- Email: [email protected]
Optimal and cost-effective industrial biomethanation of tobacco
Renewable Energy 63 (2014) 280e285
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Optimal and cost-effective industrial biomethanation of tobacco A. González-González*, F. Cuadros Department of Applied Physics, University of Extremadura, Avda. de Elvas s/n, 06006 Badajoz, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 January 2013 Accepted 20 September 2013 Available online 10 October 2013
Extremadura is a Spanish south western region located by the Portuguese border. For decades, tobacco has been traditionally cultivated in Cáceres, the north province of such region. As a result, farmers in the area have extensive experience with this crop, and a fresh tobacco annual production is therefore guaranteed. This way, tobacco plant appears as an excellent candidate as energy crop. The present work reports on the economic viability of the anaerobic digestion of tobacco plant under the two following scenarios: 1) biogas generated in the biological process to be used as fuel to produce heat, thus avoiding the consumption of any other fossil fuel; 2) electric and thermal power generated by the anaerobic digestion plant to be self-consumed or sold to nearby industries. The mesophilic anaerobic digestion experiments carried out in semicontinuous mode yielded the highest methane production (53.84 15.48 Nm3CH4/t fresh tobacco) for substrate composition 15% fresh tobacco/85% water and 16 days degradation period. However, such methane yield by anaerobic digestion was seen not to meet economic feasibility requirements. Instead, methane production rates should be increased up to 90 and 110 Nm3 methane/t fresh tobacco (1500 V/ha production costs) in order to achieve 8.91 and 9.08 years for investment return according to scenarios 1 and 2, respectively. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Tobacco Biomethanation Optimization Economic feasibility
1. Introduction The Common Agricultural Policy (CAP), through the Common Market Organization (CMO), sets the maximum output of tobacco per country and year. Provided the sale of the product was guaranteed with stable prices known in advance, tobacco has been one of the most profitable crops in Spain so far. However, 85% of the income of farmers derived from CAP subsidies. The current average annual production of dried tobacco in Spain approaches 33,400 t of which 31,400 t are grown in Extremadura (more than 94% of the total in the country) in an area of cultivation of around 10,000 ha [1]. This crop constitutes 20% of total agricultural production in Extremadura and 65% of sales of industrial crops. Moreover, 20% of farmers in Extremadura cultivate tobacco and around 20,000 families depend directly or indirectly from this crop, which makes it one of the main sources for regional economic growth [2]. Tobacco CMO has undergone three reforms (1992, 1998 and 2004) in order to meet the budgetary, environmental and health European Union requirements [2]. The latter modification set a
* Corresponding author. Tel.: þ34 685764503. E-mail addresses: [email protected] (A. González-González), cuadros1@ unex.es (F. Cuadros). 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.09.027
reduction of the outputs subjected to CAP collection subsidy. Moreover, such incentives are scheduled to expire by 2013, which might result in a drop in prices paid by tobacco companies. The abovementioned area devoted to tobacco crop, i.e. 10,000 ha, is expected to drop down to around 6000 ha by 2014, because even though only crops that improve quality and yield significant reduction of production costs will be viable, profits will probably undergo a notable decrease as compared to the current state. To illustrate the seriousness of the situation, it should be noted that tobacco production is linked to marginal and depressed areas featured by low income and high unemployment rates. The unemployment rate for Extremadura, with just over 1.1 million inhabitants, was reported as 35.56% of its workforce in the first quarter of 2013 [3], a value that exceeds the national rate as referred to the same period (28.74%) [4]. These dramatic data illustrate the need to look for alternatives to tobacco cultivation for human consumption as an effort to prevent regional unemployment, to improve social welfare as well as to promote stability of rural population in their own environment. In terms of the above considerations, the anaerobic digestion (AD) arises as a viable technology for an alternative use of tobacco plant, provided it has elsewhere been proven to be competitive with regard to other energy crops such as sugar beet silage [5], oat, ryegrass and wheat [6], maize [7]. Moreover, hemp, maize and triticale have been seen to increase the methane yield if degraded
A. González-González, F. Cuadros / Renewable Energy 63 (2014) 280e285
by a codigestion process with certain industrial wastes [8]. In addition, tobacco plant, not only for its physicochemical characteristics but also by the climatic conditions of Spain and the experience of farmers, shows the highest production per unit area (at least 135 t/ha) of all tested crops, including corn silage [9], therefore, tobacco may also be viable as an energy crop. The present work aims to show the suitability of tobacco plant to alleviate the socioeconomic difficulties derived from the hypothetical elimination of tobacco cultivation for human consumption in Extremadura. The main goal is the determination of the optimal conditions for the AD of tobacco plant on a laboratory-scale reactor, as well as the analysis of the economic feasibility of this technology at industrial scale. For such purpose, the most influential parameters on viability have been assessed and the specific conditions under which biomethanation of tobacco plant meets profitability have been determined for two different operating scenarios (heat or electricity consumption).
281
Table 1 Physicochemical characterization of the inoculum and of the tobacco leaves. Inoculum Dry matter content (%) Organic dry matter (%) Crude protein (%) Crude fat (%)
1.39 99.73 20.06 13.96
0.17 0.25 2.43 3.2
Tobacco leaves 22.82 98.24 12.34 4.00
0.85 0.10 2.08 0.96
reduction. The physicochemical characterization of the inoculum and of the tobacco leaves is shown in Table 1. 2.2. Experimental setup A schematic of the experimental setup used for the AD experiments performed in the semicontinuous mode is shown in Fig. 1. A detailed description of components and operating arrangement is reported in other study [11].
2. Materials and method 2.3. Analytical methods 2.1. Preparation of substrates and start up of the anaerobic digestion process A series of AD experiments with mixed freshly cut tobacco leaves and water were carried out. Tobacco plants were kindly supplied by a cooperative located in northern Cáceres, and samples were arranged according to the following proportions set as % fresh tobacco/% water: 30%/70%, 20%/80%, 17%/83%, 15%/85% and 10%/ 90%. Tobacco plants were initially forced to undergo a mechanical treatment to reduce particle size, provided that this feature is reported in the scientific literature as one of the most relevant parameters in AD processes [10]. The lower the particle size, the higher the efficiency of the process, given that a decrease in particle size implies an increase of the surface on which bacteria might act. Once the tobacco plants were homogenized, they were mixed up with water according to the abovementioned ratios. Due to the excessive acidity of these substrates, pH was controlled by adding small quantities of Ca(OH)2 (1.7 g/l substrate) until neutrality was approached. Such compound is very cheap, shows strong alkalinity and is chemically inert, non-toxic, and easy to handle. Tobacco plant lacks of suitable microorganisms to activate a biodigestion process. For such reason, an acclimated inoculum ought to be necessary. The sludge used as inoculum to activate biodigestion was taken from an anaerobic reactor located at the Wastewater Treatment Plant (WWTP) in Badajoz. The anaerobic reactor was completely filled with inoculum and was set to feed the substrate flow one day before the start of the process. The same flow of substrate was fed for at least three times the Hydraulic Retention Time (HRT). The first HRT interval corresponds to acclimatization, after which the reaction reaches stability, i.e. the organic content is reduced and reaches a steady value, whereas methane production is stabilized at a maximum value. This way, reliable data regarding degradation and methane production for each of the feeding flow configurations under study can be obtained. The substrate flow was gradually increased in all the experiments, so that bacterial populations can appropriately acclimatize to the substrate. Such increasing procedure was carried out until a critical flow was reached: the value at which inhibition occurs by excess organic load. After all experiments were performed, the optimal feeding flow for each substrate could be straightforwardly determined as the value simultaneously achieving the highest methane production and Chemical Oxygen Demand (COD)
Dry matter content, organic dry matter content, as well as protein and fat concentrations were analysed in the tobacco leaves an in the inoculum. A periodic sampling protocol was applied to the digesters during the experiments in order to record data concerning the following parameters: COD, Volatile Fatty Acids (VFA), alkalinity and pH. Dry matter content, organic dry matter content, protein and fat concentrations, VFA, alkalinity and pH were analysed according to the standard methods [12], whereas NanocolorÒ kits and a portable PF-12 spectrophotometer (all from Macherey-Nagel Company) were used for data analysis concerning COD. 2.4. Estimate of economic feasibility of anaerobic digestion The study of the economic aspects regarding the biomethanation plant accounted for two different scenarios: 1) biogas generated in the biological process to be used as fuel to produce
Fig. 1. Semicontinuous anaerobic digester.
282
A. González-González, F. Cuadros / Renewable Energy 63 (2014) 280e285
Table 2 Results of anaerobic digestion of fresh tobacco. Percentage of tobacco (%)
HRT (days)
OLR (mg tobacco/day)
pH
COD reduction (%)
VFA (gCH3COOH/l)
Alkalinity (gCaCO3/l)
VFA/Alkalinity
30 20 20 17 17 15 15 15 10
34 27 20 16 13.33 20 16 12 20
17.65 14.81 20.00 21.25 25.51 15.00 18.75 25.00 10.00
7.46 7.29 7.27 7.48 7.30 7.31 7.26 7.34 7.32
80.43 69.17 66.83 64.79 35.51 52.89 65.78 70.72 60.44
0.23 0.90 1.37 0.36 0.40 1.64 0.49 1.22 1.82
4.17 2.85 2.90 3.13 2.10 4.10 2.37 3.32 2.74
0.06 0.31 0.51 0.12 0.19 0.42 0.21 0.36 0.64
heat, thus avoiding the consumption of any other fossil fuel; 2) electric and thermal power generated by the anaerobic digestion plant to be self-consumed or sold to nearby industries. The AD plant was designed to treat 35,100 t fresh tobacco annually, i.e. the production corresponding to 260 ha cultivation under the assumption of 135 t fresh tobacco/ha year average production (10% lower than the actual annual production rate, reported to approach 150 t/ha and even to reach 250 t/ha for greenhouse cultivation (EUBIOM, personal communication). The following statements were assumed in order to estimate the economic viability of both scenarios: 2.4.1. Electric and thermal power calculation To calculate the electric and thermal power the AD plant was assumed to operate 8000 h per year. The efficiency of the biogas boiler was taken as 80% for scenario 1, and electric and thermal efficiencies as 39% and 44.7% for scenario 2, respectively (LUDAN GROUP, personal communication). The value considered for the calorific value of methane was 31 MJ/Nm3, although it might reach up to 35 MJ/m3 as reported elsewhere, nevertheless, a lower value was set in order to account for worst case operating scenario [9]. 2.4.2. Installation costs Construction cost has been estimated as 4000 V/kWe installed in the anaerobic digestion plant which has a biogas engine (scenario 2) [13e15], and 2560 V/installed kWe in the anaerobic digestion plant of scenario 1 which only have a boiler to generate thermal energy due to a review of the anaerobic digestion systems indicates that approximately 36 percent of the total capital cost is associated with electrical generation equipment [16]. Additional costs regarding feasibility analysis (10,000 V) and managerial performance (20,000 V) ought to be considered [17]. The main differences between both scenarios lie in the calculation of annual costs and benefits, as explained below. 2.4.3. Annual costs Engine maintenance, associated to only scenario 2 (self-consumption of thermal and electric power) was estimated to be 7 V/ MWh electricity. Given that the AD plant of scenario 1 includes no operating engine, the annual cost was associated to the electric consumption of the plant (7% of the average electric energy that could be generated by the AD plant) and estimated as 0.12 V/kWh. On another note, costs derived from operation and maintenance of the AD plant are set as 3.3% of construction costs [15,16], these costs include routine maintenance; expenses for engine oil changes and minor repairs, periodic major repairs, maintenance such as engine overhauls, sludge removal and flexible cover repair or replacement [18] while the annual repayment mortgage to pay the installation costs of the plant was calculated considering 6% interest for 15 years (installation lifetime 20 years). Besides, labour costs were estimated to be 12,000 V/year.
Finally, harvesting and transport costs (32,808 V/year) as well as other tobacco production costs like those linked to sowing, plowing, irrigation and fertilization (around 1500 V/ha) also ought to be accounted for. 2.4.4. Annual benefits Only the remaining methane volume after guaranteeing a 38 C steady temperature for the digester was assumed to be available as fuel supply in scenario 1. The required amount of heat was determined as the heat needed to raise the temperature of the substrate from 15 C to 38 C plus an additional 25% to account for heat losses in the digester and an extra 5% to ensure availability. The economic benefits were therefore obtained from the energy savings linked to another fossil fuel, i.e. diesel, for which the calorific value is reported as 35.93 MJ/l and has a price of 1.009 V/l [19]. Economic benefits for scenario 2 were associated to energy savings in electricity as well as to self-consumption of thermal power generated by cogeneration (only the thermal energy remaining after ensuring 38 C steady temperature for the digester). Thermal energy savings were calculated as for scenario 1. Electricity savings were computed under the assumption of 93% of the electric power generated by the AD plant being self-consumed or sold to nearby industries (0.12 V/kWh) and of the remaining 7% being consumed by the AD plant. 3. Results and discussion 3.1. Anaerobic digestion experiments Experiments were performed at the Laboratory for Alternative Energy Sources, in the University of Extremadura. For more than a decade, numerous experimental studies regarding biomethanation of wet residual biomass from regional agrifood industrial plants have been conducted in such laboratory facilities, some of which are available in Refs. [20] and [21]. The elimination (or significant decrease) of tobacco crop in Extremadura might lead to a major socioeconomic setback in certain areas of northern Cáceres. The biomethanation of tobacco plants has therefore become a point of interest, and some preliminary experimental works in this line have been performed at the abovementioned laboratory for the last two years. The most illustrating results achieved for the substrates referred to in Section 2.1, which will serve to assess the stability of the AD process, are shown in Table 2. In all the performed experiments, the pH of the reaction medium was set to meet the optimal values for the growth of methanogenic bacterial populations, and showed a quasi-constant value regardless of the percentage of tobacco in the substrate sample. Nevertheless, as expected, higher pH values correspond to experiments with lower ratio VFA/alkalinity and vice versa. As for such particular ratio, it is remarkable to account for the outstandingly
A. González-González, F. Cuadros / Renewable Energy 63 (2014) 280e285
283
low value associated to substrate 30%/70%, as for the extremely high values achieved for substrates 20%/80% (and 20 days HRT) and 10%/90%. The ratio VFA/alkalinity met a standard range 0.12e0.42 for the remaining experiments, as expected for the biological process under study. Regarding the behaviour of the characteristic parameters as related to changes in HRT, it was observed that a decrease of HRT during degradation of substrates 20%/80% and 17%/83% led to an increase of both VFA concentration and ratio VFA/alkalinity (provided that alkalinity remains quasi-constant during experiments conducted for each of the substrates under study). Moreover, an increment of VFA concentration was seen to originate a slight decrease in pH. This was observed only for substrate 15%/85% when the organic charge was raised from 18.75 to 25.00 mg tobacco/day, and not from 15.00 to 18.75 mg tobacco/day, provided the latter case was the most suitable for the AD of that particular substrate. If all substrates were accounted for, the lowest organic charges corresponded to the highest percentages of COD reduction, except the extremely soaring value (70.72%) obtained when degrading 25 mg tobacco/day of substrate 15%/30%. This results, together with those for which no evidence of inhibition of the AD process was observed after long degradation periods (at least three HRT), led to conclude that all substrates and all organic charges under study showed stability. However, it would be wise to determine which particular binomial substrate-HRT resulted in an optimal process from the energy standpoint, i.e. allowed the highest methane yield. For that purpose, the time dependence of methane yield for the last HRT interval (as well as for each of the substrates and HRTs under study) is illustrated in Fig. 2(aee). Note the optimal HRT for substrate 20%/80% is 27 days, given that it is linked to a production rate as high as 40.89 Nm3 methane/t tobacco, some 41.4% higher than that of HRT 20 days. Optimal HRT for substrate 17%/83% was found to be 13 days (47.04 Nm3 methane/ t tobacco), since a 27.4% increment in methane yield was observed for 16 days HRT. Finally, optimal HRT for substrate 15%/85% was seen to reach 16 days, with an associated mean production rate 53.84 Nm3 methane/t fresh tobacco. With regard to the dependence of methane production on the percentage of tobacco in substrate samples, maximum methane yields were seen to occur for substrate 15%/85%. This way, the optimal configuration of substrate and organic charge for the AD of tobacco plant was observed as 15%/85% and 18.75 mg tobacco/day. Therefore, a methane yield of 7268 Nm3 CH4/ha year would be reached under the assumption of 135 t fresh tobacco/ha mean annual production. It should be wise to compare the achievements of the present study with those of other research works regarding some other energy crops like corn silage, which is widely used in AD plants for biogas production in our neighbouring countries, like for instance Germany. Mean biogas daily generation rates for such crop are reported as 202 Nm3/t, with 52% methane presence [9]. Besides, mean annual corn silage production ranges between 58.5 and 65 t/ ha, depending on the concentration of plants per unit area [22]. Therefore, if the mean value 61.75 t corn silage/ha year is assumed, the mean methane yield that might be obtained through the AD of corn silage is calculated as 6486 Nm3 methane/ha year, i.e. 12.01% lower than that of tobacco crop. 3.2. Economic feasibility Fig. 2. Influence of substrate and Hydraulic Residence Time (HRT) in the production of methane. a) 30% fresh tobacco/70% water, b) 20% fresh tobacco/80% water, c) 17% fresh tobacco/83% water, d) 15% fresh tobacco/85% water, e) 10% fresh tobacco/90% water.
Even though the AD of tobacco plant generates significant methane yields, the efficiency of laboratory-scale facilities is not sufficient to project profitable AD industrial plants due to the high production costs associated to tobacco crop. Therefore, a sensitivity
284
A. González-González, F. Cuadros / Renewable Energy 63 (2014) 280e285
Fig. 3. Sensitivity analysis for scenario 1.
analysis to simulate the behaviour of the Period of Return of Investment (PRI) as a function of methane yield and of tobacco cultivation costs was conducted for the two scenarios under study. Such procedure allowed the determination of the particular values for methane yield and cultivation costs to end up with a PRI such that profitability was ensured. Fig. 3 illustrates the results achieved for the sensitivity analysis of scenario 1. As can be seen, even though cultivation costs were not accounted for, the construction of an AD plant at industrial scale is absolutely unfeasible provided the methane yields obtained in laboratory-scale experiments. Methane yields at least as high as 90 Nm3 methane/t fresh tobacco would be required if a PRI ensuring economic feasibility (1500 V/ha) ought to be reached. Such particular value of PRI was calculated as 8.91 years, and would reduce down to 7.59 and 6.69 years if methane yields reached 95 and 100 Nm3 methane/t fresh tobacco, respectively. The sensitivity analysis of scenario 2 (self-consumption of electric energy) is illustrated in Fig. 4. In this case, the achievement of a PRI similar to that of scenario 1 (methane yield 90 Nm3 methane/t fresh tobacco and 1500 V/ha cultivation costs) would need to meet either of the two following conditions: (i) to raise methane yield up to 90 Nm3 methane/t fresh tobacco and reduce cultivation costs down to 500 V/ha (which is indeed an markedly low value); (ii) to raise methane yield up to 110 Nm3 methane/t fresh tobacco and account for 1500 V/ha cultivation costs. The corresponding PRIs for each of those assumptions were calculated as 8.66 and 9.08 years, respectively. Moreover, if higher methane yields like 115 and 120 Nm3 methane/t fresh tobacco had been considered, values for PRI (8.43 and 7.91 years, correspondingly) higher than those obtained for scenario 1 under the same assumptions would have been achieved. The first scenario should therefore be regarded as most feasible, given that a PRI similar to that of scenario 2 is reached with a more flexible requirement for methane efficiency (in particular, a 67% increment with respect to the methane yield obtained in laboratory-scale experiments). With regard to scenario 2, it should also be noted that the thermal energy required to keep temperature inside the digester at 38 C could not be produced by cogeneration until methane yield reached 65 Nm3 methane/t fresh tobacco. The PRI was seen to strongly depend on the parameters under study for both scenarios, provided it was observed to plummet as methane yield was slightly increased and tobacco production costs were slightly reduced. In view of the abovementioned considerations, further studies on the AD of tobacco plants are strongly advisable in order to seek improvement in methane efficiency, which might be achieved by operating in continuous mode or by suitable modifications in the experimental setup (like, for instance, the use of binding agents for bacterial populations, in particular for methanogenic bacteria).
3.3. Operation and design of the anaerobic digestion plants Tobacco is a seasonal crop that for human consumption only allows an annual cut, however if the objective is to generate biomass can be grown by a high-density planting method, similar to fodder production. In this case it is possible to reach a higher productivity than with the traditional method due to that four or five cuts can be made during one growing season, which may extend to the end of October. Thus, fresh tobacco is available only for about 5 months a year, so the amount of tobacco required to be treated for the rest of the year have to be stored. The excess of tobacco will be air dried given the high temperatures recorded in Extremadura during the summer and it will be stored dehydrated to prevent natural degradation until use, in which it will be rehydrated and mixed with water in the proportion of 15% fresh tobacco/85% water. On the other hand, the digested effluent will be applied as fertilizer to the growing area. Two AD plants were designed considering the methane yields that have demonstrated to be profitable for scenario 1 and 2 (90 and 110 Nm3 methane/t fresh tobacco) and both plants were sized to treat 35,100 t fresh tobacco per year mixing with water in a proportion of 15% tobacco/85% water for 16 days. A description and the dimensions of its main components are shown in Table 3. Based on the observation of the results obtained, it can be inferred that the two plants are essentially equal. Crushing tank, mixing tank, digester and digested sludge tank have the same volumes in both plants because its size depend on the volume of substrate to be treated dairy and HRT, that are equal in both plants. Therefore, the differences lie in the equipments whose size depends on the methane yields which are storage volume of biogas and the power of the equipment installed to generate energy (a boiler in scenario 1 and a biogas engine in scenario 2). Crushing tank is designed to mash the fresh tobacco necessary daily, the digester is sized to treat the daily volume of substrate for 16 days (HRT) and a 25% more is added for safety. Finally, mixing tank and digested effluent tank have capacity to store the substrate to be treated or the digested effluent for 3 days. 4. Conclusions The results achieved in the anaerobic digestion experiments described along the present manuscript show no evidence of inhibition given the continuous monitoring of substrate’s pH via the addition of Ca(OH)2, regardless the percentage of tobacco in the substrate and organic charge. However, optimal results were reached when degrading substrate 15%/85% with 18.73 mg tobacco/ day organic charge, which allowed a methane yield as high as 53.84 Nm3 methane/t fresh tobacco as well as 65.78% COD reduction. With regard to feasibility testing, the sensitivity analyses
A. González-González, F. Cuadros / Renewable Energy 63 (2014) 280e285
285
Fig. 4. Sensitivity analysis for scenario 2.
Table 3 Dimensions of the main components of the anaerobic digestion plant. Volume (m3) Crushing tank Mixing tank Anaerobic digesters (2) Biogas storage Digested sludge tank Boiler (Scenario 1) Biogas engine (Scenario 2)
Height (m)
320 4 2110 5 7500 8 2600 scenario 1 e 3100 scenario 2 2110 5 2.7 MWth 1.62 MWe; 1.86 MWth
Diameter (m) 10 23 35 e 23
showed that the two following requirements are needed in order to achieve periods of return of investment approaching 9 years: (i) raise methane yield obtained in laboratory-scale experiments up to 67% and 104% for scenarios 1 and 2, respectively; (ii) production costs assumed as 1500 V/ha. Acknowledgements The present work was partially supported by the Government of Extremadura through projects PCJ 100201 and GR10045. A. González-González is also grateful to the Ministry of Education and Science for FPU grant referenced as AP2008-02546. References [1] Ministerio de Agricultura, Alimentación y Medio Ambiente, Avances de superficies y producciones de cultivos [Updated 2013 June 23]. Available at: http://www.magrama.gob.es/es/estadistica/temas/estadisticas-agrarias/ agricultura/avances-superficies-producciones-agricolas/; November 2012. [2] Fundación Formación y Empleo de Extremadura (Forem Extremadura). Estudio sobre el empleo e importancia social del cultivo de tabaco en Extremadura [Updated 2013 June 23]. Available at: http://www.foremextremadura.com/ userfiles/3946e52018433ebad128509dd4cba32d.pdf; 2011. [3] Instituto de Estadística de Extremadura [Updated 2013 June 23]. Available at: http://estadistica.gobex.es/; 2013. [4] Instituto Nacional de Estadística [Updated 2013 June 23]. Available at: http:// www.ine.es; 2013. [5] Demirel B, Scherer P. Bio-methanization of energy crops through monodigestion for continuous production of renewable biogas. Renew Energ 2009;34(12):2940e5. [6] Nielsen AM, Feilberg A. Anaerobic digestion of energy crops in batch. Biosyst Eng 2012;112(3):248e51.
[7] Vervaeren H, Hostyn K, Ghekiere G, Willems B. Biological ensilage additives as pretreatment for maize to increase the biogas production. Renew Energ 2010;35(9):2089e93. [8] Nges IA, Escobar F, Fu X, Björnsson L. Benefits of supplementing an industrial waste anaerobic digester with energy crops for increased biogas production. Waste Manag 2012;32(1):53e9. [9] Proyecto probiogas [updated 2013 June 23]. Available from:: www.probiogas. es [10] Izumi K, Okishio Y, Nagao N, Niwa C, Yamamato S, Toda T. Effects of particle size on anaerobic digestion of food waste. Int Biodeter Biodegr 2010;64:601e8. [11] González-González A, Cuadros F, Ruiz-Celma A, López-Rodríguez F. EnergyEnvironmental benefits and economic feasibility of anaerobic codigestion of Iberian pig slaughterhouse and tomato industry wastes in Extremadura (Spain). Bioresour Technol 2013;136:109e16. [12] MadridAPHA, AWWA, WPCF, Métodos Normalizados para el Análisis de Aguas Potables y Residuales, seventheenth ed., Díaz de Santos S.A; 1992. [13] Ministry of Agriculture and Rural Development of Alberta. Economic feasibility of anaerobic digesters, Agri-facts [updated 2013 June 23] Available at: http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex12280/$file/ 768-6.pdf?OpenElement; 2008. [14] Agencia Andaluza de la Energía. Estudio básico del biogás [updated 2013 June 23] Available at: http://www.agenciaandaluzadelaenergia.es/sites/default/ files/estudio_basico_del_biogas_0.pdf; 2011. [15] Gebrezgabher SA, Meuwissen MPM, Prins BAM, Oude Lansink AGJM. Economic analysis of anaerobic digestion. A case of green power biogas plant in The Netherlands. NJAS Wageningen J Life Sci 2010;57:109e15. [16] United States Department of Agriculture (USDA). An analysis of energy production costs from anaerobic digestion systems on U.S. livestock production facilities [updated 2013 June 23] Available at: http://www.agmrc.org/media/ cms/manuredigesters_FC5C31F0F7B78.pdf; 2007. [17] Naskeo Environment, Economy of biogas installations, [updated 2013 June 23]. Available at:http://www.biogas-renewable-energy.info/biogas_installatio ns_investments.html [18] Martin JH. A protocol for quantifying and reporting the performance of anaerobic digestion systems for livestock manures. International symposium on air quality and waste management for agriculture; Broomfield, Colorado; 2007. [19] Ministerio de Industria, Energía y Turismo (MINETUR), Precio de carburantes [updated 2013 June 23]. Available at: http://m.minetur.gob.es/Carburantes; 2013. [20] .Cuadros F, López-Rodríguez F, Ruiz-Celma A, Rubiales F, GonzálezGonzález A. Recycling, reuse and energetic valuation of meat industry wastes in Extremadura (Spain). Resour Conservat Recycl 2011;55(4):393e9. [21] Cuadros F, González-González A, Ruiz-Celma A, López-Rodríguez F. Recovery of energy and water through an eco-efficient treatment of olive mill wastewates. International Congress on Water, Waste and Energy Management. Salamanca: Mayo; 2012. [22] Roth GW. Corn silage production and management. Information and communication technologies in the college of agricultural sciences. The Pennsylvania State University; 2001.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Optimal and cost-effective industrial biomethanation of tobacco A. González-González*, F. Cuadros Department of Applied Physics, University of Extremadura, Avda. de Elvas s/n, 06006 Badajoz, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 January 2013 Accepted 20 September 2013 Available online 10 October 2013
Extremadura is a Spanish south western region located by the Portuguese border. For decades, tobacco has been traditionally cultivated in Cáceres, the north province of such region. As a result, farmers in the area have extensive experience with this crop, and a fresh tobacco annual production is therefore guaranteed. This way, tobacco plant appears as an excellent candidate as energy crop. The present work reports on the economic viability of the anaerobic digestion of tobacco plant under the two following scenarios: 1) biogas generated in the biological process to be used as fuel to produce heat, thus avoiding the consumption of any other fossil fuel; 2) electric and thermal power generated by the anaerobic digestion plant to be self-consumed or sold to nearby industries. The mesophilic anaerobic digestion experiments carried out in semicontinuous mode yielded the highest methane production (53.84 15.48 Nm3CH4/t fresh tobacco) for substrate composition 15% fresh tobacco/85% water and 16 days degradation period. However, such methane yield by anaerobic digestion was seen not to meet economic feasibility requirements. Instead, methane production rates should be increased up to 90 and 110 Nm3 methane/t fresh tobacco (1500 V/ha production costs) in order to achieve 8.91 and 9.08 years for investment return according to scenarios 1 and 2, respectively. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Tobacco Biomethanation Optimization Economic feasibility
1. Introduction The Common Agricultural Policy (CAP), through the Common Market Organization (CMO), sets the maximum output of tobacco per country and year. Provided the sale of the product was guaranteed with stable prices known in advance, tobacco has been one of the most profitable crops in Spain so far. However, 85% of the income of farmers derived from CAP subsidies. The current average annual production of dried tobacco in Spain approaches 33,400 t of which 31,400 t are grown in Extremadura (more than 94% of the total in the country) in an area of cultivation of around 10,000 ha [1]. This crop constitutes 20% of total agricultural production in Extremadura and 65% of sales of industrial crops. Moreover, 20% of farmers in Extremadura cultivate tobacco and around 20,000 families depend directly or indirectly from this crop, which makes it one of the main sources for regional economic growth [2]. Tobacco CMO has undergone three reforms (1992, 1998 and 2004) in order to meet the budgetary, environmental and health European Union requirements [2]. The latter modification set a
* Corresponding author. Tel.: þ34 685764503. E-mail addresses: [email protected] (A. González-González), cuadros1@ unex.es (F. Cuadros). 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.09.027
reduction of the outputs subjected to CAP collection subsidy. Moreover, such incentives are scheduled to expire by 2013, which might result in a drop in prices paid by tobacco companies. The abovementioned area devoted to tobacco crop, i.e. 10,000 ha, is expected to drop down to around 6000 ha by 2014, because even though only crops that improve quality and yield significant reduction of production costs will be viable, profits will probably undergo a notable decrease as compared to the current state. To illustrate the seriousness of the situation, it should be noted that tobacco production is linked to marginal and depressed areas featured by low income and high unemployment rates. The unemployment rate for Extremadura, with just over 1.1 million inhabitants, was reported as 35.56% of its workforce in the first quarter of 2013 [3], a value that exceeds the national rate as referred to the same period (28.74%) [4]. These dramatic data illustrate the need to look for alternatives to tobacco cultivation for human consumption as an effort to prevent regional unemployment, to improve social welfare as well as to promote stability of rural population in their own environment. In terms of the above considerations, the anaerobic digestion (AD) arises as a viable technology for an alternative use of tobacco plant, provided it has elsewhere been proven to be competitive with regard to other energy crops such as sugar beet silage [5], oat, ryegrass and wheat [6], maize [7]. Moreover, hemp, maize and triticale have been seen to increase the methane yield if degraded
A. González-González, F. Cuadros / Renewable Energy 63 (2014) 280e285
by a codigestion process with certain industrial wastes [8]. In addition, tobacco plant, not only for its physicochemical characteristics but also by the climatic conditions of Spain and the experience of farmers, shows the highest production per unit area (at least 135 t/ha) of all tested crops, including corn silage [9], therefore, tobacco may also be viable as an energy crop. The present work aims to show the suitability of tobacco plant to alleviate the socioeconomic difficulties derived from the hypothetical elimination of tobacco cultivation for human consumption in Extremadura. The main goal is the determination of the optimal conditions for the AD of tobacco plant on a laboratory-scale reactor, as well as the analysis of the economic feasibility of this technology at industrial scale. For such purpose, the most influential parameters on viability have been assessed and the specific conditions under which biomethanation of tobacco plant meets profitability have been determined for two different operating scenarios (heat or electricity consumption).
281
Table 1 Physicochemical characterization of the inoculum and of the tobacco leaves. Inoculum Dry matter content (%) Organic dry matter (%) Crude protein (%) Crude fat (%)
1.39 99.73 20.06 13.96
0.17 0.25 2.43 3.2
Tobacco leaves 22.82 98.24 12.34 4.00
0.85 0.10 2.08 0.96
reduction. The physicochemical characterization of the inoculum and of the tobacco leaves is shown in Table 1. 2.2. Experimental setup A schematic of the experimental setup used for the AD experiments performed in the semicontinuous mode is shown in Fig. 1. A detailed description of components and operating arrangement is reported in other study [11].
2. Materials and method 2.3. Analytical methods 2.1. Preparation of substrates and start up of the anaerobic digestion process A series of AD experiments with mixed freshly cut tobacco leaves and water were carried out. Tobacco plants were kindly supplied by a cooperative located in northern Cáceres, and samples were arranged according to the following proportions set as % fresh tobacco/% water: 30%/70%, 20%/80%, 17%/83%, 15%/85% and 10%/ 90%. Tobacco plants were initially forced to undergo a mechanical treatment to reduce particle size, provided that this feature is reported in the scientific literature as one of the most relevant parameters in AD processes [10]. The lower the particle size, the higher the efficiency of the process, given that a decrease in particle size implies an increase of the surface on which bacteria might act. Once the tobacco plants were homogenized, they were mixed up with water according to the abovementioned ratios. Due to the excessive acidity of these substrates, pH was controlled by adding small quantities of Ca(OH)2 (1.7 g/l substrate) until neutrality was approached. Such compound is very cheap, shows strong alkalinity and is chemically inert, non-toxic, and easy to handle. Tobacco plant lacks of suitable microorganisms to activate a biodigestion process. For such reason, an acclimated inoculum ought to be necessary. The sludge used as inoculum to activate biodigestion was taken from an anaerobic reactor located at the Wastewater Treatment Plant (WWTP) in Badajoz. The anaerobic reactor was completely filled with inoculum and was set to feed the substrate flow one day before the start of the process. The same flow of substrate was fed for at least three times the Hydraulic Retention Time (HRT). The first HRT interval corresponds to acclimatization, after which the reaction reaches stability, i.e. the organic content is reduced and reaches a steady value, whereas methane production is stabilized at a maximum value. This way, reliable data regarding degradation and methane production for each of the feeding flow configurations under study can be obtained. The substrate flow was gradually increased in all the experiments, so that bacterial populations can appropriately acclimatize to the substrate. Such increasing procedure was carried out until a critical flow was reached: the value at which inhibition occurs by excess organic load. After all experiments were performed, the optimal feeding flow for each substrate could be straightforwardly determined as the value simultaneously achieving the highest methane production and Chemical Oxygen Demand (COD)
Dry matter content, organic dry matter content, as well as protein and fat concentrations were analysed in the tobacco leaves an in the inoculum. A periodic sampling protocol was applied to the digesters during the experiments in order to record data concerning the following parameters: COD, Volatile Fatty Acids (VFA), alkalinity and pH. Dry matter content, organic dry matter content, protein and fat concentrations, VFA, alkalinity and pH were analysed according to the standard methods [12], whereas NanocolorÒ kits and a portable PF-12 spectrophotometer (all from Macherey-Nagel Company) were used for data analysis concerning COD. 2.4. Estimate of economic feasibility of anaerobic digestion The study of the economic aspects regarding the biomethanation plant accounted for two different scenarios: 1) biogas generated in the biological process to be used as fuel to produce
Fig. 1. Semicontinuous anaerobic digester.
282
A. González-González, F. Cuadros / Renewable Energy 63 (2014) 280e285
Table 2 Results of anaerobic digestion of fresh tobacco. Percentage of tobacco (%)
HRT (days)
OLR (mg tobacco/day)
pH
COD reduction (%)
VFA (gCH3COOH/l)
Alkalinity (gCaCO3/l)
VFA/Alkalinity
30 20 20 17 17 15 15 15 10
34 27 20 16 13.33 20 16 12 20
17.65 14.81 20.00 21.25 25.51 15.00 18.75 25.00 10.00
7.46 7.29 7.27 7.48 7.30 7.31 7.26 7.34 7.32
80.43 69.17 66.83 64.79 35.51 52.89 65.78 70.72 60.44
0.23 0.90 1.37 0.36 0.40 1.64 0.49 1.22 1.82
4.17 2.85 2.90 3.13 2.10 4.10 2.37 3.32 2.74
0.06 0.31 0.51 0.12 0.19 0.42 0.21 0.36 0.64
heat, thus avoiding the consumption of any other fossil fuel; 2) electric and thermal power generated by the anaerobic digestion plant to be self-consumed or sold to nearby industries. The AD plant was designed to treat 35,100 t fresh tobacco annually, i.e. the production corresponding to 260 ha cultivation under the assumption of 135 t fresh tobacco/ha year average production (10% lower than the actual annual production rate, reported to approach 150 t/ha and even to reach 250 t/ha for greenhouse cultivation (EUBIOM, personal communication). The following statements were assumed in order to estimate the economic viability of both scenarios: 2.4.1. Electric and thermal power calculation To calculate the electric and thermal power the AD plant was assumed to operate 8000 h per year. The efficiency of the biogas boiler was taken as 80% for scenario 1, and electric and thermal efficiencies as 39% and 44.7% for scenario 2, respectively (LUDAN GROUP, personal communication). The value considered for the calorific value of methane was 31 MJ/Nm3, although it might reach up to 35 MJ/m3 as reported elsewhere, nevertheless, a lower value was set in order to account for worst case operating scenario [9]. 2.4.2. Installation costs Construction cost has been estimated as 4000 V/kWe installed in the anaerobic digestion plant which has a biogas engine (scenario 2) [13e15], and 2560 V/installed kWe in the anaerobic digestion plant of scenario 1 which only have a boiler to generate thermal energy due to a review of the anaerobic digestion systems indicates that approximately 36 percent of the total capital cost is associated with electrical generation equipment [16]. Additional costs regarding feasibility analysis (10,000 V) and managerial performance (20,000 V) ought to be considered [17]. The main differences between both scenarios lie in the calculation of annual costs and benefits, as explained below. 2.4.3. Annual costs Engine maintenance, associated to only scenario 2 (self-consumption of thermal and electric power) was estimated to be 7 V/ MWh electricity. Given that the AD plant of scenario 1 includes no operating engine, the annual cost was associated to the electric consumption of the plant (7% of the average electric energy that could be generated by the AD plant) and estimated as 0.12 V/kWh. On another note, costs derived from operation and maintenance of the AD plant are set as 3.3% of construction costs [15,16], these costs include routine maintenance; expenses for engine oil changes and minor repairs, periodic major repairs, maintenance such as engine overhauls, sludge removal and flexible cover repair or replacement [18] while the annual repayment mortgage to pay the installation costs of the plant was calculated considering 6% interest for 15 years (installation lifetime 20 years). Besides, labour costs were estimated to be 12,000 V/year.
Finally, harvesting and transport costs (32,808 V/year) as well as other tobacco production costs like those linked to sowing, plowing, irrigation and fertilization (around 1500 V/ha) also ought to be accounted for. 2.4.4. Annual benefits Only the remaining methane volume after guaranteeing a 38 C steady temperature for the digester was assumed to be available as fuel supply in scenario 1. The required amount of heat was determined as the heat needed to raise the temperature of the substrate from 15 C to 38 C plus an additional 25% to account for heat losses in the digester and an extra 5% to ensure availability. The economic benefits were therefore obtained from the energy savings linked to another fossil fuel, i.e. diesel, for which the calorific value is reported as 35.93 MJ/l and has a price of 1.009 V/l [19]. Economic benefits for scenario 2 were associated to energy savings in electricity as well as to self-consumption of thermal power generated by cogeneration (only the thermal energy remaining after ensuring 38 C steady temperature for the digester). Thermal energy savings were calculated as for scenario 1. Electricity savings were computed under the assumption of 93% of the electric power generated by the AD plant being self-consumed or sold to nearby industries (0.12 V/kWh) and of the remaining 7% being consumed by the AD plant. 3. Results and discussion 3.1. Anaerobic digestion experiments Experiments were performed at the Laboratory for Alternative Energy Sources, in the University of Extremadura. For more than a decade, numerous experimental studies regarding biomethanation of wet residual biomass from regional agrifood industrial plants have been conducted in such laboratory facilities, some of which are available in Refs. [20] and [21]. The elimination (or significant decrease) of tobacco crop in Extremadura might lead to a major socioeconomic setback in certain areas of northern Cáceres. The biomethanation of tobacco plants has therefore become a point of interest, and some preliminary experimental works in this line have been performed at the abovementioned laboratory for the last two years. The most illustrating results achieved for the substrates referred to in Section 2.1, which will serve to assess the stability of the AD process, are shown in Table 2. In all the performed experiments, the pH of the reaction medium was set to meet the optimal values for the growth of methanogenic bacterial populations, and showed a quasi-constant value regardless of the percentage of tobacco in the substrate sample. Nevertheless, as expected, higher pH values correspond to experiments with lower ratio VFA/alkalinity and vice versa. As for such particular ratio, it is remarkable to account for the outstandingly
A. González-González, F. Cuadros / Renewable Energy 63 (2014) 280e285
283
low value associated to substrate 30%/70%, as for the extremely high values achieved for substrates 20%/80% (and 20 days HRT) and 10%/90%. The ratio VFA/alkalinity met a standard range 0.12e0.42 for the remaining experiments, as expected for the biological process under study. Regarding the behaviour of the characteristic parameters as related to changes in HRT, it was observed that a decrease of HRT during degradation of substrates 20%/80% and 17%/83% led to an increase of both VFA concentration and ratio VFA/alkalinity (provided that alkalinity remains quasi-constant during experiments conducted for each of the substrates under study). Moreover, an increment of VFA concentration was seen to originate a slight decrease in pH. This was observed only for substrate 15%/85% when the organic charge was raised from 18.75 to 25.00 mg tobacco/day, and not from 15.00 to 18.75 mg tobacco/day, provided the latter case was the most suitable for the AD of that particular substrate. If all substrates were accounted for, the lowest organic charges corresponded to the highest percentages of COD reduction, except the extremely soaring value (70.72%) obtained when degrading 25 mg tobacco/day of substrate 15%/30%. This results, together with those for which no evidence of inhibition of the AD process was observed after long degradation periods (at least three HRT), led to conclude that all substrates and all organic charges under study showed stability. However, it would be wise to determine which particular binomial substrate-HRT resulted in an optimal process from the energy standpoint, i.e. allowed the highest methane yield. For that purpose, the time dependence of methane yield for the last HRT interval (as well as for each of the substrates and HRTs under study) is illustrated in Fig. 2(aee). Note the optimal HRT for substrate 20%/80% is 27 days, given that it is linked to a production rate as high as 40.89 Nm3 methane/t tobacco, some 41.4% higher than that of HRT 20 days. Optimal HRT for substrate 17%/83% was found to be 13 days (47.04 Nm3 methane/ t tobacco), since a 27.4% increment in methane yield was observed for 16 days HRT. Finally, optimal HRT for substrate 15%/85% was seen to reach 16 days, with an associated mean production rate 53.84 Nm3 methane/t fresh tobacco. With regard to the dependence of methane production on the percentage of tobacco in substrate samples, maximum methane yields were seen to occur for substrate 15%/85%. This way, the optimal configuration of substrate and organic charge for the AD of tobacco plant was observed as 15%/85% and 18.75 mg tobacco/day. Therefore, a methane yield of 7268 Nm3 CH4/ha year would be reached under the assumption of 135 t fresh tobacco/ha mean annual production. It should be wise to compare the achievements of the present study with those of other research works regarding some other energy crops like corn silage, which is widely used in AD plants for biogas production in our neighbouring countries, like for instance Germany. Mean biogas daily generation rates for such crop are reported as 202 Nm3/t, with 52% methane presence [9]. Besides, mean annual corn silage production ranges between 58.5 and 65 t/ ha, depending on the concentration of plants per unit area [22]. Therefore, if the mean value 61.75 t corn silage/ha year is assumed, the mean methane yield that might be obtained through the AD of corn silage is calculated as 6486 Nm3 methane/ha year, i.e. 12.01% lower than that of tobacco crop. 3.2. Economic feasibility Fig. 2. Influence of substrate and Hydraulic Residence Time (HRT) in the production of methane. a) 30% fresh tobacco/70% water, b) 20% fresh tobacco/80% water, c) 17% fresh tobacco/83% water, d) 15% fresh tobacco/85% water, e) 10% fresh tobacco/90% water.
Even though the AD of tobacco plant generates significant methane yields, the efficiency of laboratory-scale facilities is not sufficient to project profitable AD industrial plants due to the high production costs associated to tobacco crop. Therefore, a sensitivity
284
A. González-González, F. Cuadros / Renewable Energy 63 (2014) 280e285
Fig. 3. Sensitivity analysis for scenario 1.
analysis to simulate the behaviour of the Period of Return of Investment (PRI) as a function of methane yield and of tobacco cultivation costs was conducted for the two scenarios under study. Such procedure allowed the determination of the particular values for methane yield and cultivation costs to end up with a PRI such that profitability was ensured. Fig. 3 illustrates the results achieved for the sensitivity analysis of scenario 1. As can be seen, even though cultivation costs were not accounted for, the construction of an AD plant at industrial scale is absolutely unfeasible provided the methane yields obtained in laboratory-scale experiments. Methane yields at least as high as 90 Nm3 methane/t fresh tobacco would be required if a PRI ensuring economic feasibility (1500 V/ha) ought to be reached. Such particular value of PRI was calculated as 8.91 years, and would reduce down to 7.59 and 6.69 years if methane yields reached 95 and 100 Nm3 methane/t fresh tobacco, respectively. The sensitivity analysis of scenario 2 (self-consumption of electric energy) is illustrated in Fig. 4. In this case, the achievement of a PRI similar to that of scenario 1 (methane yield 90 Nm3 methane/t fresh tobacco and 1500 V/ha cultivation costs) would need to meet either of the two following conditions: (i) to raise methane yield up to 90 Nm3 methane/t fresh tobacco and reduce cultivation costs down to 500 V/ha (which is indeed an markedly low value); (ii) to raise methane yield up to 110 Nm3 methane/t fresh tobacco and account for 1500 V/ha cultivation costs. The corresponding PRIs for each of those assumptions were calculated as 8.66 and 9.08 years, respectively. Moreover, if higher methane yields like 115 and 120 Nm3 methane/t fresh tobacco had been considered, values for PRI (8.43 and 7.91 years, correspondingly) higher than those obtained for scenario 1 under the same assumptions would have been achieved. The first scenario should therefore be regarded as most feasible, given that a PRI similar to that of scenario 2 is reached with a more flexible requirement for methane efficiency (in particular, a 67% increment with respect to the methane yield obtained in laboratory-scale experiments). With regard to scenario 2, it should also be noted that the thermal energy required to keep temperature inside the digester at 38 C could not be produced by cogeneration until methane yield reached 65 Nm3 methane/t fresh tobacco. The PRI was seen to strongly depend on the parameters under study for both scenarios, provided it was observed to plummet as methane yield was slightly increased and tobacco production costs were slightly reduced. In view of the abovementioned considerations, further studies on the AD of tobacco plants are strongly advisable in order to seek improvement in methane efficiency, which might be achieved by operating in continuous mode or by suitable modifications in the experimental setup (like, for instance, the use of binding agents for bacterial populations, in particular for methanogenic bacteria).
3.3. Operation and design of the anaerobic digestion plants Tobacco is a seasonal crop that for human consumption only allows an annual cut, however if the objective is to generate biomass can be grown by a high-density planting method, similar to fodder production. In this case it is possible to reach a higher productivity than with the traditional method due to that four or five cuts can be made during one growing season, which may extend to the end of October. Thus, fresh tobacco is available only for about 5 months a year, so the amount of tobacco required to be treated for the rest of the year have to be stored. The excess of tobacco will be air dried given the high temperatures recorded in Extremadura during the summer and it will be stored dehydrated to prevent natural degradation until use, in which it will be rehydrated and mixed with water in the proportion of 15% fresh tobacco/85% water. On the other hand, the digested effluent will be applied as fertilizer to the growing area. Two AD plants were designed considering the methane yields that have demonstrated to be profitable for scenario 1 and 2 (90 and 110 Nm3 methane/t fresh tobacco) and both plants were sized to treat 35,100 t fresh tobacco per year mixing with water in a proportion of 15% tobacco/85% water for 16 days. A description and the dimensions of its main components are shown in Table 3. Based on the observation of the results obtained, it can be inferred that the two plants are essentially equal. Crushing tank, mixing tank, digester and digested sludge tank have the same volumes in both plants because its size depend on the volume of substrate to be treated dairy and HRT, that are equal in both plants. Therefore, the differences lie in the equipments whose size depends on the methane yields which are storage volume of biogas and the power of the equipment installed to generate energy (a boiler in scenario 1 and a biogas engine in scenario 2). Crushing tank is designed to mash the fresh tobacco necessary daily, the digester is sized to treat the daily volume of substrate for 16 days (HRT) and a 25% more is added for safety. Finally, mixing tank and digested effluent tank have capacity to store the substrate to be treated or the digested effluent for 3 days. 4. Conclusions The results achieved in the anaerobic digestion experiments described along the present manuscript show no evidence of inhibition given the continuous monitoring of substrate’s pH via the addition of Ca(OH)2, regardless the percentage of tobacco in the substrate and organic charge. However, optimal results were reached when degrading substrate 15%/85% with 18.73 mg tobacco/ day organic charge, which allowed a methane yield as high as 53.84 Nm3 methane/t fresh tobacco as well as 65.78% COD reduction. With regard to feasibility testing, the sensitivity analyses
A. González-González, F. Cuadros / Renewable Energy 63 (2014) 280e285
285
Fig. 4. Sensitivity analysis for scenario 2.
Table 3 Dimensions of the main components of the anaerobic digestion plant. Volume (m3) Crushing tank Mixing tank Anaerobic digesters (2) Biogas storage Digested sludge tank Boiler (Scenario 1) Biogas engine (Scenario 2)
Height (m)
320 4 2110 5 7500 8 2600 scenario 1 e 3100 scenario 2 2110 5 2.7 MWth 1.62 MWe; 1.86 MWth
Diameter (m) 10 23 35 e 23
showed that the two following requirements are needed in order to achieve periods of return of investment approaching 9 years: (i) raise methane yield obtained in laboratory-scale experiments up to 67% and 104% for scenarios 1 and 2, respectively; (ii) production costs assumed as 1500 V/ha. Acknowledgements The present work was partially supported by the Government of Extremadura through projects PCJ 100201 and GR10045. A. González-González is also grateful to the Ministry of Education and Science for FPU grant referenced as AP2008-02546. References [1] Ministerio de Agricultura, Alimentación y Medio Ambiente, Avances de superficies y producciones de cultivos [Updated 2013 June 23]. Available at: http://www.magrama.gob.es/es/estadistica/temas/estadisticas-agrarias/ agricultura/avances-superficies-producciones-agricolas/; November 2012. [2] Fundación Formación y Empleo de Extremadura (Forem Extremadura). Estudio sobre el empleo e importancia social del cultivo de tabaco en Extremadura [Updated 2013 June 23]. Available at: http://www.foremextremadura.com/ userfiles/3946e52018433ebad128509dd4cba32d.pdf; 2011. [3] Instituto de Estadística de Extremadura [Updated 2013 June 23]. Available at: http://estadistica.gobex.es/; 2013. [4] Instituto Nacional de Estadística [Updated 2013 June 23]. Available at: http:// www.ine.es; 2013. [5] Demirel B, Scherer P. Bio-methanization of energy crops through monodigestion for continuous production of renewable biogas. Renew Energ 2009;34(12):2940e5. [6] Nielsen AM, Feilberg A. Anaerobic digestion of energy crops in batch. Biosyst Eng 2012;112(3):248e51.
[7] Vervaeren H, Hostyn K, Ghekiere G, Willems B. Biological ensilage additives as pretreatment for maize to increase the biogas production. Renew Energ 2010;35(9):2089e93. [8] Nges IA, Escobar F, Fu X, Björnsson L. Benefits of supplementing an industrial waste anaerobic digester with energy crops for increased biogas production. Waste Manag 2012;32(1):53e9. [9] Proyecto probiogas [updated 2013 June 23]. Available from:: www.probiogas. es [10] Izumi K, Okishio Y, Nagao N, Niwa C, Yamamato S, Toda T. Effects of particle size on anaerobic digestion of food waste. Int Biodeter Biodegr 2010;64:601e8. [11] González-González A, Cuadros F, Ruiz-Celma A, López-Rodríguez F. EnergyEnvironmental benefits and economic feasibility of anaerobic codigestion of Iberian pig slaughterhouse and tomato industry wastes in Extremadura (Spain). Bioresour Technol 2013;136:109e16. [12] MadridAPHA, AWWA, WPCF, Métodos Normalizados para el Análisis de Aguas Potables y Residuales, seventheenth ed., Díaz de Santos S.A; 1992. [13] Ministry of Agriculture and Rural Development of Alberta. Economic feasibility of anaerobic digesters, Agri-facts [updated 2013 June 23] Available at: http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex12280/$file/ 768-6.pdf?OpenElement; 2008. [14] Agencia Andaluza de la Energía. Estudio básico del biogás [updated 2013 June 23] Available at: http://www.agenciaandaluzadelaenergia.es/sites/default/ files/estudio_basico_del_biogas_0.pdf; 2011. [15] Gebrezgabher SA, Meuwissen MPM, Prins BAM, Oude Lansink AGJM. Economic analysis of anaerobic digestion. A case of green power biogas plant in The Netherlands. NJAS Wageningen J Life Sci 2010;57:109e15. [16] United States Department of Agriculture (USDA). An analysis of energy production costs from anaerobic digestion systems on U.S. livestock production facilities [updated 2013 June 23] Available at: http://www.agmrc.org/media/ cms/manuredigesters_FC5C31F0F7B78.pdf; 2007. [17] Naskeo Environment, Economy of biogas installations, [updated 2013 June 23]. Available at:http://www.biogas-renewable-energy.info/biogas_installatio ns_investments.html [18] Martin JH. A protocol for quantifying and reporting the performance of anaerobic digestion systems for livestock manures. International symposium on air quality and waste management for agriculture; Broomfield, Colorado; 2007. [19] Ministerio de Industria, Energía y Turismo (MINETUR), Precio de carburantes [updated 2013 June 23]. Available at: http://m.minetur.gob.es/Carburantes; 2013. [20] .Cuadros F, López-Rodríguez F, Ruiz-Celma A, Rubiales F, GonzálezGonzález A. Recycling, reuse and energetic valuation of meat industry wastes in Extremadura (Spain). Resour Conservat Recycl 2011;55(4):393e9. [21] Cuadros F, González-González A, Ruiz-Celma A, López-Rodríguez F. Recovery of energy and water through an eco-efficient treatment of olive mill wastewates. International Congress on Water, Waste and Energy Management. Salamanca: Mayo; 2012. [22] Roth GW. Corn silage production and management. Information and communication technologies in the college of agricultural sciences. The Pennsylvania State University; 2001.