One Century of Bioenergy in Germany: Wildcard and Advanced Technology

2.1 Biogas Production along with Wastewater Treatment

Anaerobic digestion (AD) of biomass for producing biogas is known since centuries. Primarily biogas was used to generate heat and later also electricity ²⁰, ²¹. The roots of the modern biogas technology (anaerobic technology) in Germany were marked mainly at the end of the 19th and beginning of the 20th century by the developments of urban water management due to accumulating wastewater and accompanying (drinking) water pollution ²². At the beginning, the utilization of energy was not focused in AD processes. It was rather regarded as a by-product of wastewater treatment. In the first half of the 20th century the microbial and biochemical process of AD from organic matter was intensively investigated ²³, ²⁴. First special digesters for large-scale anaerobic wastewater treatment were built in Germany by Prof. Karl Imhoff ²⁰, ²⁵ in the 1910s and 20s (cf. Fig. 2 and Tab. 1). This was the start for distributing sewage gas (later known as biogas) via gas systems in urban areas for production of heat and electricity. Based on a patent of Imhoff, the first heated digestion tanks, which were separated from the actual wastewater treatment process, were used for anaerobic sewage sludge treatment in Essen. From 1927 on, the wastewater treatment plant in Essen was the first feeding-in surplus sewage gas in the public gas network and selling the product. Further cities followed ²⁰. Due to the fuel shortage in 1937, the cities of Halle, Pforzheim, Essen, Erfurt, Pössneck, Munich and Heilbronn even replaced liquid fossil fuels in their municipal vehicle fleets with Otto engines by biogas. However, the gas had to be compressed and was delivered in heavy steel cylinders ²⁵. The German chemical company IG Farben produced light rubber tanks, which could be easily mounted on the roof of the buses and cars ²⁶. To increase the yield of sewage gas, further organic matter and waste was added to the slurry (also called as co-fermentation) of the German wastewater treatment plants. For example, in Stuttgart material from grease separators was co-fermented from 1940 on ²⁷. Nevertheless, at this time the potential of biogas production from sewage sludge and organic municipal waste was regarded as limited ²⁸.

In addition, municipal waste management became necessary. In 1906 the Charlottenburger Abfuhrgesellschaft in Berlin introduced a three-part system, separating the house waste in ash/slag, kitchen/food waste and rubbish (e.g., rags, old clothes and shoes, glass, old furniture) based on an American model. Even it was ceased in 1917 because of financial reasons and the unwillingness of the population to participate, it was the starting point for the provision of residues for biogas treatment ²⁹.

Finally, also the first biogas plant for manure was established. In general, the first biogas plants in the agriculture were very small-scale and homemade plants, constructed by farmers or agricultural engineers in Bavaria and Baden-Württemberg. This development was motivated, as large amount of semi-liquid manure along with logistical and odor problems occurred ²⁵. At that time processes were used, which are similar to co-fermentation. Hence, the rise of agricultural (biogas) plants had started. In 1944 an agricultural plant that produced biogas from manure and agricultural waste and residues was built by Imhoff ³⁰.

2.2 New Processes for a Wider Application of Solid Fuels

In the 20s leading chemists in Germany realized that due to progressive developments in science and technology new cross-border disciplines of chemistry opened up, which linked the work of chemists with the one of engineers and apparatus constructors ³¹. This initiated a lot of innovations, such as the Fischer-Tropsch process, Bergius process, wood gasification, but also an increasing fuel demand. Mainly coal, wood and peat were used for energy generation and chemical processes. To increase the availability of liquid and gaseous fuels, gasification processes were developed and became the dominant energy conversion technology, which enabled the industrialization process ³². Three pathways were mainly developed: provision of town gas, dedicated gas generators and vehicles, and generation of liquid fuels by thermochemical processes.

Provision of Town Gas

In particular, the gasification technology represented an integral feature in the public supply of gas and electricity. To assure working also in times with daylight, from the middle of the 19th century on so-called illumination gas was provided by municipal companies in the cities. Therefore, it was also referred to as town or city gas. Later the gas was also used for the operation of gas cookers and boilers. Town gas was produced mainly from coal (coal gas) in gas plants by gasification. It contains mainly hydrogen (ca. 50 %), followed by methane, nitrogen, carbon monoxide, and volatile hydrocarbons in small quantities. In times of coal scarcity also wood, peat or other organic compounds were utilized ³³. Almost all middle and larger cities run a gas plant and provided the gas via municipal gas networks. Only in the 1960s the gas supply was transferred from town to the cleaner natural gas ³⁴.

Dedicated Gas Generators and Vehicles

Due to the huge shortage of liquid fuels in Germany and other European countries, experiments were made to run vehicles on wood, coal, or even both. Generators were constructed or improved to convert timber, charcoal and coal at a high temperatures (between 1600 und 1800 °C ³⁵) into a sufficiently tar-free syngas (consisting of atmospheric nitrogen, carbon monoxide, hydrogen, traces of methane, and other gases), also called wood gas or producer gas. After cooling and purification, the gas was injected into the cylinders of a regular internal combustion car ³⁶, ³⁷. The gas was also effectively used in (stationary) industrial applications, such as saw mills and pumping stations, fire boilers for industry and home or as basic chemical feedstock, such as ammonia for fertilizers ³⁵. First technically mature wood gas generators for mobile use were developed by the Lorraine chemist and engineer Georges Imbert in the 1920s. In 1931, he sold the licenses, among others, to the Germany company of Johannes Linneborn and the mass production of the Imbert generator started ³⁶, ¹⁸⁷, ¹⁸⁸. Under the Nazi regime, and especially during Second World War, wood gas generators in vehicles were pushed mainly due to reasons of energy autarky (see Sect. 2.3).

Generation of Liquid Fuels by Thermochemical Processes

In general, at that time biofuels from renewable resources were usually used in niche markets as they tended to be too expensive. However, the main aim was to achieve independence from foreign raw materials and to save foreign exchange as part of the autarky efforts of the Nazi regime. ³⁸ Ethanol from paper processes, gases and liquids from wood pyrolysis units, as well as vegetable oils for diesel engines were common. Yet, biofuels have been increasingly chosen in case of emergency fuel shortages or agricultural surpluses ³⁹. Next to the wood gas technology, Fischer and Tropsch invented a synthesis process of diesel-like fuel, which was patented in 1925. Conventional vehicle fuel could be substituted, which ultimately covered 90 % of German consumption at that time ⁴⁰.

Another way to produce fuel from brown coal (lignite) is the Bergius Pier process. In 1913, Dr. Friedrich Bergius discovered a process for coal liquefaction, for which he was awarded with the Nobel Prize in Chemistry in 1931. Due to further developments of the BASF chemical engineer Dr. Matthias Pier in 1927, the world's first industrial-scale hydrogenation plant for producing gasoline and coal was put into operation at the Leuna plant ^41-43. BASF bought the patent and built several plants to supply Germany with synthetic fuel and lubricants ⁴¹. In the years 1943/44 about 4 million t a⁻¹ of gasoline were produced in 12 hydrogenation plants. In addition, 600 000 t a⁻¹ petrol was produced in nine Fischer-Tropsch plants ⁴⁴ and used for the aircraft fleet and other military purposes ⁴⁵. Compared to petroleum-based fuels, however, both processes were not competitive, so that they did not play a role after the war. Although these processes were based on (brown) coal (coal-to-liquid, CtL), they initiated the interest in solid-to-liquid fuel technologies, also applicable for biomass (especially wood, straw, energy crops) ⁴⁰. Most of the synthetic fuels, about 1.7 billion liters, were produced in 1942 from coal. After all, about 267 million liters of ethanol fueling German road vehicles were mainly produced from potatoes. It is estimated that 54 % of the pre-war German fuel production was derived from non-petroleum sources, of which only 8 % was ethanol from renewable sources ⁴⁶. In the 1930s, Heinrich Scholler developed a process to hydrolyze cellulose from wood chips by an acid-based process ⁴⁷. Ernst Berl, a chemical professor at the Technical University Darmstadt optimized the process and made fuel from grass, leaves, seaweed, sawdust, scrap lumber, corn, cornstalks and cotton ⁴⁸. In Europe over 20 plants converting wood wastes to ethanol via the Scholler process were constructed by 1941, mostly in Germany, Italy and Switzerland ⁴⁹.

2.3 Wood as Precious Fuel for Energy Autarky

In general, two major developments characterize the wood consumption under the Nazi regime. On the one hand, the total wood consumption increased because of the economic recovery. On the other hand, initiated by the German policy of autarky, big efforts were made to decrease the relative wood consumption (use of wood per produced unit) in order to reduce the dependency on imported timber ⁵⁰. The numbers for harvesting fuel wood is statistically proven only for few years. It was almost halved from 1927 with 24.7 M solid cubic meter ¹⁸⁹ to 1939 with 12.1 M ⁵⁰, ⁵¹, by public campaigns and administrative measures ⁵⁰. It is assumed that the use of fuel wood in rural areas is not included in these statistics. Nevertheless, wood savings were realized mainly by decreasing the demand of fuel wood and residues and waste, which should be used more effectively.

Measures were taken to support coal-, gas-, electro- and more effective wood-fired ovens in the households, as well as to improve the supply of alternative fuels as coal and gas ⁵², ⁵³. Furthermore, in 1937 a regulation was issued prohibiting the burning of raw wood that can be still used as timber (Regulation on promoting the harvesting of timber). Further strategies based on the autarky policy foresaw that no wood residues should possibly be generated or largely be utilized further, respectively ⁵⁴. Therefore, wood chips were boosted for the relatively new wood hydrolysis industry or, after particular preparation, used in wood gas generators (see Sect. 2.2). By the end of the war five wood hydrolysis factories were operated, which produced mainly alcohol as additive for petrol and diesel ⁵⁵.

The decade from 1930 to 1940 is characterized above all by the further development of small automotive and portable gas products ⁵⁶. At the end of the 1930s, about 9000 wood gas vehicles were in use in Europe ¹⁹⁰. To increase the numbers of these vehicles, from 1935 on the adaption from petrol/diesel-powered vehicles was granted by the National Socialist government and the taxes were reduced ⁵⁴, ⁵⁷. Further, the establishment of a nationwide net of wood petrol station was started ⁵⁸, ⁵⁹. In 1935, a test drive of an average of 12 000 km with 46 lorries with generators of different design and different fuels (wood, charcoal, peat coke, etc.) was carried out with the aim of further development of the generators ⁵⁴. The four-year plan of 1936 scheduled a network of about 2300 wood petrol stations ⁶⁰, ⁶¹. In 1941, special prepared wood and charcoal for gas producer vehicles were sold at over 1000 petrol stations ⁵⁶. Between 1934 and 1940, the amount of reported wood gas vehicles in Germany accounted between 1200 and 1500 ^62-64. In 1935 a rail motor coach (railcar) was powered by a wood gas generator and began to run near Cologne ³⁵.

In 1938 only 0.3 %, corresponding to 1070 trucks, were equipped with wood gas generators ⁶⁵. Practical experiences showed that wood gas vehicles could not be operated economically. Further unfavorable properties were mainly the reduced performance, relatively poor efficiency, smaller loading capacity, insufficient gas cleaning, high maintenance effort and cumbersome handling ^66-68. Nevertheless, due to the energy autarky efforts, in 1939 the German government ordered the use of gas for motive power, especially for road and farm vehicles ³⁵, ⁶⁹. However, not only private cars but also buses, trucks, artillery tractors and tanks, motorcycles, ships, rail vehicles, etc. were equipped with wood gasification units ⁵⁴, ⁷⁰. Commercial wood gas generators were produced in different design types by many prominent manufactures as Imbert, Mercedes Benz, GM, Ford, Porsche, etc. For example, conversion kits (Gazogene units) were produced to mount them on the roof of the vehicles or fastened to the sides ³⁵, ⁷⁰. In the Volkswagen (type 60, post war Beetle) and in the German Army Kübelwagen a whole wood gasification unit (type 230) was installed directly in the modified bonnet (hood), which was refilled by a hole in the bonnet ⁷¹.

Due to the strong military ambition of the Nazi regime, in 1942 the gas producer program was transferred to the Ministry of Armaments and War Productions (Zentralstelle für Generatoren). By the end of the war in Germany around 500 000 generators and producer gas vehicles, respectively were produced by the Imbert Company in Cologne, which was the largest manufacturer these days ³⁵, ⁷². In Munich, for example, the city administration was restarting waste collection after the war with wood gasifier cars ⁷³.

The renewed access to relatively cheap imported gasoline after the war immediately led to a decrease of wood-run vehicles due to their economic and technical disadvantages ³⁷, ⁶⁷, ⁶⁸. The consumption of a 5-ton car for a distance of 100 km accounted to around 100 kg beech wood. Further examples for wood consumption in wood gas vehicles are given in Box 2. The power loss of the engine was indicated with ca. 20 % ⁶⁰. In general, the application and research of the wood gasification correlates closely with the cost and availability of liquid and gaseous fossil fuels ³⁷.

3.1 Never Ending Biogas Pioneering

In West Germany, already 1947/48 the first agricultural biogas plant was established on the farm of Philipp Berthaloth in Rohrbach (Odenwald) according to the system „Darmstadt“ of the Technical University Darmstadt – a fermentation canal method with horizontal long axis stirrer after ²⁸. This plant was operated with manure from two horses and eight cows. The biogas, around 8 to 10 m³ daily, was used in the kitchen for cooking and baking. ²⁰, ²⁵. A major agricultural biogas plant with an annual biogas capacity of 40 000 m³ was built in 1948 in Allerhop (Lower Saxony in North Germany) by Ferdinand Schmidt from Verden ^77-79. The system was based on interchangeable containers according to Schmidt-Eggersglüss ²⁸. The recipe of the first so-called biological humus and gas plant (Bihuanlage/Bihu plant) included: manure of ten cows, two centner (200 kg) haulm of potatoes and vegetable waste, respectively or the same amount of other waste from plants. The ingredients are mixed and rotten together with digested sludge from the sewage plant in two digester tanks (each 60 m³). With the gas the tractors rototiller was operated. Further, the digestate was used as fertilizer for the fields, as it contained more nutrients than the dung and could be better absorbed by the plants ⁷⁹, ⁸⁰.

In East Germany the Academy of Agricultural Sciences (Akademie der Landwirtschaftswissenschaften in Jena-Zwätzen) and agricultural research facilities and faculties (among others University of Dresden, Potsdam-Bornim, agricultural faculty in Jena etc.) mainly did research on biogas technologies. Private operators were usually not involved ²⁵. The research aiming for large industrial-scale biogas plants was in particular based on the knowledge of already existing sewage gas and landfill gas technologies ²⁵. In 1953, the first industrial-scale thermophile pilot plant was operated in Thuringia, but closed in 1957 due to technical problems and high heat losses ⁸¹, ⁸².

Until the 1950s between 50 and 70 biogas plants were established in West and East Germany ²⁸. In West Germany, some of these plants were shut down because of operational defects and the decreasing oil price between the 1950s and 60s. At this time the price of heating oil was about 0.10 € L⁻¹ and dropped until 1972 to 0.04 € L^{−1 21}.

Even a lot of biogas plants closed down, two plants stand out. Out of thirteen biogas plants constructed in the 1950s according the mentioned system of Schmidt-Eggersglüss, only the Bihu plant of the monastery Benediktbeuren in Bavaria survived from 1955 until 1979. In two digester towers, each with a capacity of 240 m³, the manure of 180 livestock units could be processed and 300 m³ of biogas was produced. Around 200 m³ gas could be stored in the gas cover. The biogas was used for cogeneration of heat and electricity (CHP): one third for the heating of the digester towers, another third replacing wood for cooking in the kitchen for 200 persons and heating of the monastery; the remaining third supplied a 70-PS gas/diesel engine, which drove the central pump and a 60-kVA generator for power generation. During the operating time of 23 years, only the central pump and lately the agitators had to be maintained ⁸⁰, ⁸³. The farmer Resch from Bernloch, Swabian Alb, used one of the longest operating biogas plants, running from 1958 to 1986. In that time, around 400 000 m³ biogas and 500 000 kWh electricity were generated from the manure of 12 cows. An important feature of the plant was a special axial mixer, which produced a kind of a pulp from the liquid and solid manure. The plant generated an average of 42 m³ biogas per day. Resch calculated that per year 300 L heating oil per cow could be saved ⁸⁴.

The relatively far reaching status-quo of biogas research in wastewater treatment and agriculture after the Second World War was published by the working group “Biogas” of the German Ministry of Agriculture in ⁸⁵. For instance, it was only in 1967 that the microbial basis for the engineering process for the biogas plants was created and the central role of acid-producing bacteria in the fermentation process recognized. In 1979, the topic biotechnology was included in the CIT journal ³¹. Nevertheless, until the early 1990s, very few operators knew how the fermentation process actually works and operated their plants according to the trial and error principle ²⁵.

3.2 Reactivation of Biogas with the Fuel Crises

Under the impact of the oil crisis (1973), the awareness of expanding alternative energies in due time raised. Especially, the West German government gave impulses through research programs to wind and solar energy, as well as to the use of heat pumps. Despite the fact that the focus of energy policy was less on bioenergy, researchers intensified their investigations and several interdisciplinary working groups were established ⁸².

Nevertheless, first companies began to erect professional biogas plants ²⁰. In the 1980s the amount of biogas plants in West Germany was estimated at around 70 bigger plants and 30 smaller (testing) plants ²¹, ⁸². A share of 40 % of the plants was established by new manufactures. At the same time it is supposed that in Switzerland existed around 100 and in Austria two biogas plants ⁸². In contrast to the application of biogas technology in the 1950s, not solid manure, but slurry without litter was mainly used as substrate ²¹. Increasingly co-substrates were added, such as plant biomass (silage), dung or other organic waste and residues to increase the gas yields. In the mid-1980s, CHP plants have not been introduced to the market, yet. The main challenge was a lack of efficient feeding and stirring systems and the high sulfur content of the gas ²⁵.

After the fuel crisis abated, development of biogas use declined again from 1985 to 1990 ²⁰, ²¹. Furthermore, fertilization aspects played an important role for farmers to decide for the utilization of biogas. Not least because of that biogas was particularly connected with organic farming. In 1981, the oldest, still operating, biogas plant was constructed at an organic farm in Hessen ⁸⁶. In the eighties, further eco-pioneers followed to construct self-made biogas plants ⁸⁷. In 1990/91, the number of biogas plants in West Germany was estimated at approx. 100, of which around 70 were assumed to be operated on organic farms ⁸⁸. This trend did not continue during the biogas boom. In the early 2010s only one to maximum two percent of the produced energy from biogas came from organic farms, although it accounts for around 7 % of the total agricultural area. ⁸⁹

Also in East Germany nutrition aspects were the promotor of the biogas sector: The animal production was centralized in big agricultural production co-operatives ²⁵ and around 77 million tons of manure were generated by the industrial livestock production in 1980 ⁷⁶. Thus, the aim of the state program based of the X. party congress was the generation and storage of biogas for energy ⁹⁰. This included above all the development of concepts for processing semi-liquid manure into fertilizer with lower costs than the synthetic production of nitrogen fertilizers. The produced heat was regarded as by-product or even waste, and biogas was not seen as contributing to the big energy issues ⁹¹.

As a result, eight demonstration and pilot biogas plants in industrial scale ²⁰ with a fermenter volume from 500 to 80 000 m³ were constructed ⁸². The first big agricultural biogas plant of that type was established in 1982 in Vippachedelhausen, close to Erfurt. The plant is based on a process of the VEB “Complete Chemical Plants” (VEB, people-owned enterprise) of the Technical University in Leuna Merseburg ⁹². The biggest biogas plant was established in 1985 in Nordhausen, Thuringia with 37 m high steel containers and a volume of 8000 m³ each. Partly the gas was converted to electricity via the first CHP plant with a gas engine (reportedly, ⁸²) and a modified tractor (ZT 300) was fueled by liquid biogas. The plant is still in use today and has an installed electrical power of 2.2 MW. At least 80 % of the total biogas was produced in Thuringia ⁸². At this time the first biogas plant with a horizontal plug flow system, erected in 1985 in Rippertshausen, was unique in the world. This plant was still operated until 2004 with organic waste as co-substrate. The last biogas plant of the former GDR was built in Zoben, close to Plauen in Saxony in 1987. It was the only plant that operated via a two-stage process with an upstream hydrolysis. Another technical feature of this plant and the one in Nordhausen was that it was equipped with a system for recirculation of the solid material for further digestion. Dry chicken manure from 0.25 million laying hens was mainly used as substrate. Today the plant runs with organic waste ⁸². At the time of the reunification 1990 around 80 biogas plants were operated in Germany, some are still in use ²⁰.

3.3 Slight Developments of Liquid Biofuels and Gasification

The research on conversion technologies such as gas motors and generators for power production as well as liquid biofuels was not focused in West Germany and the former GDR ²⁵. However, CHP plants and heat pumps based on biogas offered a wide field of application, especially in agriculture ⁷⁵. Wood played an important role as cost-effective raw material for private households in West Germany; in the East, the wood industry was nationalized. In case of biofuels, the West German Government launched a program for introducing cars, which run on a mixture of 15 % methanol and 85 % petrol. At this time, the technology for producing methanol from wood was more advanced and economic than for ethanol production. As part of the overall test program, thirty petrol stations offered this fuel in 1980. ⁴⁹

In East Germany biogas was regarded as the most relevant renewable energy for substituting liquid fossil fuels, especially diesel in the agricultural sector ¹⁹¹, with an estimated potential of 0.5 to 1 % ⁹³ up to round 8 % ⁹⁴ of the diesel consumption in agriculture in the year 1986. Major technical difficulties were found in the storage of biogas in the vehicles. Therefore, biogas or biomethane, respectively was compressed (CNG) or liquefied and stored as LNG (liquified natural gas) in vacuum isolated cryotanks in the vehicles. In research projects different prototypes of LNG vehicles were constructed and tested and LNG was produced ⁹⁴. A small-scale plant for the liquefaction of methane was developed (−161 °C) and different types of automobiles were modified for the operation with LNG, such as a Wartburg, Polski FIAT (gas Otto engine), the truck W50 with a gas diesel engine or the LNG tractor ZT300 ⁹⁴. For instance, the tractor ZT300 was equipped with a special 200-L cryotank and a gas diesel engine ⁹⁴. At the same range or operating time, the cryotanks had only 1/7 of the mass of the pressure cylinders for compressed biogas or methane CNG, respectively ⁹⁴.

4.1 Power from Wood – Technologies for Efficient CHP Systems

From 1990 to the 2010s, a wide range of CHP plants was developed, including pellet stirling, micro-CHP plants and CHP plants with fuel cells ^105-107. In 1993, Germany's first municipal straw heating plant with a capacity of 3.15 MW, based on the cigar burning principle, went into operation in eastern Thuringia. The straw, 3000 t a⁻¹, is generated as waste from the surrounding cereal production ¹⁰⁸.

The majority of the biomass power plants range between 150 and 500 kW and supply the agricultural enterprises. Larger plants with an output of up to 20 MW are operated in municipal sewage treatment plants and in the wood industry. Microplants under 150 kW for the heat supply of single-family houses are mainly based on solid biogenic fuels. ¹⁰⁹

Since the EEG came into force in 2000, the number of biomass (heating and) power stations has increased more than 20-fold (taking into account estimates for the addition of wood gasifiers) and the installed electrical power has increased sevenfold ¹¹⁰. A large number of processes and variations were developed, but these did not always meet the requirements of the market ¹¹¹, ¹⁹². Wood gasification CHP plants have been increasing noticeably since 2011, especially in the output range up to 200 kW_el. Almost 700 biomass (heating and) power stations including thermo-chemical wood gasifiers with a cumulative electrical capacity of around 1511 MW_el were in operation at the end of 2014. Due to the Renewable Energy Sources Act 2014 (EEG 2014), the activities of plant manufacturers in the wood gasification sector were increasingly shifting abroad, where the current political conditions are more favorable and enable economic operation ¹¹².

Nowadays, however, the technology of some plant manufacturers has reached a state of the art that permits largely trouble-free plant operation and is acceptable from the point of view of emission control with regard to emissions of odors and benzene ¹¹³. Only a few manufacturers, mainly for systems in the low output range, offer now standardized systems manufactured in series production (Spanner, Burkhardt, etc.).

The production of wood pellets in Germany started in 1996 with pellet boilers from Austria and very small operation systems. The following year, the installed annual production capacity amounted to 6500 t ¹¹⁴, ¹¹⁵. In 1996, pellets were approved as solid fuel, but only established in the German market two years later ¹⁰¹, ¹¹⁵. Since then, wood as a raw material has literally become socially acceptable and is experiencing a renaissance as a modern energy source ¹⁰¹. In 2000, the pellet boom started, seven pellet production plants were registered ¹¹⁶. By 2017, around 437 000 pellet furnaces were in operation in Germany ¹⁰¹.

4.2 The Hype of Energy Plant Expectation

In parallel in the 1990s, the European agriculture production was more and more under pressure due to overproduction. Hence, the European Commission introduced the tool of set aside of the arable land. In 1988/89, set-aside was fixed at 15 %. This set-aside rate was adjusted annually in line with overproduction (see Fig. 3). From 1993/94 on it became compulsory and was reduced to 10 % in 1996 ¹¹⁷. Since then, renewable raw resources were allowed to be cultivated on these set-aside areas and eligible for set-aside payment. Agriculture entered the politically intended production of first-generation biofuels, i.e., ethanol and biodiesel ¹¹⁸.

Bioenergy was seen as a valve for agricultural overproduction. In 1995, 10.83 % of arable land was set aside, added by around 3 % set-aside area for the cultivation of renewable resources. In 2006, set-aside dropped to 6.24 %, whereas side-aside area for the cultivation of renewable resources increased slightly to 3.34 % (see Fig. 3). In Germany, arable land on which renewable resources were cultivated has increased more than fivefold since the beginning of the 1990s, not only including set-aside land. In 2005, more than 1.4 million ha of arable land were cultivated with renewable raw materials in 2005, which corresponds to 12 % of the total arable land ¹¹⁹. In autumn 2007, the set-aside percentage was set at 0 % and abolished in 2009 due to the increased demand for agricultural products ¹²⁰.

Under the debate of the national and international energy transition, global biomass potential assessment studies were elaborated to figure out the future role of bioenergy in a renewable energy supply. Most studies indicate the technical bioenergy potential, which describes the part of biomass that can be used considering the given technical restrictions. These include technical feasibility (e.g., salvage rate, conversion losses), structural and legal limitations (e.g., protected areas, cross-compliance regulations, waste legislation) and ecological restrictions (e.g., humus balance) and social priorities (e.g., food and fiber first) ¹²⁴.

Fig. 4 shows the range of the estimated global potential of 32 studies for the overall biomass potential including raw materials from forests and cultivated biomass from agricultural land as well agricultural residues such as straw, manure and waste. Further, the potential estimation for energy crops in the different studies is shown. The bioenergy potentials given in the studies are presented with their minimum and maximum values of their energy content in exajoules, where available. It should be considered that not all of these studies presented original data.

The range of global potentials is between 38 and 1723 EJ a⁻¹. The average global primary energy consumption, including fossil and renewable energies was about 13511.2 Mtoe (542 EJ) in 2017 ¹²⁵. Especially the studies published between 2002 and 2007 came to very ambitious expectations regarding the contribution of biomass to the energy supply. The range of potential estimates for 2050 for energy crops were also very high according to the studies 2002 to 2007. Residual materials are less affected than energy crops. The potential for residual and waste materials is much more sable and estimated in the studies at approx. 135 EJ a⁻¹ plus the use of firewood of 36 EJ a⁻¹, which means 14 GJ a⁻¹ per capita.

Considering those studies, political measures were taken to introduce agricultural-based bioenergy sources into the market in many countries. In Germany the focus was on power provision via biogas and biofuels, respectively ²⁵. However, the disillusion came after only one decade. Questions about the extent to which food crops can or should be used for fuel were increasingly raised. Climate emissions from agriculture were reassessed and the demand for agricultural products increased (2007 crisis: increase in prices, harvest failures, etc.) as well as the awareness of the finite nature of resources. Hence, energy research and policy focused especially on the impacts of biofuels and biogas on climate change, food rights, biodiversity and sustainability ³⁹, ¹²⁶.

4.3 Feed-in Act Jumpstarts Biogas Development

A third upswing in biogas utilization began at the beginning of the 1990s with the legal regulation of the legally granted feed-in tariff for electricity from biogas ¹²⁷ and the general conditions of the Closed Substance Cycle and Waste Management Act (Kreislaufwirtschafts- und Abfallgesetz) ²¹. From 1992 to 2000 the amount of biogas plants increased from 139 to 1050 ¹⁵, ¹²⁸. The co-fermentation of organic residues mainly from slaughterhouses, breweries, and food processing companies and biowaste from households further increased economic efficiency ²⁰. Innovation pressure led to optimizations primarily in the area of filling, stirring technology, desulfurization, and engine environment and consequently had a positive economic effect. For the first time, ready-made systems from manufacturers were offered.

In the first decade of the 21st century, biogas technology has undergone rapid development in the agricultural sector. Nevertheless, it was not until the “Renewable Energies Act” (EEG) in 2000 and the guaranteed remuneration for electricity and heat from combined heat and power generation that a real boom of the biogas sector began ²⁰. Energy crops were incentivized since 2003, and thus, increasingly cultivated. Since then, the biogas boom was closely linked to the agricultural sector. However, alternative fermentation processes such as dry fermentation were tested in order to further mobilize the potential of cultivated biomass, especially dry substance-rich fermentation substrates, such as maize whole plant silage ²⁵.

From 2000 to 2010 the number of biogas plants increased from 1050 to 6311 ¹²⁸ with an installed power of 65 MW to 2291 MW ⁸⁷. The cultivation area for biogas substrates, including mainly maize silage, corn and grass silage, increased from around 20 000 ha in 2000 ¹²⁹ to 650 000 ha in 2010 ¹³⁰. More efficient processes for energy were developed, the feasibility of flexible biogas production was demonstrated ¹³¹ and the substrate spectrum broadened. In addition to biogas plants, a large number of sewage gas plants and landfill gas plants contribute an additional 30 % to the supply of electricity from biogas ¹³².

The fourth EEG entered into force in 2012 and the legal framework was deteriorating. The boom phase for biogas plants was over. However, the new EEG offered for the first time the entry into the so-called direct marketing of electricity via the market premium model and the flexibility premium ⁸². With the amended of the EEG 2014, drastic new regulations for electricity generation from biomass came into force. This led to a sharp decline in new construction activities, with the exception of special plant concepts for the treatment of residual and waste materials ¹³³. From 2015, the construction of new plants was mainly concentrated on manure and waste plants (small manure plants up to 75 kW_el (§27b EEG), biowaste plants (§27a EEG)). Thus, plant system repowering and expansions mainly determined the better use of the coupled heat and the performance of biogas plants. Furthermore, flexible plant operation was subject to intensified research and practical tests. Biogas and biomethane plants, which were entitled to a flexible premium, contributed around 4 TWh_el in 2016, or 12 % of electricity generation from biogas ¹³⁴.

Furthermore, biogas plants provide another way of suppling carbon dioxide for methanation to power-to-gas (PtG) processes. In 2013, the first industrial PtG plant with 6.3 MW was operated in Werlte, in the Emsland region of Northern Germany ¹³⁵. Here, it was shown how catalytic methanization can be implemented using CO₂ from the biogas plant located at the site. In 2015, the general feasibility of biological methanization was demonstrated ⁸². By the end of 2016, 30 PtG plants with a power consumption capacity of approx. 21 MW were commissioned. First combinations of biogas plants with PtG shows, how sector coupling of electricity and gas can function with the integration of biogas plants. Moreover, the produced renewable methane as a long-term storage option makes the conversion of existing CHP plants to electricity more flexible ¹³⁶.

With the EEG amendment 2017 (EEG 2017, 2016), the amount of remuneration from biomass is determined primarily via tenders in future. This update is also linked to certain minimum requirements for flexibility and has the clear objective to make the plant portfolio more flexible in the coming years. ¹³³ Because of the low level of new constructions ¹³⁷, the activities of plant manufacturers in the biogas sector, especially for integrated biowaste fermentation and composting plants, were increasingly shifting abroad ¹³⁸.

4.4 Ramp Up of Biofuels for Transport from Alternative Resources

Biofuels were often regarded as an important element of the energy transition ^139-142. Thus, fuels from agricultural products (biofuels of the first generation) were established in the market, including biodiesel and ethanol as well as pure vegetable oil, hydrogenated vegetable oils (HVO) and biomethane. These are based on resources from agriculture, e.g., rape, cereals, sugar beet, maize, soy or oil palms. To produce larger volumes of sustainable biofuels, in addition to energy plants, residual materials, such as used cooking oil, are already used today for the production of biodiesel, HVO and biomethane ¹³⁹.

The biofuels market is highly dependent on political and legal conditions. These have changed considerably over the past few years. The generation of those biofuels increased sharply from 2000 (12 PJ) to 2007 (about 170 PJ) and were dominated by biodiesel with ca. 72.8 % and bioethanol with ca. 7.7 % ¹⁴³. In addition to tax exemptions until 2009, in 2006, the law to introduce a biofuel quota was passed ¹⁴³. Many new process facilities opened (see Fig. 5, Tab. 2). The first biogas filling station has been opened in Jameln (Wendland) ¹⁴⁴. In 2006, the first biomethane plant near Munich fed biomethane into the natural gas grid ⁸². With biomethane a wide range of using natural gas is provided, including heat, electricity, fuel and process energy. While the existing production capacities for biodiesel in Germany in 2007 of around 3.5 million tons were well above the established biodiesel proportions of around 1.5 million tones (biofuel quota 4.4 % for biodiesel), the expansion targets in the EU were not achieved ¹⁴⁵. For 2020, assuming the availability of sustainably produced raw materials, the target has been set to 10 % of the total consumption of petrol and diesel in the transport sector (KOM (2006) 848) ¹⁴³).

Further research activities have been intensified to produce biofuels from non-food feedstocks like lignocellulosic material in agricultural and forestry wastes, purpose-grown trees and grasses, as well as algae (2nd generation biofuels) via microbial and thermal processes and adapting and improving biofuels for engine compatibility and emission mitigation ⁴⁷, ¹⁴⁶. In 1998, Choren started the first pilot plant to produce methanol from wood in Freiberg, but went bankrupt in 2011 ¹⁴⁷. Commercial scale plants and notable demonstration scale plants for bioethanol from lignocellulose are in either operation or commissioning (see Fig. 5, Tab. 2). For instance, Clariant operates since 2012 a demonstration plant based on renewable lignocellulosic raw materials, such as wheat straw, corn stover and bagasse with an annual capacity of up to 1000 t ethanol ¹⁴⁸, ¹⁴⁹. In 2014, VERBIO followed by producing biogas, bioethanol and biomethane from 100 % straw (rye) at the production facility in Schwedt ¹⁵¹. However, technical barriers remain for 2nd generation biofuel production. Synthetic biofuels (BtL) still require some time to reach industrial scale. Biofuels from algae or other alcohols are at a comparatively early experimental stage and will hardly be available on the market in considerable, low-cost quantities in the medium term. In this respect, the classic biofuels biodiesel, ethanol and vegetable oil as well as biomethane are likely to remain the most important renewable alternatives in the mobility sector in the medium term ¹³⁹ (cf. Fig. 5, Tabs. 2, 3 and 5).