A Review of Key Points of an Industrial Biogas Plant

A review of key points of an industrial biogas plant. A European perspective Miguel A. Sanz-Bobi, Senior Member, IEEE, Fernando de Cuadra, Carlos Batlle Institute for Research in Technology (IIT) Comillas Pontifical University, ICAI-School of Engineering Madrid, Spain [email protected] Abstract—This paper reviews the main technical, operational and economical aspects of a biogas plant. The analysis is focused on the key points of the biogas production process, the current situation of biogas plants in Europe and the advantages, risks and new trends of the production and use of biogas. The paper offers an integrated view of the different aspects involved in a biogas plant as source of renewable energy in contrast with other more analyzed renewable energy sources based mainly on wind or solar energy. Keywords- biogas; biogas plant; organic matter; green energy production;combined-heat and power system

I.

INTRODUCTION

Nowadays, renewable energy production is an extremely important topic in many interconnected fields such as research, energy production, social acceptance, reduction of the use of non-renewable sources of energy, government strategies, etc. A great deal of effort is being made to use energy coming from renewable energy sources. The renewable energies which are most frequently used in the world are wind and solar, and in particular, energy coming from the wind is the most extended. Other renewable sources of energy are being used in wellknown operative technical processes but for different reasons they are not as extended as those previously mentioned, such as for example, the industrial biogas plants. This paper reviews the key aspects of biogas production in an industrial biogas plant in order to obtain an integrated picture about their advantages and disadvantages at present. In particular, the current situation of such plants in Europe will be considered. The paper is organized around the following sections. Section II describes the biogas production, its requirements, technology and use of the biogas obtained. Section III analyzes the basic economics of a biogas plant. Section IV describes the experience of the biogas plants throughout Europe. Section V includes the conclusions and key points of the industrial biogas plants. II.

BIOGAS PRODUCTION

A. What is biogas? How is it produced? Biogas is a mixture of mainly methane gas (CH4) and carbon dioxide gas (CO2). If this is compared with natural gas, in which the methane content is about 90-95%, it is possible to conclude that biogas is a low grade natural gas. As a

consequence of the previous assertion, the energetic potential of biogas is much lower than that of natural gas. Four essential ingredients are required for the production of biogas [1], [2]: organic matter coming from any organic source, bacteria, anaerobic conditions and heat. The process of biogas production requires a large amount of micro organisms, (bacteria), which are able to use the stored energy in carbon hydrates, fats and proteins under anaerobic conditions for their metabolism. Bacteria use the organic matter as a food source for producing methane by the decomposition of organic matter. There are many different types of sources of organic matter that can be used for biogas production. Essentially the more commonly used are manure coming from farm animals, wasted food, wastewater, wood and agricultural products such as corn silage. The amount of potential biogas produced depends on the amount of dry organic matter contained in the feedstock used. Different types of organic matter can be mixed together for biogas generation in an industrial biogas plant. Energy crops have higher contents of dry matter [3] and for this reason they have been a target source in biogas production by industrial plants over the last years. In particular, the dry organic matter content of corn silage, one of the most frequently used agricultural substrate, consists of around 35% of the total volume. The second ingredient needed for biogas production is bacteria, which are needed in order to convert the fats, carbohydrates and proteins contained in the organic matter to simple acids such as acetic and propionic acid. Then, a second type of bacteria transforms the acids to methane and carbon dioxide. These processes occur simultaneously. The bacteria are commonly present in manure, in wasted food or wastewater and in agricultural produce harvested with green parts such as corn silage or similar green plants. In order to fulfil the expected function of the bacteria in the production of biogas, two key conditions are necessary: an anaerobic atmosphere (without oxygen) and heat. The temperature has to be in the range 35-40ºC (within the mesophylic range of operation, this is typical), or between 5560°C (thermophilic range). The pH-value should be neutral up to a low alkaline value.

Biogas is produced on an industrial scale through the use of different technologies depending on the raw material to be used, the use of the biogas and the scale of production [4]. In any case, one component which is essential in all the existing configurations of biogas production plants is the digester. This is the core of any biogas plant regardless of its size. The digester is a closed recipient where the raw material

is introduced, in its atmosphere there is no oxygen and the temperature has to be constant at over 20ºC. Fig. 1 shows a scheme including the main functional relationships among components [5] in an industrial biogas plant. In this scheme the main inputs required and possible output results are also included.

GREEN CERTIFICATES

HEAT

BIOGAS PLANT HEAT MANAGEMENT: •Storage •Exchange •Delivery

ORGANIC WASTE

ENERGY CROP

FERTILIZERS

RAW MATER. MANAGEMENT: •Storage •Mixing •Handling

DIGESTER MANAGEMENT: • Local handling • Remote maintenance • Remote monitoring

RESIDUALS MANAGEMENT: •Storage •Handling •Delivery •Disposal

POWER GENERATION: • Local handling • Remote maintenance • Remote monitoring

POWER

GAS MANAGEMENT: •Storage •Mixing •Handling •Delivery

GAS

FINAL WASTE

Figure 1. Functional scheme of an industrial biogas plant

B. Features of industrial biogas plants Industrial biogas plants are located in rural sites where there is very easy access to the raw material used in the anaerobic process: agricultural products and animal manure coming from farms in the area. This suggests that these plants are not close to urban areas. In these plants wet fermentation is used because in this mode the agricultural material can be mixed with liquid substrates such as manure, complementing the energetic potential of the substrate. At any type of biogas plant, including industrial plants, the technology used is very mature and well known [5]. It is not so sophisticated, but in any case it is necessary to take care of the equipment because their components can degrade under the normal working conditions of the plant. Any biogas plant requires an important effort to monitor its behaviour. There are two main reasons for performing this important task. First, the digestion requires special conditions to be developed in an optimum way. Any deviation of these conditions can reduce the potential amount of biogas to be obtained, raw material can be wasted and non-expected residuals could appear. Another important reason for continuous monitoring is that the products resulting from the digestion are biogas, other types of gases and residuals that if they are not managed with care, they can affect the safety and environmental conditions of people and the natural environment. In particular, reliability is a particular focus of attention for agricultural biogas plants, where permanent attention and surveillance by operators are not

practical nor feasible. The optimization of biogas plants is one of the current research topics [6] that will contribute to obtain the best performance and the maximum benefit. Over the last years different concepts for the organisation of biogas plants have emerged [2] such as: • Decentralised plants on farms producing electricity and using heat (but the use of heat is often not obvious) • Decentralised plants in combination with biogas pipelines, transporting the biogas to a cogeneration unit situated in proximity to a district heating system or industries that demand heat for its productive process. Therefore the cogeneration can make full use of the heat. [7] presents an interesting case analyzing the optimal location of biogas plants in a decentralised scheme according to the geographical resources and industries in a region. • Decentralised plants delivering the raw gas in biogas pipelines to a upgrading station and injecting the biomethane in a gas grid. The biomethane can be used for cogeneration, transportation fuel or high tech process energy. • Centralised plants availability [8], [9].

in

areas

with

high

feedstock

The efficient production of biogas not only needs a biogas plant, but also an integrated infrastructure such as power lines,

biogas pipelines, upgrading stations and heat networks, in order to be able to use the energy of the cogeneration units. C. Biogas uses The produced biogas can be used in many ways (combustion for the production of electricity and heat, to be fed into a gas grid, in fuel cells or even as fuel) [10], [11]. The most common alternative is the utilisation in a gas engine for the production of electricity and heat. The produced electricity can be fed into the public electricity grid, receiving funded tariffs for it. In order to optimize the utilisation of the produced biogas and to maximize the profit of the plant, the utilisation of the excess of heat from the combustion of the gas should be considered. This heat can be used as process heat for the digestion process or for heating parts of the plant or adjacent agricultural buildings (e.g. stables). The best option for using the excess of heat is to sell it to an external heat consumer [7], [12], [13]. The typical use of biogas in a gas engine is based on a cogeneration system or a Combined Heat & Power System (CHP). In order to use the biogas as input in a gas engine, its impurities have to be removed. Particularly it is important to remove the hydrogen sulphide (H2S ) contained in the biogas. The limits for H2S for biogas used in gas engines lie between 100 and 500 ppm, depending on the manufacturer. The lower the amount of H2S, the higher is the lifespan of the gas engine. In addition to the desulphurisation, it is also necessary to dry up the biogas. Biogas is very attractive due to its versatility of use in different markets such as electricity, heat, vehicle fuels and reuse in agriculture. In fact biogas can be used for:

likewise be a different set of reduction figures if the biogas is used for transport purposes and replaces diesel or petrol. In general, if biogas is used instead of coal, it is considered that 1 m3 of biogas could produce 2.5 kWh of electricity and the CO2 reduction can be around 1600 g/m3 biogas. In the case that the heat is used, 400 g of CO2 are reduced for every m3 of biogas used for heat production instead using coal. An additional significant advantage of biogas, in comparison with other renewable energy sources, is that it is a controllable source of energy. Solar and wind power plants depend on the environmental conditions and they are operative if there is sun or wind respectively, but these condition do not have human control. This is not the case of biogas. It can be stored and used when it is required. Due to this fact, biogas is an important point of current research because it can be used in different alternative schemes as a complement of other energy sources. An attractive complementary use is the production of electricity from biogas in high peak periods of electrical demand as part of a network of distributed generation [16]. Another interesting alternative use of biogas is as complement of other renewable sources of energy such as wind and solar plants. In periods of no wind, the production of a wind turbine could be replaced by the use of biogas guarantying the continuity of this integrated source of energy [17], [18]. III.

BIOGAS BASIC ECONOMICS

While there is no question about the technical feasibility of an industrial biogas plant, its economical feasibility is always a point of important concern that must be carefully analyzed.

•

Production of electricity and heat (cogeneration)

•

Production of electricity alone

•

Production of heat alone

•

Upgrading to biomethane for: injection in the gas grid, transportation fuel, high tech process energy, raw material for the chemical industry

It is difficult to issue a general rule valid for any type and configuration of biogas plants, but under the more extended scheme of the use of biogas for electricity production, the investment required for building and maintaining an industrial biogas plant in production, hardly compensates for the revenues coming from selling the electricity produced. A simple example with real values is presented as an illustration of this assertion. The results in this example where obtained using the worksheet [19] and simplifying the information resulting from it.

The main advantage of biogas is the production of green energy, but this is not the only advantage. Biogas has other advantages for the producer and for society.

Table 1 shows the case of an industrial biogas plant that uses corn silage as the main raw material due to its high contents of organic matter.

If biogas is produced using waste raw material such as manure, wasted food or waste water, the production of biogas contributes to the reduction of pathogens and odour that can cause health problems, or at the very least, mitigate an uncomfortable environment [14]. In general, the use of biogas as an energy source is widely considered as CO2 neutral because the CO2 released during combustion of the biogas is almost the same CO2 that was assimilated during photosynthesis to create organic matter in energy crops. In this way the burning of biogas is simply a recycling of CO2 in the biosphere [15]. However in addition to this, the energy production itself also helps to reduce the emission of greenhouse gases; its magnitude depends on which fuel is being replaced. If biogas replaces oil, less oil will be used and there will be a reduction in emissions. There will

TABLE I.

ESTIMATION OF BIOGAS PRODUCED COMING FROM CORN SILAGE

Raw Material Corn silage per year Hectares required

Value 20000 444

Units tonnes per year hectares

DM amount (Total Solid)

6400000

Kg TS/year

organic Total Solid (oTS)

6080000

Kg TS/year

Possible biogas yield

4012800

m3/year

Methane in biogas

2107924

m3/year

In the case that there is cattle manure available that can be mixed with the corn silage into the digester, the potential production of biogas is presented in Table II. According to the information included in Tables I and II, and considering typical ratios of a CHP system performance, the expected results of the plant for this case example are included in Table III. An average period of operation of 8000 hours per year and an average price for selling electricity of 0.035 € per kWh were considered. TABLE II.

ESTIMATION OF BIOGAS PRODUCED COMING FROM CATTLE MANURE

Raw Material

Value

Available cattle manure

Unit

5000

tonnes per year

DM amount (Total Solid)

1100000

Kg TS/year

organic Total Solid (oTS)

935000

Kg TS/year

Possible biogas yield

355300

m3/year

Methane in biogas

195415

m3/year

TABLE III.

ESTIMATION OF THE MAIN RESULTS OBTAINED IN THE BIOGAS PLANT IN THE CASE EXAMPLE

Plant Data Methane in biogas corn silage

Amount

Unit

2107924

m3/year

195415

m3/year

Average biogas yield

2303339

m3/year

Average energy yield

12140899

kW/year

Methane in biogas cattle manure

estimated electrical power output

639

kWh/h

estimated thermal power output

634

kWh/h

19

kWh/h

14936

t/year

7416

t/year

estimated electrical power consumption produced amount of compost TS=25% produced amount of digested liquid TS=5% Revenues on energy Revenues from selling electricity

173600

€

the price will depend on the type of agreements established between farmers and the biogas plant. Very often the farmers are also stakeholders of the plant and the prices could be different if there is a long time guarantee for them that the whole production of corn is acquired by the plant. References [20], [21], [22] evaluate prices for corn silage between 21 and 36 € per tonne of which the range is due to different reasons such as variability of the prices of corn grain, seasonal factors and type of selected hybrid. The most important costs involved in an industrial biogas plant correspond to the initial investment required for building and developing the plant, and in a longer perspective, the required maintenance and operation costs. According to different sources of information coming from several manufacturers, the cost of building an industrial biogas plant ranges among different values depending on the size of the plant. For a medium size biogas plant, such as the one in the example, the cost of the plant is between 4 and 5 M€ and its operational and maintenance costs can be around 50 and 70 k€ per year. The previous data shows that the equilibrium between costs and revenues could be reached after many years of operation. This does not mean that an industrial biogas is not profitable, because in fact the number of biogas plants is increasing continuously each year. In order to shorten this time where the equilibrium between costs and revenues is reached, governments, regional agencies and municipalities around the world, and in particular in Europe, are proposing different incentives to promote the development of biogas plants. The most important reason for promoting the construction of biogas plants is based on its renewable nature that can substitute part of the conventional energy sources, but this is not the only reason. As a differential feature of biogas in comparison with other renewable energies, it offers in parallel other benefits affecting the environment and society. Biogas plants can use as raw material residual products such as agricultural products, manure, livestock, food processing residuals, etc. which if they are not reused or eliminated can suppose an important problem with environmental and health consequences. All these primary benefits are difficult to evaluate, but certainly contribute to the revenues of the biogas plant. IV.

As it can be observed, an important amount of heat is being generated from which the possible revenue from selling it was not considered. This is one of the worse scenarios where the heat cannot be reused. Also revenues from products resulting from the digestion of the organic matter, mainly compost that can be used as fertilizer, were not taken into account. In our example the main costs are coming from the initial investment for building and starting the biogas plant, its maintenance and the cost of the raw materials required as inputs for the plant. The cost of the required animal manure normally is reduced to the cost of transport from the farm to the plant and in some cases it does not exist due to the proximity or agreements between the parts. In the case of the use of corn silage as raw material in the biogas plant, as was considered in the example, its cost has to be taken into account. Once again

BIOGAS PRODUCTION IN EUROPE

In Europe, biogas production has increased progressively over the last years driven by the EU (European Union) target of meeting 20 % of the final energy consumption with renewable energies and the guidelines set forth in the EU Directive 2008/98/EC concerning the handling of waste. On the basis of these political objectives, many countries in Europe have introduced incentive programmes for the generation of power from biogas (feed-in tariffs for electricity from biogas, green certificates, tenders or grants for the use of energy crops) [23]. In Europe, the general objective of any RES-E (electricity generation from renewable energy sources) support mechanism is to enhance and speed up the learning curves of the currently known RES-E, in order to fully accomplish the objective of mitigating -as soon as economically possible- global warming. In the particular

context of the European Union, the chosen path to contribute to this objective has been to commit to, among others, exigent renewable targets. As it was mentioned, the European Commission Directive 2009/28/EC establishes “mandatory national targets consistent with a 20% share of energy from renewable sources and a 10 % share of energy from renewable sources in transport in Community energy consumption by 2020.” Although the situation could change, to date, the still comparatively higher cost of RES-E technologies has made it virtually impossible for them to grow without regulatory intervention. Thus, different support mechanisms have been implemented in Europe and worldwide. In this context, the best way to design subsidy regimes to ensure the proper development of RES-E is still an unsolved issue. Currently, there are two different types of support methods: • Indirect methods. Implicit payments or discounts as well as institutional support tools that include: research and development funding, below-cost provision of infrastructure or services, and positive discriminatory rules (regulations facilitating grid access for RES-E power, RESE dispatch priority in the EU and others: net metering, building codes, etc.). • Direct methods. Investment supports, such as capital grants, tax exemptions or reductions on the purchase of goods and operating support mechanisms, i.e. price subsidies, obligations, tenders and tax exemptions on production. These latter alternatives are often classified into two different categories, the price-based supports, which fix the price to be paid for renewable electricity (e.g. feed-in tariffs or feed-in premiums), and quantity-based supports, which determine a specific amount of electricity to be produced by RES-E. Tradable Green Certificates (TGC) also referred to as Renewable Obligations or Renewable Portfolio Standards (RPS) in the US, establish quota requirements for consumers, suppliers, and/or generators to ensure that a portion of their electricity comes from RES-E. Tradable certificates are awarded for every unit produced from RES-E; these certificates are then bought by those required to comply with the RES-E quota. Several options exist in the design and implementation of TGC and, also, of the tariffs in Europe [24]. In this context, power generation from biogas grew between 2008 and 2009 by almost 18 % and accounted for a total of 25,170 GigaWatt hours (GWh) in 2009. The total energy extraction from biogas rose over the same period by some 4.3 % to 8,346 ktoe (kilotonnes of oil equivalent). Around 52% of the plants produced biogas from agricultural waste, while landfills and sewage treatment plants generated 36 %, or 12 % of the biogas in the EU. In 2009, the largest biogas producers in Europe (in thousands of tonnes of oil equivalent or ktoe) were Germany, the United Kingdom, France, Italy and the Netherlands. Rapid and dynamic growth is being observed in Greece, Slovenia, Slovakia, the Czech Republic and Belgium.

Reference [25] includes interesting data corresponding to the production of biogas, electricity and heat in Europe in 2008/2009. They offer a good idea about the development of biogas production in Europe. The five European countries with most biogas plants are: Germany, Austria [26], the Netherlands [27], Denmark [28] and Italy. Also, the data shows an important growth of electricity and heat production in almost all the European countries listed. The most relevant points that can be outlined after an analysis of the biogas production in Europe can be summarized as follows: • There is a great deal of experience in biogas plants including its use and operation in many European countries. • The number of biogas plants in Europe varies according to the country. • The biogas plants have been stimulated by important programmes of subsidies facilitated by the respective governments. When these programmes were reduced, the growth of the biogas sector was null. According to experiences in some countries, if the subsidies are removed or mitigated, the biogas sector does not progress [16]. • There are different types of incentives for biogas production according to the country [24]. • Some strategies in some European countries harmonize agricultural interest with energy goals providing excellent results. • Biogas production in some countries solves a double problem: processing animal manure and production of renewable energy. • The typical profile of biogas plant owners in Europe are cooperatives including mainly farmers and public organisations, but also, some shareholders such as companies or private investors. V.

CONCLUSIONS: KEY POINTS OF BIOGAS PLANTS FROM AN EUROPEAN PERSPECTIVE

A biogas plant is a local and sustainability-based business. Its profitability depends on national/European policies (renewable and sustainable development criteria, energy markets) and the local economy (co-generation, gas and fertilizers production, waste processing). From a technological point of view, and according to the amount of existing positive experiences, there are no doubts regarding its feasibility. Improvements in biogas plants are possible in many aspects that are under different fields of investigation, but they are not negative factors affecting the suitable operation of a biogas plant. Some risk factors to take into account are related to the engagement of the local partners (i.e. farmers) for the longterm supply of raw materials. Other current external risk factors existing in some countries or regions are related to regulatory changes in the energy market and the market

volatility that can affect the electricity prices and the number and price of the European TGC. The main external threats are the increasing development of more popular green energy sources such as wind farms and the pressure of electricity tariffs on customers [27], [28]. With respect to the increasing development of more competitive sources of green energy, the subsequent possible drop of prices in both TGC and electricity could be compensated by new strategies of operation of the biogas plants. In this context, the need for back-up and peak-demand units could be satisfied by biogas plants since biogas is storable and can produce electricity when needed. Also, biogas could be used in order to compensate for the deficient production of green energy, for instance, as a back-up for wind-prediction deviations or periods of no wind. In any case, the external risks [16] can be mitigated if adaptive policies are foreseen at the beginning of the project, including heat usage, Organic Rankine-cycle, variable power generation, waste processing, fertilizer and even gas production. The main internal risk is the dependence on the local supply of raw materials at a reasonable price. The best actions that are possible to suggest in order to overcome this risk factor are to assure long-term management of local land, to sign long-term supply contracts, and make local agents very interested in the long-term profitability of the plant by multiple business relations. REFERENCES [1]

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