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INTRODUCTION
In accordance with the Guide on Biogas, from production to the use, 2013 (Guía sobre el biogás, 2013), the technology of the plant covers a very wide spectrum. Practically, limits do not exist in component terms and in equipment combinations. Consequently, technical examples are given in this paper to illustrate articles about equipment. However, it should be noticed that it is always necessary to make a particular expert analysis for each case about the convenience and adaptation of the capacity of plants and systems.
Dry Matter Content of Substrates for the Digestion: The consistency of the substrate depends on its dry matter content. This is the reason why the biogas technology is subdivided in processes of humid and dry digestion. The humid digestion uses substrate of consistency susceptible of pumping. The dry digestion uses piling substrate. The Law of Renewable Energy Sources (EEG) of 2004, specifies a content of dry mass of at least 30% for mass in the feed stock and a rate of organic load of at least 3,5 kgVS / (m3.d) in the digester. The dry matter content in the liquid of the digester in the humid digestion process can be until 12% of the mass. Generally, a limit of 15% of the mass to pump the medium is predetermined, but the value is qualitative and it is not viable for all the feed stocks. Some substrates that have a distribution of finely dispersed particles and discharges proportions of dissolved substances can even be pumped when the dry matter content reaches 20% of the mass. This is the case of the residuals of discharged dispersed feed stock of transports in tanks. Other substrates like fruit and vegetables peels are piling when the dry matter content (DM) is 10 or 12% of the mass.
Feeding Type: The load or feeding regime of the biogas recovery plant determines greatly the readiness of fresh substrate for the microorganisms and it has a corresponding effect in the biogas generation. Wide distinctions are made among the continuous, semi-continuous and intermittent supply.
Continuous and Semi-Continuous Supply: An additional distinction can be made between the methods of continuous flow and a combination of continuous flow with tank tampon. The feeding method that uses only the tank tampon is not mentioned here because the current economic considerations and engineering of processes virtually impede its use, although it is still mentioned in the literature. In contrast with the continuous supply, the semi-continuous supply implies adding to the digester, a not fermented lot of substrate at least once for working day. Additional advantages exist in adding the substrate in several small lots along the day.
In Germany, at the end of 2017, there were more than 8000 biogas plants of medium and great format in production. Using the co-fermentation of cow and pig excretes, with substrates of agricultural origin, standing out for these ends varieties of energy corn, previously fermented in bunker silos, bio-products of the food industry and canteen residuals.
According to Suarez et al. (2018), in the sectors of Ministry of Agriculture (MINAG), Ministry of Food Industry (MINAL) and Ministry of Sugar Industry (AZCUBA), the potential of biogas production is of 674 609 m3/day, mainly from pig and poultry production. This potential means a production of energy of 1 477 394 MWh/year, equivalent to 132 856 t of diesel whose import costs Cuba 48 615 065 USD. On the other hand, the emissions avoided by the substitution of this fossil fuel by biogas are considered in 440 778 t of CO2eq/year.
In Cuba, according to NTV, 2020, cited by Oechsner et al. (2020), 3000 biogas plants of small format and 70 plants of medium format exist in the state and private sectors. In their majority, they use the residual (excrete) of the pig and cow production in mono-fermentation (excrete+water without the addition of other substrate). Due to the potential of existent residuals at country level (in the cattle, poultry and pig branches) it is considered that it is possible to elevate the number in next year’s up to 7000 biogas facilities. If the above-mentioned were achieved, the perspective agenda to develop renewable sources of energy for the decade 2020-2030 in the country would be supported, since Cuba has foreseen to improve its energy matrix giving 24% participation to the renewable sources of energy in that decade.
The pig production of Villa Clara province stands out at national level in this activity. In this county, more than 400 biodigesters of small format have been installed at state and private level, in order to give treatment to these residual and to obtain biogas, however some problems related with the underemployed of the biogas facilities, as for the electricity production, the use and final disposition of the effluents and the little use of these as organic fertilizer have gotten the attention of the sanitary authorities of the territory. In works previous of Francesena (2016); Martinez et al. (2017); Martínez et al. (2017); Martínez & Francesena (2018), it has been meditated on the control parameters and monitoring of biogas plants of medium and small format, the environmental impact caused by the effluents of these plants, the non-execution of the Cuban norm of pouring NC-27:12 (2012) and the underemployed of these effluents like organic fertilizer. In the face of the possibility of creating biogas facilities of great format in the future, this work has as objective to present the state of the art in this thematic.
This work has been elaborated taking bibliographical references from 1998 to 2020. The mentioned fundamental sources are of German origin, although works carried out in Cuba are also mentioned. The geographical places referred are from Germany and Cuba. The selection approaches kept in mind in this work have been related with the technologies and uses of biogas facilities of great format.
DEVELOPMENT OF THE TOPIC
In Germany several methods are used for the biogas production. Some of these are presented next:
Method of Continuous Flow: In the past, most of systems of biogas recovery were built to operate on the principle of continuous flow. Substrate is pumped several times a day from a pre-digester tank or from a pre-digester well to the reactor. The same quantity of fresh substrate that is added to the digester is expelled or extracted from the tank of digested storage (Figure 1).
Process of Combination of Continuous Flow and Tampon Tank: The biogas recovery plants that operate the combination of continuous flow/tank-tampon, also use facilities covered of digested storage. This allows capturing and using the product of the biogas digestion. The tank of digested storage works as a tank tampon. The current above the unit of this tank tampon, that is part of the plant, is a digester of continuous flow. If there were an urgent necessity of a great quantity of pre-digested substrate as fertilizer, substrate could be taken out from the digester of continuous flow. Figure 2 is a diagrammatic list of the process. The process allows a permanent production of gas. The time of staying inside the tank cannot be determined with accuracy due to the possibility of short circuits of the flow in the continuous digester of flows (Weiland & Rieger, 2001). This process is the most advanced at the present time. The investment expenses for the storage tank of the digested can be recovered from the incomes for extra yields of gas.
Source: Weiland & Rieger (2001).
FIGURE 2 Outline of the combined process of continuous flow/tampon tank.
Intermittent Feeding: The intermittent feeding, for lots, implies to fill the digester completely with fresh substrate and then to place a hermetic closure. The feed stock remains inside the tank until the selected time of staying passes, without adding or eliminating any substrate. When this time concludes, the digester is emptied and filled with a fresh lot of feed stock again, with the possibility that a small proportion of the digested can stay as material seed to inoculate the fresh substrate. The process of filling of the lot digester accelerates by placing a supply tank, with a recipient of discharge storage for the same purpose next to the exit. The intermittent feeding for lots is characterized by a rate of gas production that changes along the time. The production of gas begins slowly after the reactor has been filled, it reaches a pick in some days (depending on the substrate) and then it goes diminishing continually. As a unique digester, it cannot assure the constancy in gas production or the quality of gas, the filling up by stages of several digesters has to be adapted (method of filling of lots in battery) so that the net production is more homogeneous. The time of minimum staying is kept with accuracy. (Weiland & Rieger, 2001). The feeding for lots of unique digesters is not practical. The principle of the feeding of lots in battery is used for the dry digestion, what is sometimes known as "digesters garages" or "digesters in modulate boxes."
Number of Phases of the Process and Stages of the Process: A phase of the process is considered like the biological medium, hydrolysis phase or methane phase with specific conditions of the process as pH value and temperature. When the hydrolysis and the methane process happen in a unique tank, the utilized term is process management in unique phase. A process in two phases is that in which the hydrolysis and the methane process happen in separate tanks. Stage is the term that is used for the process tank, independently of the biological phase. As a consequence, the configuration of the plant with a well pre-digester, a digester tank and a tank of digested storage that is often found in agriculture, is of unique phase, but in three stages. The well open pre-digester as such is not a phase separated in it. The sealed recipient of retention or reception, on the other hand, is considered like a separate phase (hydrolysis phase). The main and secondary digesters belong both to the methane phase. Generally, the agricultural biogas recovery plants have a design of unique phase or two phases, being the plants of unique phase the more communes (Schulz & Eder, 2006).
Engineering of the Process: In general terms, independently of the operative principle, agricultural biogás plant can be subdivided in four different processing steps:
Manage of the substrate (delivers, storage, preparation, transport and feeding)
Recovery of the biogas
Storage of the digested, treatment and spread in the field
Storage, treatment and biogas use
DEVELOPMENT OF THE TOPIC
The individual steps are shown in Figure 3. The four steps of the process are not independent to each other. The link between steps two and four are particularly narrow because the step four generally provides the heat that is needed for the step two in the process. The treatment and use of the biogas that correspond to the step four are referred separated in Chapter 6. While Chapter 10 refers to digested processing and treatment (Guides on the biogas. From the production until the use, 2013) in Guía sobre el biogás (2013). The following information is related to the technology and the techniques used in steps 1, 2 and 3. The election of the processing equipment depends mainly on the nature of the available substrate. The sizes of all the plants and containers have to be based on quantities of substrate. The quality of the substrate (dry matter content, structures, source, etc.) is the decisive factor in terms of designing of process engineering. Depending on the composition of the substrate, it can be necessary to eliminate substances that interfere or to humidify them with extra liquid to obtain a capable mixture for pumping. If substances that require sanitation are utilized, the planning has to allow a sanitation stage. After pre-treatment, the substrate is transferred to the digester where it is fermented. The plants of humid digestion generally have a design of two stages and they operate under the principle of continuous flow. A design in two stages consists of a digester and a secondary digester. The substrate moves from the first digester, the primary one, to the secondary digester, where the most resistant substances can be bio-degraded. The digested is stored in sealed tanks of digested storage with biogas extraction or tanks open of digested and then it is generally discarded, spreading it as liquid fertilizer in agricultural soil. The biogas produced by the biodegradation of the feed stock is stored and purified. Generally, it is used for combustion in a combined unit of heat and energy (CHP) to co-generate electricity and heat. Figure 4 shows the most important components in the plant, the sub-assembling and the units of a plant of agricultural biogas recovery of unique stage for co-substrate with sanitation.
According to Oechsner & Lemmer (2007), already in the 2007, there were 3500 biogas plants in Germany with capacity to generate more than 1 GW of electricity. That shows clearly the German government's policy in this sphere, which reports electricity, heat and organic fertilizer. These plants can operate in mesophilic (37-42 0C) or thermophilic (46-55 0C) regime, in mono-fermentation and co-fermentation, being these last ones the most utilized. According to Oechsner & Lemmer (2007), the co-fermentation has the following advantages: it optimizes the relationship C/N, increases the biogas yield, influences in the quantity and quality of the biogas, stabilizes the process, adds value to the treatment of the residuals and closes the cycle of the nutrients. As disadvantages, these authors outline: risk of introducing toxic substances (industrial and municipal residuals) and pathogen organisms into the system. For such a reason, when this type of substrate is used, it is necessary to decontaminate it. In the continuous development, investigation and introduction of improvements in these plants in Germany, different productive and academic institutions have worked and some of the merits and flaws reported have been published in the work of Schlegel et al. (2009).
Sources: (Institute Leibniz for the Agricultural Engineering Potsdam-Bornim, ATB).
FIGURE 4 Outline of a plant of agricultural biogas recovery for co-substrate.
These authors outline that the sensibility of a biogas plant able to generate up to 500 kW-h/day of electricity is provided by achieving an appropriate relationship among factors like hours of daily operation, biogas operating cost (cultivate of energy plants, preparation of the substrate and their transport), cost of the initial investment, efficiency of the plant and utility for the sale of the electric power). Likewise Schlegel et al. (2009), mention that the most common problems are presented in the groups: motor-generator, apparatuses for substrate removing, pipes for heating the substrate, mensuration and control apparatuses, plastic cover to retain, store and evacuate the biogas produced. These authors analyze annual and daily hours of total load and the relative load in the years 2004 and 2005, obtaining the values presented in Table 1.
TABLE 1 Total load hours per year, per day and relative load Source: Schlegel et al. (2009)
| Year | Annual electricity production | Daily electricity production | Total load hours per year | Total load hours per day | Relative loads |
|---|---|---|---|---|---|
| kWhel/a | kWhel/d | h/a | h/d | % | |
| 2004 | 2 537 593 | 6 952 | 5 106 | 14,0 | 58,29 |
| 2005 | 3 001 995 | 8 225 | 6 040 | 16,5 | 68,95 |
From the analysis of Table 1, it is inferred that the quantity of working hours daily determines the electricity production and the relative load of these facilities. According to Schlegel et al. (2009), in these plants the remove apparatus has the following functions: to avoid floating and deposited layers, to mix fermented and fresh substrates, to homogenize the substrate temperature and to make biogas flow. These authors mention that optimizing the process requires to avoid breakdowns of the motor (risk for lack of heat supply), to avoid the supply of great quantity of cold substrate (risk of reduction of temperature), to avoid failures in the remove apparatus (appearance of bubbles, risk of obstruction of the pipes) and to provide external energy to the beginning of the process. In these plants, a constant monitoring of the quantity of biogas produced and its contents of methane, dioxide of carbon and sulfur is carried out. It is also foreseen the cleaning or H2S elimination from the biogas to prevent that this element causes damages and interference in the process of electricity generation, since the motor that activate the generator works with a part of the biogas obtained in the plant.
A biogas plant of great format involves a group of facilities that involve a closed technological flow, in which the feeding with animal and agricultural substrates is interconnected with the animals’ facilities and silos type bunker where the agricultural substrate is fermented. This group of facilities should have a very connected infrastructure with the necessary and specific agricultural machinery for the storage and processing of the substrate, electricity and heat productions, as well as those for gathering and application of the obtained bio fertilizer (digested) into the areas of agricultural production. In Figure 5, an outline of the process is shown in this type of plants (Lemmer, 2009). The technical data of this installation are: Use of fermentable substrate: liquid excretes: 7,200 kg/day; solid excretes: 3,500 kg/day. Use of agricultural co-substrate: field fruits-GPS: 3,000 kg/day; grass silage: 1,500 kg/day; grains: 1,000 kg/day. Specifications of the installation: Volume of the fermenter: 2 x 923 m³. Volume of the procedural digestate tank: 1 x 923 m³. Electric power generated in the biogas plant: 186 kilowatts-h. Thermal power generated: 207 kilowatts-h. The cost of this type installation oscillates among the 2 to 2, 5 million Eurus.
Source: (Biogasanlage "Unterer Lindenhof" Poster. Hohenheim University. Stuttgart, 2009).
FIGURE 5 Scale model of the biogas plant.“Unterer Lindenhof”.
On the other hand, the investigations to productive scale and in centers of investigations allow maintaining these facilities in constant change and development. All that helps to optimize these plants in the productive and economic aspects. In recent works, Oechsner et al. (2020) show some results obtained while comparing the results obtained to laboratory scale with regard to real scale, which confirms that an appropriate transferability exists from the obtained results to laboratory scale to real scale, the above-mentioned allows an improvement and optimization of the biogas plants continually.
Treatments of the Digested: The number and the size of the biogas plants in Germany are rising in an abrupt way according (Guides on the biogas. From the production until the use, 2013) (Guía sobre el biogás, 2013). The plants of multiple stages exhibit a lower residual gas potential at 20 - 22 °C and to 37 °C. This is mainly due to the fact that a plant of multiple stages has a higher time of retention that has the effect of reducing the residual gas potential. Since the high potential of hothouse of CH4 (1 g CH4 is equivalent to 23 g CO2), it is desirable to reduce or to avoid the emissions of CH4 of the digested storage tanks. The plants without final storage, safe against gas leakages, besides the operation in multiple stages (cascade digesters), satisfy, at least, one of the following requirements:
Average of hydraulic retention time of the total substrate volume of at least 100 days to a continuous temperature along the year at least of 30 °C or rate of organic load of the digester <2.5 kg VS/m3 d.
The calculation of the volume of the substrate should take into account all the inputs in the tank (s) of digestion (including, for example, water and/or recycled). If the aforementioned requirements are not fulfilled, it should be expected that the methane emissions exceed the values average. In these cases, it is advisable to recondition the tank (s) of digested storage with a safe closure against leaks of gas for at least the first 60 days of required storage of digested. The tank (s) of digested storage should fulfill the following requirements: a) it should not have active control of the temperature and b) the tank should be connected to the system for gas transporting. An effective prevention of emissions of CH4 of the digested is achieved covering for the first 60 days of storage of required digested because, as it is known for experience, the methane formation under the prevalent conditions in a plant of the real world will have culminated in that period.
Particularities of the German Case
In accordance with the Law of Renewable Sources of Energy (EEG) of 2009, the covering of tanks of digested storage is a pre-requirement to receive the NawaRo voucher (energy cultivations), in the cases in which the plant can be authorized, according with the Law of Contamination Control of Germany. This includes all the plants whose capacity of total combustion exceeds 1 MW (equivalent to approximately 380 kWel) or whose capacity of dung storage exceeds 2,500 m³. Although this is applied to all the new plants, the interpretation of the Law continues being in discussion regarding the existent plants, since in many cases, the rebuilding of digested storage tank is not possible or it is possible solely to a certain extent.
Use of Biogas Plants in Germany
As it is shown in the work, this is a mature technology and it is ready to start. There are several German companies that are in charge of their installation, setting up, technical support and guarantee services during their exploitation (after-sales).
Analysis of the Cuban Case. In Cuba several scientific institutions and Universities have studied the technology of biogas production during years. It was introduced for the first time in 1985, however, a peak in their study and implementation has been recently reached due to the proliferation of contracts with private pig producers, which are demanded for an appropriate treatment of the residual generated. Some investigations referred to the use of biogas (biodigesters) have been carried out. Guardado (2016), created the national Movement of Biogas Users in Cuba (MUB), which includes state and private producers that possess biodigester of medium and small format. That author refers that, in Cuba, it is possible to generate 27 MW of electricity from biogas. The constructive types of these biodigesters are: fixed dome, mobile dome and plastics or flexible. Fifteen provinces participate and around 3000 members belong to the Movement of Biogas Users (MUB). Guardado (2016) details the advantages and disadvantages of the biogas technology as follows:
Damages: contamination of the water bodies, transmission of illnesses, deterioration of the sanitary-hygienic conditions, emissions of carbon dioxide (CO2) and methane (CH4) to the atmosphere.
Benefits: cooking, illumination, heating, welding, incubation of eggs, transportation, motors of internal combustion, refrigeration, conservation of grains, fish culture, watering, fertilization to foliate, plagues control, soil improver, organic fertilizer, animal feed, worm culture and cultivation of mushrooms.
Economic-Environmental Feasibility of the Use of Biogas Plants of Great Format
ANALYSIS. In Cuba, until the present, the co-digestion of excrete of animal origin and biomasses of agricultural origin is not used. The existent biodigesters are single-stage types, of small and medium format, using mono-fermentation (pig or bovine excrete with water);
Cuban biodigesters are generally constituted for: ramp to feed the biomass (caw or pig excrete and water), load well, stabilization channel, gas meter, key in passing and valve for the exit of the biogas, biodigester and well of discharge of the effluents. The existent constructive types in Cuba (fixed dome, mobile dome and plastics or flexible) possess a cost of around 1500 at 2000 $(CUP)/m3 of the biodigesters volume. The most widespread Cuban biodigesters oscillates among 10-12 m3, although other less utilized superior volumes exist.
The biodigesters installed in Cuba cannot be considered like biogas plants, for not possessing the whole necessary infrastructure for such a definition. However, in Cuba until the present, only a minimum quantity of systematic studies referred to biodigesters of great format are reported, of them, the most significant are located in Havana (100 Street drain), biodigester that uses the urban solid residuals poured in that place, to produce methane and to give energy to a motor that works an electric generator facilitating the production and electricity distribution to the electric net of the territory. The Agro Industrial Company Guayos, in Sancti Spíritus, has settled a biodigester of 740 m3, it uses substrate from a pig multiplier center, four rabbit units, a slaughterhouse of pigs, cows and birds and a fish plant. The power of electric generation is of 1 MW-h. The produced biogas is compressed, it is stored in metallic cylinders and it feeds to internal combustion motors that put in action an electric generator. The effluents of the biodigestion process are spilled to a lagoon of oxidation (stabilization), from where they are taken to be used as fertilizer in grass areas utilized for feeding bovine livestock. In Martí Municipality, Matanzas Province, a biogas plant has been built with the technology of covered lagoon whose technical characteristics are: capacity of processing 3854 m3/day, volume of operation of the biodigester 1542 m3, total volume of the biodigester 1850 m3, biogas production per day 771 m3, volume of the secondary lagoon 617, 92 m3, reduction of emissions 5, 39 teq/day CO2, reduction of DBO5, and DOO> 95%, daily generation of electricity 1,542 MW-h. In Cuba, until the present, there is a little use of this type of biodigesters, hence it is necessary to think about the possibility of introducing and using in Cuba, biogas plants of great format like those built in Germany. The answer to this query is given in the use of this technology under a deep technical and economic analysis that ponders the advantages and disadvantages of its introduction in the country, which could represent an important opportunity to obtain several productive results like the production of electric power and heat and the use of the effluents of these plants like bio fertilizers and soil improvers; as well as to diminish the environmental contamination that cause when they are poured indiscriminately to the environment. The development of each one of these technologies is based on protocols and norms, which are of strict fulfilment according with the foreign investment laws and the sanitary requirements established in Cuba by the Ministry of Science Technology and Environment (CITMA). However, it is necessary to have the essential training, financing and equipment to decide about its possible technical-economic feasibility. In spite of some of these premises already exist, the authors of this review consider, the current economic conditions in Cuba do not allow this type of investment. Nevertheless, studies and constant learning should continue in order to evaluate and execute some variant of those installations as soon as the necessary financial capacity is available.
CONCLUSIONS
In accordance with that reported in the precedent investigations, it can be concluded that:
In the Cuban case, the biodigesters installed in the cow and pig farms to state and private scale, use generally the treatment type mono fermentation and it is of small and medium format. The residual of the production of biogas (digested), have little treatment and it is used very little as organic fertilizer and soil improver.
The work shows that to introduce in the country a biogas plant of great format of German type, diverse factors should be pondered and in the authors´ opinion, the country is not still in conditions of assuming such a challenge. Nevertheless, it is possible and pertinent to have the knowledge for, in a non-distant future, carrying out some variant of this installation type and to evaluate it under the real conditions of Cuba.
