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The contribution of anaerobic digestion to Europe’s renewable energy supply is growing year on year. The Biochemical Methane Potential test is a useful tool in optimising the anaerobic digestion process, as Dr Elaine Groom and Dr Angela Orozco explain.
In 2009, the European Union (EU) agreed a package on climate change which included three key targets for 2020: a 20% reduction in greenhouse gas emissions compared to levels in 1990; a 20% increase in the share of renewables in the energy mix; and a 20% cut in energy consumption. The requirement to meet these targets, coupled with rising energy costs and concerns about security of energy supply, is driving greater investment in renewable energy and alternative uses for waste resources.
To meet these targets, governments across Europe have offered a variety of financial incentives in order to aid the introduction of low carbon energy technologies, such as wind, tidal, solar and energy from biomass – including anaerobic digestion. Anaerobic digestion is a naturally occurring process where organic materials decay in the absence of oxygen. It is an established energy from biomass technology which produces the gaseous fuel biogas, a mixture containing typically 50 – 75% methane with the majority of the remainder being carbon dioxide, although small volumes of nitrogen, hydrogen, hydrogen sulphide and oxygen are also found.
A residue called digestate remains at the end of the digestion process. Made up of undigested material and dead micro-organisms, digestate is high in nutrients and is suitable for use as a fertiliser and soil conditioner. Biogas is a renewable energy source that could make a valuable contribution to the energy mix required to meet the 2020 targets. It also has an important role to play in diverting organic wastes from landfill and in the diversification of the rural economy. It is a key component of the EU strategy for meeting the 2020 goals, with a target contribution of up to 14% of the energy mix and 10% for use in transport. There are an estimated 12,000 commercial anaerobic digesters in Europe, providing more than 2,000 megawatts of electrical power.
This number is predicted to grow to more than 40,000 by 2020, with an increase in biogas production from wastewater treatment sludges, farm wastes and waste food as organic wastes are diverted from landfill. It is estimated that there are sufficient organic resources for biogas to contribute one terawatt hour (TWh) of energy per million people in Europe.
Biogas production
At an industrial scale, biogas production normally takes place in large, stirred, sealed tanks. Inside these reactors a complex series of biological reactions occur to convert feedstock to biogas. These tanks are temperature controlled and operated in either the mesophilic range of 30 to 40° C, or less frequently, in the thermophilic range of approximately 55° C. Biogas accumulates in the headspace of the tank and may be retained in a gas storage vessel before use.
The biogas produced can be used for combustion in a gas boiler to produce renewable heat, or in a combined heat and power (CHP) engine to produce both electricity and heat. Alternatively, the biogas can undergo upgrading to biomethane (>99% methane) for compression and injection into the natural gas grid or for use as a transport fuel. Plant operators generate income from biogas by exporting electricity into the grid, by selling heat to the communities surrounding the plant and by direct sales of biogas or biomethane. Income can also be generated by charging a gate fee for the waste materials they process.
The biological reactions are carried out by a range of anaerobic bacteria which interact in a complex, dynamic system which must be carefully controlled to achieve optimum biogas production. For plant operators the detail of these interactions is not critical, but process conditions should be monitored to ensure stable plant operation.
Process monitoring
Conversion of organic matter to biogas takes place in four stages, mediated by different groups of bacteria (see Figure 1 showing the anaerobic digestion process). Firstly, large carbohydrate, fat and protein molecules are broken into smaller sugars, fatty acids and amino acids respectively. These chemicals then undergo a series of reactions and are transformed to acetic acid, which is finally converted to biogas by methanogenic bacteria.
The bacterial groups in an anaerobic digester are sensitive to changes in environmental conditions and monitoring is vital. Knowledge and control of parameters such as temperature, pH, oxidation reduction potential, total and volatile solids concentration, ammoniacal nitrogen concentration and volatile fatty acids (VFA) concentration is important. Most modern digesters have simple online measurement systems for parameters such as temperature, biogas volume and quality. Use of more sophisticated equipment for direct measurement of parameters, such as VFA and ammonia concentration, is limited due to reliability and cost issues.
Development of new, low cost probes which can make reliable measurements of these parameters in the harsh conditions in a digester is a focus of current research.
Feedstocks
A wide range of different materials are suitable as feedstocks for biogas plants. These range from freshly harvested crop materials and residues to municipal and industrial wastes. The food versus fuel debate has been a contentious issue for many biofuels; however, biogas is a means of treating agricultural residues, manures and organic wastes diverted from landfill, and to recover nutrients for return to the land, thereby displacing synthetic fertilisers.
A plant owner will build their business plan around achieving the highest biogas yield and best digestate quality, and so the composition of the incoming feedstock is critical. The biogas plant is a reliable source of income for the plant owner and a proven treatment process for waste products; however, it is a biological system and is therefore prone to variation and fluctuation in performance. While processing high amounts of variable feedstock may initially seem economically attractive, stable operation of the plant is the key to maintaining profitability. Many suitable feedstocks are non-homogenous materials or wastes with variable composition.
The quality of the feedstock material impacts on biogas quality, quantity and stability of plant operation. It is important for plant operators to perform a number of analytical tests on this material to assess: • Variability and suitability for digestion • Potential biogas yield • Presence of interfering substances Stable plant operation with high biogas yields can be achieved through co-digestion of two or more feedstocks. Designing the optimal feed regime requires the operator to learn about the biogas yields of individual components and how to blend and mix these in a complementary fashion.
Quantifying methane potential
The Biochemical Methane Potential (BMP) test is a lab based test to determine the gas quality and quantity from a feedstock for an anaerobic digester. The BMP test is essentially a small scale replication of the processes happening in an anaerobic digestion plant. Although time consuming, it is a thorough and complete microcosm of the digestion process and therefore offers the most accurate estimate of gas yield and digestibility of samples.
For a plant in the planning stage, the information provided by the BMP test is valuable when deciding on plant sizing and throughputs, and provides information on potential gas yield from a range of available feedstocks, assisting in feedstock selection. For established plants, the BMP test offers operators the opportunity to trial new feedstocks and feedstock blends without causing disruption to the plant. The revenue generated by anaerobic digestion plants through electricity and renewable heat sales is significant, and therefore any changes to the plant which negatively affect biogas production must be avoided.
The test can also be used to benchmark plant performance, allowing comparisons to be made between ‘actual’ plant performance and the maximum biogas production possible from a given feedstock.
Results of the BMP test
The BMP test is complete when the sample stops producing gas, typically after a period of 20 days. From the gas volume and quality measurements, the cumulative methane production is calculated. Plotting this versus time gives a curve with the characteristic shape seen in Figure 3 – BMP results graph. The BMP result is equivalent to the highest point on the curve, normally expressed as the volume of methane obtained per kilogram of sample.
The shape of the graph also provides information on the rate at which the sample can be digested, which aids digester design with regard to feedstock throughput and retention time. Those feedstocks with the highest biogas yield may not necessarily be the most suitable for some plant designs, while others will require long retention times. Feedstocks which produce less biogas but digest fully more quickly may be preferred; shorter retention times mean smaller equipment with associated savings on capital costs. The time required for complete digestion to occur is also important with regard to digestate quality. Digestate must have low residual biogas potential in order to meet the requirements of quality protocols such as the UK BSI PAS 110.
Meeting the standard allows the biogas plant operator to label digestate as biofertiliser, which can provide an extra and valuable revenue stream. Additionally, the shape of the curve can also indicate the presence of inhibitory compounds in a sample. If inhibition is sufficiently high to totally prevent digestion the sample will produce little or no methane.
Conclusion
Anaerobic digesters are making an increasingly important contribution to waste management and renewable energy production. These developments are driving increased operational efficiency and higher feedstock throughputs. Knowledge of the expected feedstock performance can be gained through the BMP test. This contributes to better process knowledge, more efficient designs and greater quantities of higher quality gas, ultimately boosting energy production and improving profits.
The level of technical support available for biogas plant operators has increased in recent years, and teams of researchers are working with companies to develop the next generation of efficient biogas plants.
Acknowledgments
The authors wish to acknowledge assistance from colleagues in the QUESTOR Applied Technology Unit and funding from the Government of Ireland Department of Agriculture, Food and the Marine for the GreenGrass Project (RSF 07 557) provided under the Irish Agricultural Research Stimulus Fund. GreenGrass (2007-2012) is led by the Agriculture and Food Development Authority (TEAGASC) in collaboration with University College Cork and the QUESTOR Centre at Queen’s University of Belfast. The project focuses on the utilisation of Ireland’s significant grassland resources for energy production through anaerobic digestion. http://www.greengrassproject.ie/
Published: 01st Sep 2012 in AWE International
