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INTRODUCTION
Anaerobic digestion technologies for biogas production contribute to reduce environmental pollution, lessen greenhouse gas emissions, decrease the use of fossil fuels and chemical fertilizers, and improve the quality of life of the settlers in rural and suburban areas. In addition, it has had a growing boom in recent years for being an economically feasible solution, enabling nutrients recirculation and reducing pollution (Zhang et al., 2016).
Biogas is used as fuel for internal combustion engines, gas turbines, fuel cells, water heaters, as well as industrial heaters, among many other processes. It can also be used as fuel for electricity generation, where the overall conversion efficiency is around 10 - 16% (Fantin et al., 2015).
In China, India and other developing countries, family biodigesters for biogas production play an important role in rural energy programs. Their design depends on climatic conditions, available organic waste, local materials and operators´ skills. Animal and human manures combined with food, fruit and vegetable residues, are suitable materials to obtain biogas. Generally, the sizes of family biodigesters are between 8 - 10 m3 and produce between 0,3 - 0,9 m3 of biogas per m3 of biodigesters per day; however, there are other standardized designs with volumes between 15 - 60 m3 (Rajendran et al., 2012; Kaur et al., 2017)
Unlike the existing international experience with natural or liquefied gasoline, the relative supply of biogas in a distribution network for homes consumption is uncommon. The updated literature only reports the injection of biogas to natural gas networks in Sweden, Luxembourg and Spain (Bekkering et al., 2010; Díaz-Trujillo y Nápoles-Rivera, 2019; Khishtandar, 2019). In Cuba, there are limited experiences on biogas distribution, mainly for its use in cooking food and other domestic uses. In these cases, the biogas is obtained from the anaerobic treatment of pig excreta using two digesters (López & Suárez, 2018). However, this study does not carry out an in-depth analysis of the energy evaluation of the biogas supply for the distribution network. According to the abovementioned, the current investigation proposes a biogas distribution network, based on technical, economic, energy and environmental criteria, which allows an adequate distribution of biogas for cooking food in Cuban rural communities.
MATERIALS AND METHODS
Determination of the Biogas Production Potential in the Farm ´Los Hermanos
Amount of Residuals Generated in the Farm
To calculate the amount of residuals, the number of housed animals is determined, and the volume of excreta calculated. In addition, according to the data provided by the owner of the farm, the amount of animals corresponds to 50 kg of weight considering a per capita contribution of BOD5 per animal, which allows determining an average contribution of 0,39 kg day-1.
Design Methodology for Fixed Dome Biodigesters
To calculate an anaerobic biodigester design parameters, it is necessary to know the input data, and those that must be calculated (Table 1). The daily amount of material (Am) is in direct function to the biomass amount generated, whether it is domestic, agricultural or animal waste. Besides, it is essential to take into account the maximum amount obtained and the future productive increase plans.
TABLE 1 Input and output data required for an anaerobic digester design
| Parameters | Unit |
|---|---|
| Input data | |
| Daily biomass amount generated (Md) | kg d-1 |
| Dilution rate (Td) | L kg-1 |
| Biogas productivity (Y) | m3 kg-1 |
| Hydraulic retention time (HRT) | d |
| Biogas containment coefficient (k) | |
| Output data | |
| Daily material volume (manure and water mixture) (Sd) | kg d-1 |
| Biodigester volume (Vbiodig) | m3 |
| Biogas production capacity (G) | m3 d-1 |
| Biogas containment volume (Vbiogás) | m3 |
| Buffer tank volume (Vtc) | m3 |
The daily amount of input material (Sd) is the sum of the residual and the dilution of the biomass (residual and water).
While, the volume of the biodigester (Vbiodig) is calculated taking into account the value of Sd that enters to the biodigester and HRT.
Likewise, the biogas production capacity (G) is calculated through Equation 3.
In addition, the biogas containment volume is obtained from Equation 4.
Therefore, substituting G in Equation 4, the result is:
Biogas Purification System
As a fundamental part of the structure, it has a biogas filtering system, using the absorption purification method. The device has two filter tanks with the following characteristics: 1,30 m high and 0,30 m in diameter and contain iron filings inside. When the biogas has small amounts of air, the iron corrodes and the sulfur partially deposits itself on the iron filings, which can be washed away avoiding the corrosion caused by H2S in the cookers.
Determination of the Amount of Network Beneficiaries
The amount of the network beneficiaries is calculated taking into account the exclusive use of the biogas for cooking. According to Guardado (2007), for Cuban conditions and food culture, there is a daily consumption between 0,38-0,42 m3 d-1 per person. That is why, the existing production reserves are also taken into account.
Distribution Network Design
The design of the distribution network is based on the analysis of each of the factors evaluated in the field by the technical group, to define the most appropriate way according to technical, economic and social standards, as well as the criteria of the users, highlighting the geographical disposition of the houses and the development plans at the local level.
To define the criteria for selecting the houses, several aspects are taken into account according to a certain order of priority. 1st) the needs of the farm. 2nd) the houses of the owner and his relatives. 3rd) the houses located to the south since the winds circulate in that direction and may be affected by bad odors; 4th) the nearby houses where the workers reside; and 5th) the housing of the most vulnerable people, such as the physically and visually disabled, the elderly, and young children.
The biodigesters will conduct the biogas produced to the places of use through plastic pipes. PVC plastic pipes are suitable for this purpose, as they have the following advantages: corrosion resistant, easy to install, and lower prices. Their disadvantage lies in the necessary protection against the sunbeam and the movement of animals and heavy transport (López & Suárez, 2018).
The required diameter of a pipe depends on the distance from the plant to the place of gas consumption, as well as the maximum gas flow required and the allowable pressure loss (Table 2). The maximum gas flow is obtained by adding the consumptions of the equipment operating simultaneously (Guardado, 2007).
TABLA 2 Pressure loss in mm water flow for every 10 m of pipe
| Flow (m3/h) | Pipe diameter | ||||
|---|---|---|---|---|---|
| 17 mm | 23 mm | 30 mm | 43 mm | 54 mm | |
| Losses of pressure every 10 m | |||||
| 0,5 | 1,0 | 0,3 | 0,1 | - | - |
| 1,0 | 2,5 | 0,7 | 0,2 | - | - |
| 2,0 | 7,0 | 1,8 | 0,6 | - | - |
| 2,5 | 9,9 | 2,5 | 0,8 | - | - |
| 3,0 | 13,1 | 3,3 | 1,0 | 0,2 | - |
| 4,0 | 20,7 | 5,2 | 1,6 | 0,3 | - |
| 5,0 | 29,6 | 7,4 | 2,2 | 0,4 | - |
| 6,0 | 29,7 | 9,8 | 2,9 | 0,6 | - |
| 7,0 | - | 12,6 | 3,7 | 0,7 | - |
| 8,0 | - | 15,7 | 4,6 | 0,9 | 0,3 |
| 9,0 | - | 19,0 | 5,6 | 1,0 | 0,4 |
| 10,0 | 22,6 | 6,6 | 1,3 | 0,5 | |
Energy Contribution of the Biogas Produced
The energy contribution of the biogas produced is determined from the saving of electrical energy that will no longer be consumed by network users when cooking food. For this, the analysis was carried out in a period of one year and considering what was reported by Argota (2013), determining that in Cuba approximately 60% of electricity consumption in an average home is used for cooking food. For the study, three evaluations were made, considering the consumption of 60%, 50%, and 40% of the total consumed.
Gas Analyzer Features
To verify the biogas composition, a gas analyzer model Gas Board-3200L was used. This device is powered by rechargeable Lithium (Li) batteries, and is used to measure the concentration of CH4, CO2, H2S and O2 in biogas. The gas determination process consists in the detection of CH4 and CO2 concentrations by non-scattered infrared beams (NDIR) and the use of a fuel cell (ECD) to determine H2S and O2 gases.
Economic Analysis
The most used methods to verify the economic profitability of the project are the Net Present Value, (NPV) and the Internal Rate of Return, (IRR). The NPV method, also known as Net Present Value (NPV), is one of the most widely used economic criteria in the evaluation of an investment project. It consists in determining the equivalence in time zero of the future cash flows generated by a project, and comparing this equivalence with the initial disbursement, then it is recommended that the project be accepted (Barta et al., 2010).
The investor finances taking into account two factors, first, the project’s profits must be such that they compensate for the inflationary effects, and secondly, it must be a premium or overrate to risk his money in a certain investment. So before investing, the investor always has to take into account a minimum rate of return on the proposed investment called the Minimum Acceptable Rate of Return (TMAR) (Eq. 6).
The evaluation acceptance criteria are if NPV ≥ 0, then the project is acceptable; meanwhile, if the NPV <0 the project is rejected. Any project with negative NPV becomes underfunded as time passes, and therefore, it is no sustainable.
Likewise, the IRR is the rate that equals the sum of the flows discounted to the initial investment, that is, the NPV is equal to 0. As a criterion it is considered that if the IRR> TMAR, then the investment is accepted.
RESULTS AND DISCUSSION
Characterization of Biogas Quality in the Biodigesters
The quality of the biogas produced (Table 3) in each of the biodigesters is adequate, since it is within the values established for this type of biomass technology.
TABLE 3 Composition of biogas in the biodigesters
| Components | Digesters | Average | |||
|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | ||
| CH4 (%) | 64,04±3,2 | 63,80±2,4 | 62,0±2,5 | 63,85±1,9 | 63,2±1,2 |
| CO2 (%) | 31,05±2,1 | 32,00±1,5 | 31,80±2,2 | 30,00±1,3 | 31,3±1,1 |
| O2 (%) | 2,60±0,5 | 1,80±0,3 | 2,00±0,2 | 2,30±0,7 | 2,00,3 |
| H2S (ppm) | 640,0±21 | 636,00±24 | 600,00±41 | 650,00±36 | 631,5 |
It is important to point out that the methane concentration values are excellent for use in domestic cookers, which coincide with what Guardado (2007) proposes. However, the H2S values are slightly high, which implies that maintenance of the purification system is necessary to avoid the hydrogen sulfide formation and the accelerated corrosion of the metallic elements of the cookers.
Table 4 shows the results of the daily biogas measurements in each of the biodigesters for 5 days. It is observed that the average daily gas production value is 101,48 m3 d-1. Considering it is very unlikely that all users are connected and using the maximum capacity of the biogas produced, it is estimated that approximately 100 m3 d-1 is the theoretical biogas flow to determine the number of users that the distribution network will support.
TABLE 4 Biogas flow in the digesters
| Biodigesters | Flow (m3/d) | |||||
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | Average | |
| 1 | 24,60 | 25,03 | 25,65 | 24,85 | 26,00 | 25,22±0,6 |
| 2 | 25,00 | 26,85 | 24,20 | 25,03 | 23,65 | 24,94±1,2 |
| 3 | 25,88 | 25,85 | 27,02 | 24,75 | 26,08 | 25,91±0,8 |
| 4 | 24,30 | 25,66 | 26,01 | 25,20 | 25,80 | 25,39±0,7 |
| Total | 99,78 | 103,39 | 102,88 | 99,83 | 101,53 | 101,48±1,7 |
Figure 1 shows the biogas quality measurements made. These measurements were performed on 5 different days in 2 weeks of evaluation.
The variation range of H2S is between 80-140 ppm; while the concentrations of CH4, CO2 & O2 are between 52,98 - 57,74%, 27,90 - 30,65% and 2,70 - 5,82%, respectively. The variation in the biogas components concentrations shows a fluctuation in its quality during the days evaluated.
The range of H2S values shows that the maximum value observed is 140 ppm. These values are far below those obtained by Zapata (1998) where he reported hydrogen sulfide contents between 0,125% and 0,176% (1,250 ppm - 1,760 ppm) in the biogas produced in biodigesters fed with pig manure. An anaerobic biodigester can produce biogas with different proportions in the content of CH4 & CO2, where the percentage of methane will be between 40% and 70%, depending on the organic material with which the biodigester is fed (Duque et al., 2008).
In the stored biogas, a reduction in the concentration of H2S and a slight increase in the percentages of methane are observed. These results show that, in the filtration stage, H2S removal efficiencies of 50,4% and an improvement in methane concentrations are obtained because of the slight reduction in CO2. The literature is contradictory concerning the allowable amount of H2S in biogas to operate engines without damage. According to Hanoi Energy Institute specialists’ criteria, it should not exceed 50 ppm (Schirmer et al., 2014).
The CO2 levels remain without significant variation. This result seems to be related to the biogas accumulation in the storage containers, which causes the increase in CH4 levels, in addition to the effect of the filters used. (Quesada et al. 2007) reported similar results, where an electrical energy generation system from biogas is evaluated.
Gas Distribution Network
The technical group proposes the branched network design considering the following advantages:
i) Better technical operation and maintenance according to the established priorities.
ii) Due to the layout of the houses, it is the most economically attractive variant.
iii) The highest-pressure values are obtained at the critical points, which are the furthest points from the network, reaching values of 120 mm c.a., sufficient for the work of the cookers.
Energy and Environmental Impact of the Distribution Network in the Community
Before the current proposal for the use of biogas through a distribution network, the most widely used energy sources for cooking on the farm and households were electricity and firewood. For this reason, a study of energy consumption is carried out to determine electricity consumption taking into account the groupings of homes by priorities. For this, the information provided by the municipal electricity company is used. It includes a history of electricity consumption in each home in the selected period, from January to December 2019.
López & Suárez (2018) report that the biogas network installation allows reducing between 30 and 60% the electric power consumption in all of the houses. Taking this result into account, the reduction of the electric consumption in the 50 houses selected is calculated (Figure 2).
FIGURE 2 Behavior of electricity consumption in homes considering reductions between 30% - 60% in consumption.
When analyzing electricity consumption by groups of dwellings, according to the defined priority levels considering reductions in electricity costs between 30% - 60%, it can be stated that the use of the biogas network in the community El Almirante, will allow obtaining the following energy consumption benefits. i) the electricity consumption of the 50 households has an average value of 6,977 kW/month, and considering the reduction abovementioned on account of cooking, consumption will decrease between 2 093 kW / month and 4 186 kW/month. ii) Once the distribution network has been installed, the current average electricity consumption per household of 139 kW/month will decrease to a range between 41 kW/month and 83 kW/month; iii) the network will reduce energy consumption in each house between 55 kW / month and 97 kW/month.
However, in the visits made it was evidenced the use of firewood in several households to cook in order to save electricity. Therefore, the biogas supply could positively influence the life quality of these inhabitants. On the other hand, the daily cooking of food with firewood for 15 workers on the farm generates smoke and significantly affects working conditions. Likewise, the expenses for the search, preparation and transportation of firewood are high, so there will be financial resources saving and a significant impact on workers' wages.
There are also other important environmental impacts: i) Stopping the felling of 24 ha/year by the daily consumption of 100 m3 of biogas for cooking food. ii) Reducing the emission of 59,8 t of CO2 eq/year, that is, 1 255 t of methane. iii) Producing approximately 4 t/year of anaerobic digested, used as organic fertilizers for the improvement and fertilization of soils. iv) Developing the hygienic-sanitary conditions of the cookers in both the farm and the 50 homes. v) Enhancing the 15-employees´ working conditions by replacing firewood with biogas. vi) Ameliorating the quality of life of 149 inhabitants.
Economic Analysis
The project financial feasibility analysis is made under the three most used indicators for this type of study: NPV, IRR and the investment recovery index (IR).
These indicators are considered taking into account the average range of savings in electricity consumption resulting from the application of this technology, already systematized in different investigations (Abbasi et al., 2012; Garfí et al., 2016; Masebinu et al., 2018).
Table 5 shows the economic analysis of the biodigesters investment and the biogas distribution network. In general, for the two cases analyzed (reduction of electricity consumption between 30% and 60%) the implementation of the project is feasible. The financial indicators NPV and IRR show favorable results; moreover, as energy savings increase, the expected results improve.
TABLE 5 Economic analysis of the investment
| Indicador | Year 1 | Year 2 | Year 3 | Year 4 | Year 5 |
|---|---|---|---|---|---|
| Biodigester cost 1 1 | 50 000 | - | - | - | - |
| Biodigester cost 2 | 50 000 | - | - | - | - |
| Biodigester cost 3 | 50 000 | - | - | - | - |
| Biodigester cost 4 | 50 000 | - | - | - | - |
| Network installation cost | 20 000 | - | - | - | - |
| Total investment cost | 220 000 | - | - | - | - |
| Useful life (years) | 20 | - | - | - | - |
| Taxes (%) | 0 | 0 | - | - | - |
| Interest rate (%) | 0 | 0 | - | - | - |
| Electricity cost (CUP/kW h) | 5,25 | 5,25 | 5,25 | 5,25 | 5,25 |
| Energy no longer consumed (30%) (kW h/month) | 2 093 | 2 093 | 2 093 | 2 093 | 2 093 |
| Energy no longer consumed (60%) (kW h/month) | 4 186 | 4 186 | 4 186 | 4 186 | 4 186 |
| Savings for OBE (30%) (kW h/year) | 25 116 | 25 116 | 25 116 | 25 116 | 25 116 |
| Savings for OBE (60%) (kW h/year) | 50 232 | 50 232 | 50 232 | 50 232 | 50 232 |
| Average savings in homes (30%) (kW h/month) | 41 | 41 | 41 | 41 | 41 |
| Average savings in homes (60%) (kW h/month) | 83 | 83 | 83 | 83 | 83 |
| Savings from wood replacement (CUP/year) | 15 000 | 15 000 | 15 000 | 15 000 | 15 000 |
| Total savings (30%) (CUP/year) | 146 859 | 146 859 | 146 859 | 146 859 | 146 859 |
| Total savings (60%) (CUP/year) | 278 718 | 278 718 | 278 718 | 278 718 | 278 718 |
| Depreciation (Year) | 11 000 | 11 000 | 11 000 | 11 000 | 11 000 |
| Savings-Depreciation (Cash Flow) (30%) (CUP/year) | 135 859 | 135 859 | 135 859 | 135 859 | 135 859 |
| Savings-Depreciation (Cash Flow) (60%) (CUP/year) | 267 718 | 267 718 | 267 718 | 267 718 | 267 718 |
| Accumulated cash flow (60%) (CUP/year) | -84 141 | 51 718 | 187 577 | 323 436 | 459 295 |
| Accumulated cash flow (60%) (CUP/year) | 47 718 | 315 436 | 583 154 | 850 872 | 1 118590 |
| NPV (30%) | 295 012,5 | ||||
| IRR (30%) | 326% | ||||
| NPV (60%) | 393 276,9 | ||||
| IRR (60%) | 1 225% |
The total cost of the investment, which includes the cost of the four biodigesters, the biogas filtration system and the distribution network is 220,000 CUP; while the total expected savings per year of operation of the biogas distribution network is between 146,859 CUP and 278,718 CUP, for saving values of 30% and 60%, respectively. For both values, the investment recovery is between the first and second year of the system operation. In addition, both NPV values are greater than zero, indicating that the investment is justified from an economic point of view.
According to López & Suárez (2018), the total investment cost of two biodigesters and a biogas distribution network, which supplies 110 inhabitants for cooking tasks, with a value of 121 213 CUP is recovered by the end of the second year of operation. The authors conclude that the NPV is superior to zero for all the variants evaluated, evidencing the validation of the investment from the economic point of view, allowing the rapid recovery of the investment. This work achieves similar results.
CONCLUSIONS
The amount of substratum to be fermented requires a volume of approximately 360 m3 of digestion, which would allow the generation of more than 200 m3 of biogas daily.
The current total volume of the treatment system requires an increase of 100 m3; however, the 100 m3d-1 of biogas generated by the four biodigesters built allows providing gas to 149 people who live in the 50 homes closest to the farm.
The efficiency of the filtration system is adequate; however, the iron filings contained in the tanks require periodic reactivation, approximately every 3 months, in order not to reduce the purifying capacity of the filters.
For this study, the branched biogas distribution network presents several advantages compared to a circular distribution network according to the six established priority orders.
Biogas distribution networks have important environmental and social benefits; likewise, the favorable results of the financial indicators show the economic feasibility of these systems considering how significant electrical energy saving is to these houses.
