaningful actual state can be defined at all. If a change of concept is implemented at the plant, the process targets must be adapted accordingly.

Technical Optimization

The optimisation of technical procedures in a biogas plant is aimed at raising the availability of the technology, in other words at minimising downtimes and ensuring smooth management of the process. This objective also has indirect consequences for the economics of the plant, of course, because the plant can only meet its performance target if it has a high capacity utilisation rate. On the other hand, a high level of technological input means high cost, so a cost-benefit analysis should be performed in the context of economic optimisation. As a general rule, in order to assess the availability of the plant as a whole it makes sense to record and document the operating hours and full-load hours . If in addition to that the downtimes and the associated causes of the malfunctions are documented, together with the hours worked and financial cost of correcting the malfunctions, the weak points in the process can be identified.

In very general terms, the availability of technical facilities can be increased by adopting the foll owing regime:

• Keep to maintenance intervals
  
• Perform predictive Maintenance
  
• Install measuring equipment to detect disturbances
• Stock important spare parts
• Ensure service from the manufacturer or regional workshops is available at short notice
• Use redundant design for critical components
• Use low-wear technologies and materials.

A prerequisite for a stable decomposition process is that the technology remains functional. If outages occur during charging of the digester or during mixing, the biological process is directly affected. For more details of optimising the biological process, see chapter 2 and the relevant sections of this chapter. If the plant is operating at a high utilisation rate, in certain circumstances it may be possible to increase efficiency by looking at the plant's power demand and investigating and if possible reducing any energy losses. It makes sense here to consider the plant as a whole in order to identify the key energy flows and weak points.

The following separate areas need to be taken into consideration:

• Substrate supply (quantity and quality of the substrate, quality of ensilage, feeding of the substrate)
  
• Silage loss (quality of ensilage, feed rate, size of cut surfaces, seepage water)
• Process biology (feeding intervals, degree of degradation achieved, specific biogas production rate and composition, stability of the plant, substrate composition, acid concentrations)
• Gas utilisation (efficiency of the CHP unit (electrical and thermal), methane slip, engine settings, maintenance intervals)
  
• Fermentation residue (residual gas potential of the fermentation residue, utilisation of the fermentation residue)
  
• Methane losses (emissions from leakage)
• Workload for plant operation and troubleshooting, downtimes
• On-site energy consumption:
  • Regular acquisition of meter readings (energyconsumption, running times)
  • Clear demarcation between power consumers (e.g. agitators, loading system, CHP unit …)
  • Adjustment of agitator systems, agitator running times and agitation intensity to the conditions
  • No pumping of unnecessary quantities
  • Efficient and economical substrate treatment and loading technologies
• Heat recovery concept.

It should always be remembered that each biogas plant is a system that consists of a large number of individual components that have to be fine-tuned to each other. Efforts must therefore be made as early as the planning phase to ensure that the chain works as a unified whole: purchasing individual components that work does not necessarily produce a working biogas plant.

It is often seen in practice that somewhere along the process chain there is a bottleneck that restricts performance and hence the economic efficiency of the downstream plant components. It may be the case, for example, that gas generation output does not reach the capacity of the CHP unit, but by taking steps such as changing the substrate mixture or improving capacity utilisation in the second digester stage it could be possible to achieve the required level of gas production. In addition to the balancing of energy flows, therefore, balancing material flows is also an appropriate means of discovering deficiencies in plant operation. Analysing the efficiency of the plant as a whole)

Environmental Protection

Hygienisation requirements
The aim of hygienisation is to deactivate any germs and pathogens that may be present in the substrate and thus ensure that it is harmless from the epidemiological and phytohygienic standpoint. This becomes necessary as soon as biogenic wastes from other lines of business are used in addition to raw materials and residues from agriculture. The relevant underlying legal texts that should be mentioned in this connection are EC Regulation No. 1774/2002 and the Ordinance on Biowastes. The EC Regulation includes health rules dealing with the handling of animal by-products not intended for human consumption. In biogas plants, subject
to official approval category-2 material can be used after high-pressure steam sterilisation (comminution < 55 mm, 133 °C at a pressure of 3 bar for at least 20 minutes , manure and digestive tract content can be used without pretreatment, and category-3 material (e.g. slaughterhouse waste) can be used after hygienisation (heating to a minimum of 70 °C for at least 1 hour). This regulation is rarely applied to agricultural biogas plants, however. If the only animal by-products used are catering waste, the regulation is not applicable. If substances are used that are subject to the regulations of the Ordinance on Biowastes, hygienisation is a requirement. In these cases it is necessary to ensure a minimum temperature of 55 °C and a hydraulic dwell time in the reactor of at least 20 days .

Air pollution control
Various air pollution control requirements need to be observed in relation to the operation of biogas plants. These requirements relate primarily to odour, pollutant and dust emissions. The overarching legal basis is provided by the Federal Pollution Control Act (Bundesimmissionsschutzgesetz – BImSchG) and its implementing regulations together with the Technical Instructions on Air Quality Control (TA Luft). The purpose of the legislation is to protect the environment from harmful effects and to prevent the emergence of such harmful effects. These statutory provisions are applied only within the context of the licensing procedure for large-scale biogas plants with a total combustion capacity of 1 MW or more and for plants designed to treat biowastes.

Water pollution control
Harmful impacts on the environment should be avoided if at all possible when operating biogas plants. In relation to water pollution control, in very general terms this means that the biogas plant must be constructed in such a way as to prevent the contamination of surface waters or groundwater. The legal provisions tend to differ from one region to another, since the specific water pollution control requirements depend on the natural conditions at the location in question (e.g. water protection area) and authorities issue approval on a case-to-case basis. The substances that occur most often at agricultural biogas plants, such as slurry, liquid manure and silage effluent, are categorised in water hazard class 1 (slightly hazardous to water); energy crops are similarly classified. The contamination of groundwater and surface water by these substances must there be avoided along the entire process chain. For practical purposes this means that all storage yards, storage tanks and fermentation vessels as well as the pipes and pump feed lines connecting them must be liquid-tight and be of approved design. Particular attention must be paid to silage storage sites, because silage effluent can arise in considerable quantities if harvest conditions are unfavourable and compacting pressures are very high. There is an obligation to collect and make use of the fermentation liquids and effluents escaping from the equipment. As these generally contain considerable quantities of organic materials, it is advisable to feed them into the fermentation tanks. So as not to add unnecessarily large quantities of unpolluted water to the process, especially after heavy precipitation, it makes sense to separate contaminated and uncontaminated water. This can be achieved with separate drainage systems,
which use two separate piping systems with manual changeover to divert uncontaminated water to the outfall and contaminated water and effluent to the biogas plant. Furthermore, special attention must also be paid to the interfaces between the individual process stages. These include above all the substrate delivery point (solids and liquids) and the discharge of digestates to the transport/application vehicles. The unwanted escape of material (for example overflows or residual quantities of material) must be avoided, or it must be ensured that any contaminated water from these areas is trapped.

In addition, the installation sites for the CHP unit must comply with the relevant regulations, as must the storage locations for new oil, used oil and if applicable ignition oil. It must be possible to identify and eliminate potential leaks of gear oil or engine oil, for
example.

Noise abatement
The most common source of noise in relation to biogas plants is traffic noise. The frequency and intensity of the noise generated is mostly dependent on overall plant layout and the input materials used. In the majority of agricultural biogas plants, traffic noise arises in connection with the delivery of substrates (transport, storage and metering system) for a period of 1-2 hours on an almost daily basis. A larger volume of traffic and hence also more noise is to be expected during harvesting and when the substrates are being brought in, and when the fermentation residues are being taken away. Other noisy machines, for example those operated in connection with the use of gas in a CHP unit, are normally installed in enclosed, soundproofed areas.

Economic Optimisation

Economic optimisation is aimed at reducing costs and increasing yields. Like technical optimisation, economic optimisation can be applied to all sub-processes. In this case, too, the first step is to identify the substantial cost factors so that the related costs can be reduced accordingly. Specific variables such as electricity generation costs (e.g. in €/kWh) or specific investment costs (in €/kWel inst.) serve as the basis for an initial guide to plant performance as a whole. There are comparative studies for these (for example German biogas measuring programme, ), thus enabling the overall economic performance of the plant to be graded.

To conduct an in-depth study it is advisable to analyse and compare the following economic data:

• Operating costs:
  • Personnel costs
  • Maintenance costs
  • Repair costs
  • Energy costs
  • Cost of upkeep
• Investment costs (depreciation), repayment, interest
• Substrate costs (linked to substrate quality and substrate quantities)
• Revenue for generated electricity and heat
• Revenue for substrates
• Revenue for fermentation residues/fertiliser

Further Information

• Monitoring, Controll and Maintenance Presentation
  
• Microbiological Process Optimization of Biogas Synthesis (DBFZ)
• Service companies for optimization
• Research planning – The Biogas Optimization Project
• Monitoring and optimizing of biogas production