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Comparison of feedstocks and technologies for biodiesel production: An environmental and techno-economic evaluation
Renewable Energy 69 (2014) 479e487
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Comparison of feedstocks and technologies for biodiesel production: An environmental and techno-economic evaluation L.E. Rincón, J.J. Jaramillo, C.A. Cardona* Instituto de Biotecnología y Agroindustria, Departamento de Ingeniería Química, Universidad Nacional de Colombia sede Manizales, Cra. 27 No. 64-60, Manizales, Colombia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 January 2012 Accepted 28 March 2014 Available online 4 May 2014
Due to their high productivity in both crops and algae, tropical countries are likely to be the future world suppliers of feedstocks as well as biofuels such as biodiesel. In this work five feedstocks: palm, jatropha, microalgae, tallow and waste cooking oil were analyzed and compared using techno-economic and environmental criteria. For each feedstock, technological configurations currently used in the industry were taken into account (acid catalysis, basic catalysis and cogeneration). In this work, it was found that productivities for the basic catalyzed process were comparatively higher (1.010 kg biodiesel/kg crude oil), than those catalyzed by acid (0.85e0.95 kg biodiesel/kg crude oil). After the simulation of the selected processes, the lowest production costs were obtained for jatropha (USD 0.15/L, basic catalysis) and for waste cooking oils (USD 0.23/L, acid catalysis). The PEI (Potential Environmental Impact) generated for basic catalyzed process ranged from 0.04 to 0.09, while the acid catalyzed case 0.020 and 0.06 PEI per kg of product. The jatropha and microalgae oil using basic catalyzed configuration with energy cogeneration were the best process alternative from the environmental and economical points of view. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Integral analysis Biodiesel WAR algorithm Economic evaluation Tropical feedstocks
1. Introduction Biodiesel is defined as a clean-burning fuel with low viscosity and pour point, non-toxic, biodegradable and environmentally friendly due to its comparative low emissions and reduction of SO2 production [1]. The main advantages of producing and using biodiesel come from the fact that foreign oil imports can be reduced, using the current installed distribution networks and the current engine technologies. The use of renewable sources for this biofuel industry helps to increase not only job generation and incomes, but also promoting energy self-sufficiency in rural areas [2,3]. Biodiesel is composed by fatty alkyl esters produced using different chemical routes according to the initial feedstock [4]. Transesterification and fatty acid esterification reactions are currently the most used [5,6]. However, other routes such as interesterification and thermal cracking can also be employed [7]. The increase in the petroleum prices and the diesel fuel consumption as well as the promotion of several bioenergy fuels policies and consumption incentives, project the biodiesel production
* Corresponding author. Tel.: þ57 6 8879300x50199; fax: þ57 6 8879300x50452. E-mail addresses: [email protected] (L.E. Rincón), [email protected] (J.J. Jaramillo), [email protected] (C.A. Cardona). http://dx.doi.org/10.1016/j.renene.2014.03.058 0960-1481/Ó 2014 Elsevier Ltd. All rights reserved.
above 50 billion liters by 2030 [8,9]. As a result, biodiesel has a rising potential market, that can be classified according to its end-use applications in: transportation, non-road applications (mining, forestry, construction, etc.), marine and heating [10]. Biodiesel can be produced from different oleochemical feedstocks, including animal fat, vegetable oils and algae oils, among others. These feedstocks are mainly composed by (85e98%wt) triglycerides, three long fatty acid chains joined to the glycerol molecule. Today the most common feedstocks are edible vegetable oils. United States, and Argentina mostly employees soybean oil, European Union countries use rapeseed oil; and tropical countries as Malaysia, Indonesia, Nigeria and Colombia prefer palm oil [5,11]. The main advantage of edible oil as a feedstock is that the plantations and the infrastructure are well established in most of these countries, making it easier the production of these edible oil crops to be expanded to meet the increasing demand. However, base biodiesel industry on edible feedstocks can be inconvenient due to their competition with food, and this may lead to food shortages and increase in food prices. Moreover, an expansion of these edible oil crops, would require monoculture plantations, affecting water resources and biodiversity [12,13]. Indeed, a good biodiesel feedstock should be easily available, without impacting negatively food security and environment. Therefore, most of the countries are involved into a difficult decision regarding to what kind of feedstocks should be used to
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sustain their national biofuels policies. For instance, in Argentina soybean is the preferred raw material for biodiesel production due to its low cost, while in China this feedstock it is not accepted as biofuel due to the high demand of soybean oil from traditional Chinese food [14]. This situation has driven to promote the use of other feedstocks as jatropha, castorbean and karanja in countries such as India and Brazil [15]. In the same way, other alternative feedstocks different from edible oil crops are used in Japan (waste cooking oils), Canada and Australia (tallow and animal fats) [16]. Non-edible oils make possible to use degraded and waste lands, preserving most productive lands for food production [17]. Meanwhile, the use of animal fats and waste cooking oil is still limited at large scale due to their high free fatty acid content and, solidification points at room temperature, causing difficulties during their production and use. However, these non-conventional raw materials are also promising options for biodiesel production due to low cost [18,19]. On the other hand, microalgae are species with a high comparative potential to produce biodiesel given their high production rates and low land requirements [20,21]. In this work five promising feedstocks, under two different process configurations (basic catalysis with cogeneration and acid catalysis) available in tropical countries, were simulated and compared using techno-economic and environmental criteria. As a result, two feedstocks and one technological configuration were found to be the more convenient for biodiesel production in tropical countries. 2. Biodiesel feedstocks Oil palm is a high-yield crop that requires small areas to be cultivated. Currently, it is one of the largest suppliers of edible oil in the world. It is produced in countries throughout the tropics, being Malaysia, Indonesia and Colombia the higher producers with almost 83% of total global production [22]. Crude palm oil (CPO) and crude palm kernel oil are obtained from the extraction process. Although CPO can be directly used in industrial activities, in order to be used on human food industries, palm oil must be refined, blanched and deodorized [23]. The obtained oil is known as refinedebleachededeodorized (RBD) oils and has free fatty acid content lower than 0.1%. Today, 70e90% of palm oil production is used for food and cosmetic industries, while the remaining 10e20% for other industrial applications. However, these values change according to fluctuations on demand and large competition with soybean oil [24]. Jatropha curcas is a bush which belongs to the family of Euphorbiaceae, distributed in the wild or semi-cultivated areas. It is a tropical and subtropical plant, and can grow at latitudes 30 north and south of the equator. This crop is a highly resistant plant able to survive in fallowed agricultural lands and from low to high rainfall areas, being easily cultivated with little efforts [25,26]. Jatropha oil finds different possible applications in lubricants, illumination, soaps, cosmetics, medicinal uses, biopesticides (phorbol esters) industries [24]. For biodiesel, low-jatropha production costs and possibilities to be grown in different lands can ensure competitiveness for this feedstock [27]. Microalgae are the most primitive form of plants. They have a huge range of genetic diversity and can exist as unicellular plants, colonies or extended filaments. Microalgae grow under the widest possible variety of conditions [28]. These microscopic aquatic plants, carry out the same process and mechanism of photosynthesis as higher plants converting sunlight, water and carbon dioxide into biomass, lipids and oxygen [10,24]. Nevertheless, they have more efficient access to water, CO2 and other nutrients due to their simpler cellular structure and high specific surface area [12]. A microalgae with high oil productivity is desired for producing biodiesel, depending on the species,
microalgae produce different kinds of lipids, hydrocarbons and complex oils [24]. The species of microalgae with higher oil content includes Chlorella vulgaris, Chlorella protothecoides, Spirulina maxima, Nannochloropsis sp., Neochloris oleabundans, Scenedesmus obliquus and Dunaliella tertiolecta among others [29]. Waste oil, includes residues from deep frying processes, such as soap stocks, yellow and brown greases, obtained from restaurants, hotels and industries. There are enough waste cooking oils and fats generated in the United States annually, to produce about 18 billion liters of biodiesel [6]. In United States the biodiesel made from waste cooking oil is known as McDiesel, because one large source of this oils is McDonald’s restaurants [30]. It has been reported that waste oils still conserve still conserves most of its triglycerides groups, with chemical and physical properties very similar to those of its source oils [31]. The free fatty acid content of waste cooking oils ranges between 10 and 25%, as a result of the frying process where heating in presence of air and light increase the viscosity and specific heat [32]. Waste cooking oils can be used as a fuel, either directly in the engine after filtering or can be transesterified using short-chain alcohol to produce biodiesel and glycerin [33e35]. In biodiesel production process, fats, moisture, proteins, and animal fragments extracted from meats during the frying process are an important component of waste cooking oils affecting the process in terms of additional pretreatment is required [36]. Tallow oil is a rendered from beef fats obtained in slaughterhouses [24]. Human consumption of tallow has a negative effect on health; consequently, these feedstocks are addressed fundamentally to industry. Tallows are used in the manufacture of products such as cosmetics, soaps, shampoo, candles, lubricants, paints, tires, perfumes, textiles, plastics, inks, polishes, cleaners and solvents. Different grades of tallow are produced to meet the varying needs. They are also an important source of fatty acids and glycerol for the chemical industry [37]. However, when its market is overloaded, this oil is incinerated or disposed in landfills [38,39]. Therefore, biodiesel production from these feedstocks has been considered, since they are potential sources for biodiesel due to their high cetane number (typically 56e62), good stability and low price. The drawback of these feedstock is that still requires a pretreatment in order to remove impurities and treat those with high saturated fatty acids content, to avoid yield methyl esters with poor cold temperature properties. 3. Technologies for biodiesel production As most of chemical processes, the biodiesel production can be generalized as three main sequential stages: Pretreatment, reaction and purification. 3.1. Pretreatment In this stage, those elements in feedstock oil, which may have an undesired effect over the transformation reactions, are withdrawn. Particles, colloidal mater, pigments, extraction residues and other impurities can be removed using filtration. When water content (>0.06%) and free fatty acid (FFA) content (>4%) are high, a saponification reaction can be induced, generating a gel soap instead of biodiesel [40]. To avoid this it is necessary to dry the oil first, proceeding then to FFA elimination, using: neutralization or pre-esterification of FFA. Esterification as pretreatment method in biodiesel production, can be combined with transesterification to obtain almost a complete conversion to biodiesel [6]. There are two general methods used for esterification: the batch process and the continuous process. Esterification can be performed under batch operation at a temperature of 200e250 C. As long as, it is an equilibrium reaction, the water must be removed continuously to
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obtain a high-ester yield. The conversion rate is also strongly influenced by the reaction temperature. However, giving enough time, the reaction will also proceed to near completion even at room temperature. Generally, the reaction is conducted close to the methanol boiling point [41]. 3.2. Reaction Biodiesel is mainly produced using transesterification reactions with short-chain alcohols. Nevertheless, interesterification can also be employed using short-chain esters, such as methyl acetate [42]. Transesterification or alcoholysis is a reaction of fatty acid groups from the triglycerides using an alcohol to produce three molecules of alkyl esters and one of glycerol. Low molecular weight alcohols such as methanol, ethanol, propanol and butanol are suitable to be used in biodiesel production; however, methanol is the most extensively used because of its low-cost as well as physical and chemical properties [43]. The function of catalysts on biodiesel production is to modify the kinetics, improving reaction rates and selectivity to the desired products. In this way, the reaction time and loads sent to purification sections are reduced, saving energy and capital costs. Different types of catalysts are employed in biodiesel production (basic, acid, heterogeneous and enzymes). The homogeneous basic catalysts most used in biodiesel industry for transesterification reaction are NaOH, KOH, due to their low market costs, comparative high reaction rates and the mild reaction temperatures required (55e75 C) [44]. However, the use of these catalysts may lead to undesirable reactions if FFA and moisture are not low enough (FFA (>1%) and moisture (<0.06%)) [45]. Industrially, this situation is managed using alkoxides instead of hydroxides such as sodium methoxide, sodium ethoxide, or sodium propoxide, according to the alcohol employed in the transesterification reaction [46]. Homogeneous acid catalysis is employed for transesterification reactions involving oleochemical feedstocks with high free fatty acid content, such as waste cooking oils, yellow and brown greases and animal rendering residues (tallow) [6]. Acid catalysis uses strong acids such HCl and H2SO4. The advantage of acid catalysts is to convert free fatty acids into biodiesel esters through esterification, while simultaneously catalyze the transesterification of triglycerides to biodiesel, all in a single step [47]. However, acid catalysis has comparatively slower reaction rates than basic and requires higher rates of methanol [46]. Heterogeneous catalysts use a different kind of catalytically active solids (MgOeLa2O3, zeolites (ETS-10), KI/Al2O3, TiO2) to produce fatty acid methyl esters. This catalyst is easily recovered and can be regenerated after reaction ends [6]. This feature simplifies and economizes the downstream treatment of the products [46]. Enzymatic catalysis is carried out using lipases which implies mild conditions and a biologically closed cycle (biocatalysts for a bioproduct). Finally, it helps to avoid some problems of conventional methods related to saponification reactions, glycerol recovery, and removal of inorganic salts. Moreover, FFA can be totally esterified. Enzymes typically need small amounts of water for activity and constraints of water in conventional systems may be avoided, thus it is possible to save some energy by the reduction of separation loads. However enzymes are still very expensive to use at large biodiesel production scales [48,49]. 3.3. Separation and purification The reaction mixture obtained after the transesterification process contains alkyl esters, glycerol, non-converted alcohol, catalyst and mono-, di- and tri-glycerides. This mixture must be purified, in order to obtain a high-quality biodiesel according to the
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European standard (EN14214) or United States standard (ASTM D6751) [50,51]. For this reason separation and purification stages are important for final product quality, affecting consequently the operative costs. The main step of conventional biodiesel purification can be performed using two main routes: i) Performing liquide liquid extraction first, recovering alkyl esters in light phase, while glycerin is in heavy phase; then non-converted alcohol is separated [52,53]. ii) Vacuum distillation is first used to separate non-converted alcohol and then liquideliquid extraction to separate glycerin and biodiesel; last step is known as water washing [54,55]. Apparently, first alternative is the most convenient option. Under an initial analysis from the point of view of chemical process design, this route reduces in a first stage the separation load sent to reaming stages. However, this alternative is operationally not convenient as a number of disadvantages are reported in the practice: 1) A low amount of glycerol remains in biodiesel phase, making difficult its further purification 2) The glycerol separation rate is lower due to changes on solubility as a result of the effect of alcohol. 3) Still an additional alcohol recovery stage is needed, increasing operation costs. Conversely, the second alternative can be slightly more expensive because of vacuum distillation technology. However the initial separation of alcohol makes easier further separation phases. After this point, both for homogeneous alkaline and acid processes, catalyst must be neutralized, producing salts that could be removed later by filtration or centrifugation. For instance, on the alkaline case, the neutralization makes easier water washing. But if the basic catalyst is not neutralized an emulsion of non-converted oil-water may be formed, increasing energy consumption of the separation process. After neutralization, the ester-enriched phase is purified, removing residual alcohol, catalyst, neutralization salts, soaps and residual glycerol. Finally, biodiesel is dried using distillation or vacuum flash. The main objective of this final stage is to improve the smell and color of final product. To select a technology for biodiesel production includes determining: a) main process variables, such as: reaction time, phase, catalyst and additional reactants; b) pretreatment-reaction technologies and c) capital and operational costs, as well as energy consumption. An improvement to biodiesel production process is the biomass fired cogeneration. This option allows to generate the process requirements of heat and power using oil extraction residues as fuel [56]. Cogeneration technologies as a way for combined production of mechanical and thermal energy have remarkable cost and energy savings, operating also with higher efficiency compared to systems, which produce heat and power separately. The use of this technology increases the energy utilization improving process economy. Last is achieved given, the reduction of external utilities requirements, while generated electricity can be sold to central grid. Among available technologies for biomass fired cogeneration, the combined-cycle gas turbine (CCGT), is considered as a promising alternative due to its efficiency [57]. The CCGT configurations are composed by the following elements: gas turbine, where chemical energy is converted to mechanical work [56]. Heat steam recovery generator (HSRG), that is a high-efficiency steam boiler that uses hot gases from a gas-turbine or reciprocating engine to generate steam, in a thermodynamic Rankine cycle; generating steam at different pressure levels required by a chemical process. Finally, steam turbine, where steam produced in HSRG system is employed to generate additional power (see Fig. 1). Two process configurations were used in this work. The first option uses basic catalysis (Fig. 2) improved with biomass fired
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Fig. 1. Process flow-sheet diagram for cogeneration system based on extraction cake residues. 1. Direct use heat exchanger, 2. Air divisor, 3. Compressor, 4. Turbine, 5. Dryer, 6. Combustion chamber, 7. Cyclone, 8. High pressure superheater, 9. High pressure evaporator, 10. High pressure economizer, 11. High pressure drum, 12. Low pressure economizer I, 13. Flow divisor, 14. Low pressure economizer II, 15. Intermediate pressure economizer, 16. Intermediate pressure evaporator, 17. Intermediate pressure superheater, 18. Intermediate pressure drum, 19. Flow divisor, 20. Low pressure evaporator, 21. Low pressure superheater, 22. Low pressure drum, 23. Steam turbine, 24. Flow mixer.
cogeneration (Fig. 1), while second employs acid catalysis (Fig. 3) without any cogeneration (waste cooking and tallow oil are not associated to any lignocellulosic residue). Basic catalysis was used for vegetable oils with low free fatty acid content (jatropha, palm and microalgae); whereas acid catalysis was chosen for feedstocks with high free fatty acid content (waste cooking oil, tallow oil). Both options considered vacuum distillation as a first step for biodiesel purification. 4. Methodology The simulations were carried out in Aspen Plus V7.1 (Aspen Technology, Inc., USA). This approach allows obtaining data of mass and energy balances, basic engineering estimations of equipment size and its energy consumption. Feedstocks (palm, jatropha, microalgae, tallow and waste cooking oils) for biodiesel production
were modeled as a sort of pseudocomponents, created to represent triglycerides and methyl esters, according to Chang and Liu’s methodology [10]. Conversely, extraction residues used as a fuel in biomass fired cogeneration, were included into the simulation according to chemical compositions reported by Akintayo, 2004 (jatropha cake) [58], Piarpuzán et al., 2011 (palm cake) [59], and Phukan et al., 2011 (Microalgae paste) [60]. Physicochemical properties for pseudocomponents, were estimated using the Marrero and Gani method [11]. UNIFAC Dortmund for liquid phase, Soave Redlich Kwong with the Boston Mathias modification (RKSBM) for the vapor phase. The water enthalpy was calculated with NBS steam tables. UNIFAC Dortmund, allows to model accurately binary and ternary liquideliquid equilibriums of systems containing vegetable oils, biodiesel, glycerol for which NRTL data are not available. RKS-BM model is recommended for simulations involving complex compounds such as triglycerides. For instance,
Fig. 2. Process flow diagram for biodiesel production using basic catalysis. 1. Neutralization reactor I, 2. Transesterification reactor, 3. Liquideliquid extraction column I, 4. Distillation tower I, 5. Liquideliquid extraction column II, 6. Distillation tower II, 7. Neutralization reactor II, 8. Solid separator, 9. Flash.
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Fig. 3. Process flow diagram for biodiesel production using acid catalysis. 1. Flash I, 2. Pre-esterification reactor, 3. Transesterification reactor, 4. Distillation tower I, 5. Liquideliquid extraction column, 6. Distillation tower II, 7. Neutralization reactor, 8. Solid separator, 9, Flash.
this model has been successfully used to model refinery and petrochemical processes [61]. The kinetic model for acid and basic catalysis, employed in this work, was reported by Granjo et al. [12], as first order and second order expressions, respectively. The functional basis used in this work for the simulation procedures was 1000 kg/h of biodiesel production. Based on this value was calculated a residue flow rate for palm oil cake, jatropha oil cake and microalgae paste; using it as an input for the biomass fired cogeneration system. The economic analysis was performed using Aspen Icarus Process Evaluator package (Aspen Technology, Inc., USA), to calculate a mean cost in US dollar per liter of biodiesel produced with the selected feedstocks. This analysis was performed using the design information provided by Aspen Plus, under the economic conditions of Colombia (annual interest rate of 17% and tax rate of 33%). A straight line depreciation method, at 12 years of the analysis period, was considered. Prices of commercial oils and other reagents (methanol, sodium hydroxide, sulfuric acid, etc.) reported on ICIS pricing were used as a base for the raw materials [62,63]. For microalgae oil as a non-commercial oil, it was employed values reported in literature by Campbell et al. [64] for microalgae production in ponds using flue gas as substrate. Operative charges such as operator and supervisor labor cost were defined as USD 2.14/h and USD 4.29/h, respectively. Electricity, potable water, low and high steam pressure costs were USD 0.03/ kWh, USD 1.25/m3, USD 8.18/ton. The environmental impact was assessed with WAR, waste reduction algorithm (EPA, USA), to estimate the potential environmental impact (PEI) generated in the biodiesel production process. Considering eight environmental impact categories: Human toxicity potential by ingestion (HTPI), Human toxicity potential by exposure both dermal and inhalation (HTPE), Terrestrial toxicity potential (TTP), Aquatic toxicity potential (ATP), Global warming potential (GWP), Ozone depletion
potential (ODP), Photochemical oxidation potential (PCOP), and Acidification potential (AP). The mass flow rate of each component in the process streams is multiplied by its chemical potency; determining its contribution to the potential environmental impact categories [65]. To compare the environmental profiles for all process, total PEI was determined by the sum of all (eight) potential P environmental impact categories as follows: ni¼ 1 ai 4i , where ai is the weighting factor for potential environmental impact category i, and 4i represents the potential environmental impact for category i. In this work all of the weighting factors were set equal to 1 [66,67] 5. Results Results of the simulation are summarized in a table (see Table 1). It was found how productivities for the process catalyzed with NaOH are comparatively higher (1.01 kg of biodiesel/kg of crude oil), than those catalyzed with H2SO4 (0.85e0.95 kg of biodiesel/kg of crude oil). Meanwhile, methanol consumption as well as global energy consumption (heating and electricity) for acid catalyzed processes were 1.58e1.77 kg of methanol/kg of crude oil and heating of 53.39e72.76 MW respectively. These values are higher than methanol and energy required by the basic catalyzed process (0.16e0.21 kg of methanol/kg of crude oil and heating of 24.73e29.96 MW respectively). Results for process residues, demonstrated that the acid catalyzed processes also have a higher production rate for waste water 1.84e2.16 kg of water/kg of crude oil compared to 0.01e0.94 kg of water/kg of crude oil for basic catalyzed process (see Table 2). Simulation results for the biomass fired cogeneration plant using palm oil cake, jatropha oil cake and microalgae paste as fuels (see Table 3), reveal how the heating energy production from jatropha cake (31.03 MW) is higher than the energy produced from palm oil cake (20.39 MW) and a little
Table 1 Feedstocks and process configurations used in this work. Feedstock
Process configuration
Catalyst
Pretreatment
Energy source
Jatropha Palm Microalgae Waste cooking oils Tallow
Basic catalysis þ cogeneration Basic catalysis þ cogeneration Basic catalysis þ cogeneration Acid catalysis
NaOH NaOH NaOH H2SO4
FFA FFA FFA FFA
Cogeneration Cogeneration Cogeneration External source
Acid catalysis
H2SO4
FFA esterification
neutralization neutralization neutralization esterification
External source
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Table 2 Simulation results for biodiesel production using acid and basic catalysis.
Materials (kg/h) Crude oil FFA content (%wt) Impurities content (%wt) Water content (%wt) Methanol NaOH H2O H2SO4 CaO Energy (electricity, Pumping, and recycling Energy (heat, MW) Heating Cooling Products (kg/h) Biodiesel @>99%wt Glycerol @>88%wt Waste water CaSO4 Na2SO4
Palm oil
Jatropha oil
Microalgae oil
Tallow oil
Waste cooking oil
992.59 6% 0%
990.41 4% 0%
985.59 4% 0%
1052.79 15% 3%
1179.76 10% 3%
0%
0%
0%
8%
12%
212.69 9.51 1386.57 21.24 e
204.78 9.15 1379.82 20.45 e
1662.43 e 1315.99 36.85 31.58
2091.06 e 1651.66 13.21 11.33
0.58
0.64
0.46
0.55
26.39 39.15
29.96 25.64
24.73 36.63
72.76 90.66
62.99 80.92
1000.00 112.21 1003.45 e 10.56
1000.00 116.21 933.73 e 16.87
1000.00 125.50 1046.89 e 16.79
1000.00 86.27 2277.22 110.03 e
1000.00 96.82 2167.03 65.20 e
159.02 9.33 1389.63 20.84 e kW) 0.55
lower than the energy from microalgae paste (33.33 MW). Among the three residues considered, microalgae paste cogenerates more electricity (8.22 MW) than jatropha oil cake (7.58 MW) and palm oil cake (4.84 MW). The economic evaluation results for biodiesel production are summarized in Table 4. As significant results, raw material costs, (including costs of all feedstocks used in biodiesel production such as oil, methanol, catalyst and acids) for basic catalyzed processes (USD 0.32e0.50/L) were higher than those for acid catalyzed processes (USD 0.23e0.30/L). Utilities costs for acid catalyzed processes (USD 0.05e0.06/L) were higher than basic catalyzed process (USD 0.02e0.02/L). The remaining costs for both processes were similar. Production costs for basic catalyzed processes (USD 0.39e 0.58/L) were higher than those for acid catalyzed processes (USD 0.23e0.30/L). However, considering potential incomes by electricity selling at average price in Colombia, it can be seen how total
production costs for jatropha, palm and microalgae biodiesels were reduced to USD 0.15/L, USD 0.32/L and USD 0.31/L, respectively. The environmental analysis using the war algorithm for the 5 feedstocks revealed that, the generated emissions as PEI generated per kg of product ranged from 0.04 to 0.09 for basic catalyzed process, whereas for acid catalyzed case 0.02 and 0.06 (see Fig. 4a). In the same way, outlet emissions as PEI per kg of product vary between 0.04 and 0.15 for basic catalyzed process, while for acid catalyzed process the values ranged over 0.05 and 0.31 (see Fig. 4b). 6. Discussion Results from chemical processes simulation, reveal how both basic and acid catalyzed processes have reaction yields ranging 96e 98%. Reaction yield values were calculated based on the defined biodiesel production (1000 kg/h) for all processes and the net oil (after pretreatment) incoming with feedstock, free of FFA and other impurities (jatropha oil 950.79 kg/h, palm oil 933.04 kg/h, microalgal oil 946.16 kg/h, tallow oil 779.07 kg/h, waste cooing oil 884.82 kg/h). The obtained yields for tallow oil (97.7%) and waste cooking oil (98.7%) agree with values ranging over 92e98% reported for acid catalysis using H2SO4 by West et al., 2008 [68] and Van Kasteren et al., 2007 [69]. In the same way obtained yields for jatropha oil (93.95%), palm oil (95.80%) and microalgae (96.07%) agree with the values reported for basic catalyzed process (92e 97%) reported by Glisic et al., 2003 [70] and Haas et al., 2006 [53]. These results indicate how low quality feedstock with high content of FFA and other impurities after an adequate pretreatment can produce high-quality biodiesel (>99 %wt). Cogeneration results gave an advantage to basic catalyzed processes due to the possibility of employing the extraction residues to generate heat and power. Both higher heating and power potential were released by microalgae paste followed by jatropha cake. As it can be seen in Table 3, the high protein and low fiber content of these feedstocks apparently help to increase the calorific value of these residues. As long as, they have comparative higher calorific values and higher available rates, these residues could generate more steam and consequently, produce more heat and power (see Table 3). However, the above results were reflected on capacity of cogeneration systems to cover heating requirements of biodiesel processes. Thus, only microalgae paste and jatropha cake could
Table 3 Cogeneration results for extraction residues based on biomass fired cogeneration system.
Available residue [kg/h] Moisture Protein Oil Fiber Ash Calorific value [MJ/kg] Cogenerated heating Heat fired [MW] Heat direct use [MW] Heat exchangers [MW] Total heating [MW] Cogenerated electricity Pumping [MW] Gas turbine [MW] Steam turbine [MW] Total electricity [MW] a b c
Jatropha oil cakea
Palm oil cakeb
Microalgae oil pastec
3062.33 27.28% 21.30% 27% 18.28% 6.25% 15.62 Consumed
2281.89 30.00% 2.52% 23.48% 41.00% 3.00% 12.20 Consumed
3906.87 6.80% 43.22% 28.82% 9.46% 5.93% 15.88 Consumed
Generated 13.42
1.14
Consumed
7.8 0.978
18.75 31.03 Generated
0.012
Chemical composition from Akintayo, 2004 [58]. Chemical composition from Mata et al., 2010 [51]. Chemical composition from Phukan et al., 2011 [60].
Generated
Consumed
15.42 1.17
13.57 20.39 Generated
0.009 5.59 2.01 7.58
Generated
Consumed
19.07 33.33 Generated
0.011 3.45 1.41 4.84
6.17 2.06 8.22
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Table 4 Economic evaluation results for biodiesel production using basic catalysis. Basic Catalysis
Raw material cost Total utilities cost Operating labor Maintenance Operating charges Plant overhead General and administrative cost Subtotal cost Credit by electricity selling Total cost with cogeneration
Acid Catalysis
Jatropha oil, USD/L
Palm oil, USD/L
Microalgae, USD/L
Waste cooking oil, USD/L
Tallow oil, USD/L
$ $ $ $ $ $ $ $ $ $
$ $ $ $ $ $ $ $ $ $
$ $ $ $ $ $ $ $ $ $
$ $ $ $ $ $ $ $ $ $
$ $ $ $ $ $ $ $ $ $
0.322 0.022 0.008 0.005 0.002 0.006 0.029 0.394 0.249 0.145
0.406 0.021 0.008 0.004 0.002 0.006 0.036 0.482 0.159 0.324
cover 100% of heating requirements of biodiesel processes, while palm oil cake only covered 77.27% of total heating requirements. All extraction residues could cover 100% of electricity requirements of biodiesel processes, generating also an excess of electricity able to be sold to central grid. Production costs for basic catalyzed processes from jatropha oil and palm oil, were lower than values reported on literature by Elbersen et al., 2008 (USD 0.39/L, palm oil) Carriquiry et al., 2011 (USD 0.44e0.72/L, jatropha oil) [71]. This result is explained because the processes simulated in this work use cogeneration to supply process energy and generate an additional income used as credit to reduce total production costs. For biodiesel from microalgae oil, at the moment, there are not commercial values reported in the open literature; however, authors such as Demirbas, 2011 [72] and Campbell et al., 2011 [64] agree that production cost of this oil should be lower than diesel oil. In this sense obtained biodiesel from microalgae oil covers this requirement. In the same way acid catalyzed production costs were lower than values reported by West, 2008 [68] and Zhang, 2003 [31] (USD 0.45e0.54/L). It can be explained due to the low costs considered for these feedstocks. Comparing economic evaluations among the biodiesel production processes, it can be seen how initially basic catalyzed process costs were higher than acid catalyzed and how only after including cogeneration credits this situation change positively. This situation can be explained based on difference of raw material costs (as
0.497 0.022 0.008 0.005 0.002 0.006 0.037 0.577 0.269 0.307
0.146 0.055 0.009 0.002 0.002 0.006 0.008 0.230 0.000 0.230
0.219 0.052 0.009 0.002 0.002 0.006 0.012 0.303 0.000 0.303
major cost). Vegetable oils such as palm oil, jatropha oil and microalgae oil have higher costs, mainly due to extraction process and market competition for other usages, especially in palm oil case. Conversely, waste cooking oil and tallow oil are residues from other industries with a representative content of impurities and FFA. This feature reduces its costs, but also reduces its uniformity and quality as feedstock, requiring the use of acid catalysis for its processing. Acid catalysis as modification for biodiesel production has a higher methanol consumption rate and generates more effluents. This situation drives to processes with a low production cost but potentially contaminant. The environmental analysis results using WAR algorithm revealed how all considered processes and feedstocks have a negative PEI generated per kg of product. This means from the generation point of view that all the alternatives have the potential to generate a mitigation of incoming environmental load by transforming feedstocks into added value products. However, it can be seen in Fig. 4a that the feedstocks processed using basic catalyzed routes have higher mitigation potentials (more negative PEI) compared to acid catalyzed routes. Above statement was confirmed analyzing the environmental impact by outlet PEI per kg of product (see Fig. 4b). In this analysis, it can be seen how all outlet streams contain pollutant elements, which can affect the ecosystem. Therefore, it would be desirable for these streams to have a PEI close to zero, in order to reduce the potential environmental
Fig. 4. PEI analysis for biodiesel production from different feedstocks. a) Generated PEI per mass of product. b) Outlet PEI per mass of product.
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impacts of products. In this case acid catalyzed processes have higher PEI and are less environmentally friendly than those catalyzed with bases. This effect is explained by the fact that sulfuric acid is used as catalyst, increasing the acidification potential (AP) and photochemical oxidation potential (PCOP). Thus, the emitted sulfur materials might acidify surrounding lands and rivers, and increase the formation of SOx, contributing to acid rain formation and global warming. Additionally, palm and jatropha oils had comparatively lower PEIs, with reduced impacts on human toxicity potential by ingestion (HTPI), Terrestrial toxicity potential (TTP), Aquatic toxicity potential (ATP). Consequently, basic catalyzed processes arise as a comparatively cleaner technology, where feedstocks are processed to obtain high-value products in an environmentally friendly way. 7. Conclusions Based on the results, the most convenient configuration for biodiesel production in tropical countries (in the framework of this analysis) employs jatropha oil in a basic catalyzed scheme, integrated to a cogeneration plant where jatropha oil cake is fired. This configuration can produce high rates of biodiesel with the lower production costs, improved by electricity selling. Furthermore, this configuration proves, to be the most environmentally friendly with lesser potential emissions and climate-change effect, as well as reduced land use given its ability to be growth in marginal lands. Microalgae oil is called to be the future biodiesel feedstock, due to its low land requirements and high potential to capture CO2; nevertheless, this feedstock still requires improvements on its production technologies, in order to reduce production costs and increase its economic sustainability. Acknowledgments To the Colombian Institute for Development of Science and Technology (Colciencias) and Universidad Nacional de Colombia at Manizales, at Amazonas and at Orinoquia at for the financial support of this work.
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Comparison of feedstocks and technologies for biodiesel production: An environmental and techno-economic evaluation L.E. Rincón, J.J. Jaramillo, C.A. Cardona* Instituto de Biotecnología y Agroindustria, Departamento de Ingeniería Química, Universidad Nacional de Colombia sede Manizales, Cra. 27 No. 64-60, Manizales, Colombia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 January 2012 Accepted 28 March 2014 Available online 4 May 2014
Due to their high productivity in both crops and algae, tropical countries are likely to be the future world suppliers of feedstocks as well as biofuels such as biodiesel. In this work five feedstocks: palm, jatropha, microalgae, tallow and waste cooking oil were analyzed and compared using techno-economic and environmental criteria. For each feedstock, technological configurations currently used in the industry were taken into account (acid catalysis, basic catalysis and cogeneration). In this work, it was found that productivities for the basic catalyzed process were comparatively higher (1.010 kg biodiesel/kg crude oil), than those catalyzed by acid (0.85e0.95 kg biodiesel/kg crude oil). After the simulation of the selected processes, the lowest production costs were obtained for jatropha (USD 0.15/L, basic catalysis) and for waste cooking oils (USD 0.23/L, acid catalysis). The PEI (Potential Environmental Impact) generated for basic catalyzed process ranged from 0.04 to 0.09, while the acid catalyzed case 0.020 and 0.06 PEI per kg of product. The jatropha and microalgae oil using basic catalyzed configuration with energy cogeneration were the best process alternative from the environmental and economical points of view. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Integral analysis Biodiesel WAR algorithm Economic evaluation Tropical feedstocks
1. Introduction Biodiesel is defined as a clean-burning fuel with low viscosity and pour point, non-toxic, biodegradable and environmentally friendly due to its comparative low emissions and reduction of SO2 production [1]. The main advantages of producing and using biodiesel come from the fact that foreign oil imports can be reduced, using the current installed distribution networks and the current engine technologies. The use of renewable sources for this biofuel industry helps to increase not only job generation and incomes, but also promoting energy self-sufficiency in rural areas [2,3]. Biodiesel is composed by fatty alkyl esters produced using different chemical routes according to the initial feedstock [4]. Transesterification and fatty acid esterification reactions are currently the most used [5,6]. However, other routes such as interesterification and thermal cracking can also be employed [7]. The increase in the petroleum prices and the diesel fuel consumption as well as the promotion of several bioenergy fuels policies and consumption incentives, project the biodiesel production
* Corresponding author. Tel.: þ57 6 8879300x50199; fax: þ57 6 8879300x50452. E-mail addresses: [email protected] (L.E. Rincón), [email protected] (J.J. Jaramillo), [email protected] (C.A. Cardona). http://dx.doi.org/10.1016/j.renene.2014.03.058 0960-1481/Ó 2014 Elsevier Ltd. All rights reserved.
above 50 billion liters by 2030 [8,9]. As a result, biodiesel has a rising potential market, that can be classified according to its end-use applications in: transportation, non-road applications (mining, forestry, construction, etc.), marine and heating [10]. Biodiesel can be produced from different oleochemical feedstocks, including animal fat, vegetable oils and algae oils, among others. These feedstocks are mainly composed by (85e98%wt) triglycerides, three long fatty acid chains joined to the glycerol molecule. Today the most common feedstocks are edible vegetable oils. United States, and Argentina mostly employees soybean oil, European Union countries use rapeseed oil; and tropical countries as Malaysia, Indonesia, Nigeria and Colombia prefer palm oil [5,11]. The main advantage of edible oil as a feedstock is that the plantations and the infrastructure are well established in most of these countries, making it easier the production of these edible oil crops to be expanded to meet the increasing demand. However, base biodiesel industry on edible feedstocks can be inconvenient due to their competition with food, and this may lead to food shortages and increase in food prices. Moreover, an expansion of these edible oil crops, would require monoculture plantations, affecting water resources and biodiversity [12,13]. Indeed, a good biodiesel feedstock should be easily available, without impacting negatively food security and environment. Therefore, most of the countries are involved into a difficult decision regarding to what kind of feedstocks should be used to
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sustain their national biofuels policies. For instance, in Argentina soybean is the preferred raw material for biodiesel production due to its low cost, while in China this feedstock it is not accepted as biofuel due to the high demand of soybean oil from traditional Chinese food [14]. This situation has driven to promote the use of other feedstocks as jatropha, castorbean and karanja in countries such as India and Brazil [15]. In the same way, other alternative feedstocks different from edible oil crops are used in Japan (waste cooking oils), Canada and Australia (tallow and animal fats) [16]. Non-edible oils make possible to use degraded and waste lands, preserving most productive lands for food production [17]. Meanwhile, the use of animal fats and waste cooking oil is still limited at large scale due to their high free fatty acid content and, solidification points at room temperature, causing difficulties during their production and use. However, these non-conventional raw materials are also promising options for biodiesel production due to low cost [18,19]. On the other hand, microalgae are species with a high comparative potential to produce biodiesel given their high production rates and low land requirements [20,21]. In this work five promising feedstocks, under two different process configurations (basic catalysis with cogeneration and acid catalysis) available in tropical countries, were simulated and compared using techno-economic and environmental criteria. As a result, two feedstocks and one technological configuration were found to be the more convenient for biodiesel production in tropical countries. 2. Biodiesel feedstocks Oil palm is a high-yield crop that requires small areas to be cultivated. Currently, it is one of the largest suppliers of edible oil in the world. It is produced in countries throughout the tropics, being Malaysia, Indonesia and Colombia the higher producers with almost 83% of total global production [22]. Crude palm oil (CPO) and crude palm kernel oil are obtained from the extraction process. Although CPO can be directly used in industrial activities, in order to be used on human food industries, palm oil must be refined, blanched and deodorized [23]. The obtained oil is known as refinedebleachededeodorized (RBD) oils and has free fatty acid content lower than 0.1%. Today, 70e90% of palm oil production is used for food and cosmetic industries, while the remaining 10e20% for other industrial applications. However, these values change according to fluctuations on demand and large competition with soybean oil [24]. Jatropha curcas is a bush which belongs to the family of Euphorbiaceae, distributed in the wild or semi-cultivated areas. It is a tropical and subtropical plant, and can grow at latitudes 30 north and south of the equator. This crop is a highly resistant plant able to survive in fallowed agricultural lands and from low to high rainfall areas, being easily cultivated with little efforts [25,26]. Jatropha oil finds different possible applications in lubricants, illumination, soaps, cosmetics, medicinal uses, biopesticides (phorbol esters) industries [24]. For biodiesel, low-jatropha production costs and possibilities to be grown in different lands can ensure competitiveness for this feedstock [27]. Microalgae are the most primitive form of plants. They have a huge range of genetic diversity and can exist as unicellular plants, colonies or extended filaments. Microalgae grow under the widest possible variety of conditions [28]. These microscopic aquatic plants, carry out the same process and mechanism of photosynthesis as higher plants converting sunlight, water and carbon dioxide into biomass, lipids and oxygen [10,24]. Nevertheless, they have more efficient access to water, CO2 and other nutrients due to their simpler cellular structure and high specific surface area [12]. A microalgae with high oil productivity is desired for producing biodiesel, depending on the species,
microalgae produce different kinds of lipids, hydrocarbons and complex oils [24]. The species of microalgae with higher oil content includes Chlorella vulgaris, Chlorella protothecoides, Spirulina maxima, Nannochloropsis sp., Neochloris oleabundans, Scenedesmus obliquus and Dunaliella tertiolecta among others [29]. Waste oil, includes residues from deep frying processes, such as soap stocks, yellow and brown greases, obtained from restaurants, hotels and industries. There are enough waste cooking oils and fats generated in the United States annually, to produce about 18 billion liters of biodiesel [6]. In United States the biodiesel made from waste cooking oil is known as McDiesel, because one large source of this oils is McDonald’s restaurants [30]. It has been reported that waste oils still conserve still conserves most of its triglycerides groups, with chemical and physical properties very similar to those of its source oils [31]. The free fatty acid content of waste cooking oils ranges between 10 and 25%, as a result of the frying process where heating in presence of air and light increase the viscosity and specific heat [32]. Waste cooking oils can be used as a fuel, either directly in the engine after filtering or can be transesterified using short-chain alcohol to produce biodiesel and glycerin [33e35]. In biodiesel production process, fats, moisture, proteins, and animal fragments extracted from meats during the frying process are an important component of waste cooking oils affecting the process in terms of additional pretreatment is required [36]. Tallow oil is a rendered from beef fats obtained in slaughterhouses [24]. Human consumption of tallow has a negative effect on health; consequently, these feedstocks are addressed fundamentally to industry. Tallows are used in the manufacture of products such as cosmetics, soaps, shampoo, candles, lubricants, paints, tires, perfumes, textiles, plastics, inks, polishes, cleaners and solvents. Different grades of tallow are produced to meet the varying needs. They are also an important source of fatty acids and glycerol for the chemical industry [37]. However, when its market is overloaded, this oil is incinerated or disposed in landfills [38,39]. Therefore, biodiesel production from these feedstocks has been considered, since they are potential sources for biodiesel due to their high cetane number (typically 56e62), good stability and low price. The drawback of these feedstock is that still requires a pretreatment in order to remove impurities and treat those with high saturated fatty acids content, to avoid yield methyl esters with poor cold temperature properties. 3. Technologies for biodiesel production As most of chemical processes, the biodiesel production can be generalized as three main sequential stages: Pretreatment, reaction and purification. 3.1. Pretreatment In this stage, those elements in feedstock oil, which may have an undesired effect over the transformation reactions, are withdrawn. Particles, colloidal mater, pigments, extraction residues and other impurities can be removed using filtration. When water content (>0.06%) and free fatty acid (FFA) content (>4%) are high, a saponification reaction can be induced, generating a gel soap instead of biodiesel [40]. To avoid this it is necessary to dry the oil first, proceeding then to FFA elimination, using: neutralization or pre-esterification of FFA. Esterification as pretreatment method in biodiesel production, can be combined with transesterification to obtain almost a complete conversion to biodiesel [6]. There are two general methods used for esterification: the batch process and the continuous process. Esterification can be performed under batch operation at a temperature of 200e250 C. As long as, it is an equilibrium reaction, the water must be removed continuously to
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obtain a high-ester yield. The conversion rate is also strongly influenced by the reaction temperature. However, giving enough time, the reaction will also proceed to near completion even at room temperature. Generally, the reaction is conducted close to the methanol boiling point [41]. 3.2. Reaction Biodiesel is mainly produced using transesterification reactions with short-chain alcohols. Nevertheless, interesterification can also be employed using short-chain esters, such as methyl acetate [42]. Transesterification or alcoholysis is a reaction of fatty acid groups from the triglycerides using an alcohol to produce three molecules of alkyl esters and one of glycerol. Low molecular weight alcohols such as methanol, ethanol, propanol and butanol are suitable to be used in biodiesel production; however, methanol is the most extensively used because of its low-cost as well as physical and chemical properties [43]. The function of catalysts on biodiesel production is to modify the kinetics, improving reaction rates and selectivity to the desired products. In this way, the reaction time and loads sent to purification sections are reduced, saving energy and capital costs. Different types of catalysts are employed in biodiesel production (basic, acid, heterogeneous and enzymes). The homogeneous basic catalysts most used in biodiesel industry for transesterification reaction are NaOH, KOH, due to their low market costs, comparative high reaction rates and the mild reaction temperatures required (55e75 C) [44]. However, the use of these catalysts may lead to undesirable reactions if FFA and moisture are not low enough (FFA (>1%) and moisture (<0.06%)) [45]. Industrially, this situation is managed using alkoxides instead of hydroxides such as sodium methoxide, sodium ethoxide, or sodium propoxide, according to the alcohol employed in the transesterification reaction [46]. Homogeneous acid catalysis is employed for transesterification reactions involving oleochemical feedstocks with high free fatty acid content, such as waste cooking oils, yellow and brown greases and animal rendering residues (tallow) [6]. Acid catalysis uses strong acids such HCl and H2SO4. The advantage of acid catalysts is to convert free fatty acids into biodiesel esters through esterification, while simultaneously catalyze the transesterification of triglycerides to biodiesel, all in a single step [47]. However, acid catalysis has comparatively slower reaction rates than basic and requires higher rates of methanol [46]. Heterogeneous catalysts use a different kind of catalytically active solids (MgOeLa2O3, zeolites (ETS-10), KI/Al2O3, TiO2) to produce fatty acid methyl esters. This catalyst is easily recovered and can be regenerated after reaction ends [6]. This feature simplifies and economizes the downstream treatment of the products [46]. Enzymatic catalysis is carried out using lipases which implies mild conditions and a biologically closed cycle (biocatalysts for a bioproduct). Finally, it helps to avoid some problems of conventional methods related to saponification reactions, glycerol recovery, and removal of inorganic salts. Moreover, FFA can be totally esterified. Enzymes typically need small amounts of water for activity and constraints of water in conventional systems may be avoided, thus it is possible to save some energy by the reduction of separation loads. However enzymes are still very expensive to use at large biodiesel production scales [48,49]. 3.3. Separation and purification The reaction mixture obtained after the transesterification process contains alkyl esters, glycerol, non-converted alcohol, catalyst and mono-, di- and tri-glycerides. This mixture must be purified, in order to obtain a high-quality biodiesel according to the
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European standard (EN14214) or United States standard (ASTM D6751) [50,51]. For this reason separation and purification stages are important for final product quality, affecting consequently the operative costs. The main step of conventional biodiesel purification can be performed using two main routes: i) Performing liquide liquid extraction first, recovering alkyl esters in light phase, while glycerin is in heavy phase; then non-converted alcohol is separated [52,53]. ii) Vacuum distillation is first used to separate non-converted alcohol and then liquideliquid extraction to separate glycerin and biodiesel; last step is known as water washing [54,55]. Apparently, first alternative is the most convenient option. Under an initial analysis from the point of view of chemical process design, this route reduces in a first stage the separation load sent to reaming stages. However, this alternative is operationally not convenient as a number of disadvantages are reported in the practice: 1) A low amount of glycerol remains in biodiesel phase, making difficult its further purification 2) The glycerol separation rate is lower due to changes on solubility as a result of the effect of alcohol. 3) Still an additional alcohol recovery stage is needed, increasing operation costs. Conversely, the second alternative can be slightly more expensive because of vacuum distillation technology. However the initial separation of alcohol makes easier further separation phases. After this point, both for homogeneous alkaline and acid processes, catalyst must be neutralized, producing salts that could be removed later by filtration or centrifugation. For instance, on the alkaline case, the neutralization makes easier water washing. But if the basic catalyst is not neutralized an emulsion of non-converted oil-water may be formed, increasing energy consumption of the separation process. After neutralization, the ester-enriched phase is purified, removing residual alcohol, catalyst, neutralization salts, soaps and residual glycerol. Finally, biodiesel is dried using distillation or vacuum flash. The main objective of this final stage is to improve the smell and color of final product. To select a technology for biodiesel production includes determining: a) main process variables, such as: reaction time, phase, catalyst and additional reactants; b) pretreatment-reaction technologies and c) capital and operational costs, as well as energy consumption. An improvement to biodiesel production process is the biomass fired cogeneration. This option allows to generate the process requirements of heat and power using oil extraction residues as fuel [56]. Cogeneration technologies as a way for combined production of mechanical and thermal energy have remarkable cost and energy savings, operating also with higher efficiency compared to systems, which produce heat and power separately. The use of this technology increases the energy utilization improving process economy. Last is achieved given, the reduction of external utilities requirements, while generated electricity can be sold to central grid. Among available technologies for biomass fired cogeneration, the combined-cycle gas turbine (CCGT), is considered as a promising alternative due to its efficiency [57]. The CCGT configurations are composed by the following elements: gas turbine, where chemical energy is converted to mechanical work [56]. Heat steam recovery generator (HSRG), that is a high-efficiency steam boiler that uses hot gases from a gas-turbine or reciprocating engine to generate steam, in a thermodynamic Rankine cycle; generating steam at different pressure levels required by a chemical process. Finally, steam turbine, where steam produced in HSRG system is employed to generate additional power (see Fig. 1). Two process configurations were used in this work. The first option uses basic catalysis (Fig. 2) improved with biomass fired
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Fig. 1. Process flow-sheet diagram for cogeneration system based on extraction cake residues. 1. Direct use heat exchanger, 2. Air divisor, 3. Compressor, 4. Turbine, 5. Dryer, 6. Combustion chamber, 7. Cyclone, 8. High pressure superheater, 9. High pressure evaporator, 10. High pressure economizer, 11. High pressure drum, 12. Low pressure economizer I, 13. Flow divisor, 14. Low pressure economizer II, 15. Intermediate pressure economizer, 16. Intermediate pressure evaporator, 17. Intermediate pressure superheater, 18. Intermediate pressure drum, 19. Flow divisor, 20. Low pressure evaporator, 21. Low pressure superheater, 22. Low pressure drum, 23. Steam turbine, 24. Flow mixer.
cogeneration (Fig. 1), while second employs acid catalysis (Fig. 3) without any cogeneration (waste cooking and tallow oil are not associated to any lignocellulosic residue). Basic catalysis was used for vegetable oils with low free fatty acid content (jatropha, palm and microalgae); whereas acid catalysis was chosen for feedstocks with high free fatty acid content (waste cooking oil, tallow oil). Both options considered vacuum distillation as a first step for biodiesel purification. 4. Methodology The simulations were carried out in Aspen Plus V7.1 (Aspen Technology, Inc., USA). This approach allows obtaining data of mass and energy balances, basic engineering estimations of equipment size and its energy consumption. Feedstocks (palm, jatropha, microalgae, tallow and waste cooking oils) for biodiesel production
were modeled as a sort of pseudocomponents, created to represent triglycerides and methyl esters, according to Chang and Liu’s methodology [10]. Conversely, extraction residues used as a fuel in biomass fired cogeneration, were included into the simulation according to chemical compositions reported by Akintayo, 2004 (jatropha cake) [58], Piarpuzán et al., 2011 (palm cake) [59], and Phukan et al., 2011 (Microalgae paste) [60]. Physicochemical properties for pseudocomponents, were estimated using the Marrero and Gani method [11]. UNIFAC Dortmund for liquid phase, Soave Redlich Kwong with the Boston Mathias modification (RKSBM) for the vapor phase. The water enthalpy was calculated with NBS steam tables. UNIFAC Dortmund, allows to model accurately binary and ternary liquideliquid equilibriums of systems containing vegetable oils, biodiesel, glycerol for which NRTL data are not available. RKS-BM model is recommended for simulations involving complex compounds such as triglycerides. For instance,
Fig. 2. Process flow diagram for biodiesel production using basic catalysis. 1. Neutralization reactor I, 2. Transesterification reactor, 3. Liquideliquid extraction column I, 4. Distillation tower I, 5. Liquideliquid extraction column II, 6. Distillation tower II, 7. Neutralization reactor II, 8. Solid separator, 9. Flash.
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Fig. 3. Process flow diagram for biodiesel production using acid catalysis. 1. Flash I, 2. Pre-esterification reactor, 3. Transesterification reactor, 4. Distillation tower I, 5. Liquideliquid extraction column, 6. Distillation tower II, 7. Neutralization reactor, 8. Solid separator, 9, Flash.
this model has been successfully used to model refinery and petrochemical processes [61]. The kinetic model for acid and basic catalysis, employed in this work, was reported by Granjo et al. [12], as first order and second order expressions, respectively. The functional basis used in this work for the simulation procedures was 1000 kg/h of biodiesel production. Based on this value was calculated a residue flow rate for palm oil cake, jatropha oil cake and microalgae paste; using it as an input for the biomass fired cogeneration system. The economic analysis was performed using Aspen Icarus Process Evaluator package (Aspen Technology, Inc., USA), to calculate a mean cost in US dollar per liter of biodiesel produced with the selected feedstocks. This analysis was performed using the design information provided by Aspen Plus, under the economic conditions of Colombia (annual interest rate of 17% and tax rate of 33%). A straight line depreciation method, at 12 years of the analysis period, was considered. Prices of commercial oils and other reagents (methanol, sodium hydroxide, sulfuric acid, etc.) reported on ICIS pricing were used as a base for the raw materials [62,63]. For microalgae oil as a non-commercial oil, it was employed values reported in literature by Campbell et al. [64] for microalgae production in ponds using flue gas as substrate. Operative charges such as operator and supervisor labor cost were defined as USD 2.14/h and USD 4.29/h, respectively. Electricity, potable water, low and high steam pressure costs were USD 0.03/ kWh, USD 1.25/m3, USD 8.18/ton. The environmental impact was assessed with WAR, waste reduction algorithm (EPA, USA), to estimate the potential environmental impact (PEI) generated in the biodiesel production process. Considering eight environmental impact categories: Human toxicity potential by ingestion (HTPI), Human toxicity potential by exposure both dermal and inhalation (HTPE), Terrestrial toxicity potential (TTP), Aquatic toxicity potential (ATP), Global warming potential (GWP), Ozone depletion
potential (ODP), Photochemical oxidation potential (PCOP), and Acidification potential (AP). The mass flow rate of each component in the process streams is multiplied by its chemical potency; determining its contribution to the potential environmental impact categories [65]. To compare the environmental profiles for all process, total PEI was determined by the sum of all (eight) potential P environmental impact categories as follows: ni¼ 1 ai 4i , where ai is the weighting factor for potential environmental impact category i, and 4i represents the potential environmental impact for category i. In this work all of the weighting factors were set equal to 1 [66,67] 5. Results Results of the simulation are summarized in a table (see Table 1). It was found how productivities for the process catalyzed with NaOH are comparatively higher (1.01 kg of biodiesel/kg of crude oil), than those catalyzed with H2SO4 (0.85e0.95 kg of biodiesel/kg of crude oil). Meanwhile, methanol consumption as well as global energy consumption (heating and electricity) for acid catalyzed processes were 1.58e1.77 kg of methanol/kg of crude oil and heating of 53.39e72.76 MW respectively. These values are higher than methanol and energy required by the basic catalyzed process (0.16e0.21 kg of methanol/kg of crude oil and heating of 24.73e29.96 MW respectively). Results for process residues, demonstrated that the acid catalyzed processes also have a higher production rate for waste water 1.84e2.16 kg of water/kg of crude oil compared to 0.01e0.94 kg of water/kg of crude oil for basic catalyzed process (see Table 2). Simulation results for the biomass fired cogeneration plant using palm oil cake, jatropha oil cake and microalgae paste as fuels (see Table 3), reveal how the heating energy production from jatropha cake (31.03 MW) is higher than the energy produced from palm oil cake (20.39 MW) and a little
Table 1 Feedstocks and process configurations used in this work. Feedstock
Process configuration
Catalyst
Pretreatment
Energy source
Jatropha Palm Microalgae Waste cooking oils Tallow
Basic catalysis þ cogeneration Basic catalysis þ cogeneration Basic catalysis þ cogeneration Acid catalysis
NaOH NaOH NaOH H2SO4
FFA FFA FFA FFA
Cogeneration Cogeneration Cogeneration External source
Acid catalysis
H2SO4
FFA esterification
neutralization neutralization neutralization esterification
External source
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Table 2 Simulation results for biodiesel production using acid and basic catalysis.
Materials (kg/h) Crude oil FFA content (%wt) Impurities content (%wt) Water content (%wt) Methanol NaOH H2O H2SO4 CaO Energy (electricity, Pumping, and recycling Energy (heat, MW) Heating Cooling Products (kg/h) Biodiesel @>99%wt Glycerol @>88%wt Waste water CaSO4 Na2SO4
Palm oil
Jatropha oil
Microalgae oil
Tallow oil
Waste cooking oil
992.59 6% 0%
990.41 4% 0%
985.59 4% 0%
1052.79 15% 3%
1179.76 10% 3%
0%
0%
0%
8%
12%
212.69 9.51 1386.57 21.24 e
204.78 9.15 1379.82 20.45 e
1662.43 e 1315.99 36.85 31.58
2091.06 e 1651.66 13.21 11.33
0.58
0.64
0.46
0.55
26.39 39.15
29.96 25.64
24.73 36.63
72.76 90.66
62.99 80.92
1000.00 112.21 1003.45 e 10.56
1000.00 116.21 933.73 e 16.87
1000.00 125.50 1046.89 e 16.79
1000.00 86.27 2277.22 110.03 e
1000.00 96.82 2167.03 65.20 e
159.02 9.33 1389.63 20.84 e kW) 0.55
lower than the energy from microalgae paste (33.33 MW). Among the three residues considered, microalgae paste cogenerates more electricity (8.22 MW) than jatropha oil cake (7.58 MW) and palm oil cake (4.84 MW). The economic evaluation results for biodiesel production are summarized in Table 4. As significant results, raw material costs, (including costs of all feedstocks used in biodiesel production such as oil, methanol, catalyst and acids) for basic catalyzed processes (USD 0.32e0.50/L) were higher than those for acid catalyzed processes (USD 0.23e0.30/L). Utilities costs for acid catalyzed processes (USD 0.05e0.06/L) were higher than basic catalyzed process (USD 0.02e0.02/L). The remaining costs for both processes were similar. Production costs for basic catalyzed processes (USD 0.39e 0.58/L) were higher than those for acid catalyzed processes (USD 0.23e0.30/L). However, considering potential incomes by electricity selling at average price in Colombia, it can be seen how total
production costs for jatropha, palm and microalgae biodiesels were reduced to USD 0.15/L, USD 0.32/L and USD 0.31/L, respectively. The environmental analysis using the war algorithm for the 5 feedstocks revealed that, the generated emissions as PEI generated per kg of product ranged from 0.04 to 0.09 for basic catalyzed process, whereas for acid catalyzed case 0.02 and 0.06 (see Fig. 4a). In the same way, outlet emissions as PEI per kg of product vary between 0.04 and 0.15 for basic catalyzed process, while for acid catalyzed process the values ranged over 0.05 and 0.31 (see Fig. 4b). 6. Discussion Results from chemical processes simulation, reveal how both basic and acid catalyzed processes have reaction yields ranging 96e 98%. Reaction yield values were calculated based on the defined biodiesel production (1000 kg/h) for all processes and the net oil (after pretreatment) incoming with feedstock, free of FFA and other impurities (jatropha oil 950.79 kg/h, palm oil 933.04 kg/h, microalgal oil 946.16 kg/h, tallow oil 779.07 kg/h, waste cooing oil 884.82 kg/h). The obtained yields for tallow oil (97.7%) and waste cooking oil (98.7%) agree with values ranging over 92e98% reported for acid catalysis using H2SO4 by West et al., 2008 [68] and Van Kasteren et al., 2007 [69]. In the same way obtained yields for jatropha oil (93.95%), palm oil (95.80%) and microalgae (96.07%) agree with the values reported for basic catalyzed process (92e 97%) reported by Glisic et al., 2003 [70] and Haas et al., 2006 [53]. These results indicate how low quality feedstock with high content of FFA and other impurities after an adequate pretreatment can produce high-quality biodiesel (>99 %wt). Cogeneration results gave an advantage to basic catalyzed processes due to the possibility of employing the extraction residues to generate heat and power. Both higher heating and power potential were released by microalgae paste followed by jatropha cake. As it can be seen in Table 3, the high protein and low fiber content of these feedstocks apparently help to increase the calorific value of these residues. As long as, they have comparative higher calorific values and higher available rates, these residues could generate more steam and consequently, produce more heat and power (see Table 3). However, the above results were reflected on capacity of cogeneration systems to cover heating requirements of biodiesel processes. Thus, only microalgae paste and jatropha cake could
Table 3 Cogeneration results for extraction residues based on biomass fired cogeneration system.
Available residue [kg/h] Moisture Protein Oil Fiber Ash Calorific value [MJ/kg] Cogenerated heating Heat fired [MW] Heat direct use [MW] Heat exchangers [MW] Total heating [MW] Cogenerated electricity Pumping [MW] Gas turbine [MW] Steam turbine [MW] Total electricity [MW] a b c
Jatropha oil cakea
Palm oil cakeb
Microalgae oil pastec
3062.33 27.28% 21.30% 27% 18.28% 6.25% 15.62 Consumed
2281.89 30.00% 2.52% 23.48% 41.00% 3.00% 12.20 Consumed
3906.87 6.80% 43.22% 28.82% 9.46% 5.93% 15.88 Consumed
Generated 13.42
1.14
Consumed
7.8 0.978
18.75 31.03 Generated
0.012
Chemical composition from Akintayo, 2004 [58]. Chemical composition from Mata et al., 2010 [51]. Chemical composition from Phukan et al., 2011 [60].
Generated
Consumed
15.42 1.17
13.57 20.39 Generated
0.009 5.59 2.01 7.58
Generated
Consumed
19.07 33.33 Generated
0.011 3.45 1.41 4.84
6.17 2.06 8.22
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Table 4 Economic evaluation results for biodiesel production using basic catalysis. Basic Catalysis
Raw material cost Total utilities cost Operating labor Maintenance Operating charges Plant overhead General and administrative cost Subtotal cost Credit by electricity selling Total cost with cogeneration
Acid Catalysis
Jatropha oil, USD/L
Palm oil, USD/L
Microalgae, USD/L
Waste cooking oil, USD/L
Tallow oil, USD/L
$ $ $ $ $ $ $ $ $ $
$ $ $ $ $ $ $ $ $ $
$ $ $ $ $ $ $ $ $ $
$ $ $ $ $ $ $ $ $ $
$ $ $ $ $ $ $ $ $ $
0.322 0.022 0.008 0.005 0.002 0.006 0.029 0.394 0.249 0.145
0.406 0.021 0.008 0.004 0.002 0.006 0.036 0.482 0.159 0.324
cover 100% of heating requirements of biodiesel processes, while palm oil cake only covered 77.27% of total heating requirements. All extraction residues could cover 100% of electricity requirements of biodiesel processes, generating also an excess of electricity able to be sold to central grid. Production costs for basic catalyzed processes from jatropha oil and palm oil, were lower than values reported on literature by Elbersen et al., 2008 (USD 0.39/L, palm oil) Carriquiry et al., 2011 (USD 0.44e0.72/L, jatropha oil) [71]. This result is explained because the processes simulated in this work use cogeneration to supply process energy and generate an additional income used as credit to reduce total production costs. For biodiesel from microalgae oil, at the moment, there are not commercial values reported in the open literature; however, authors such as Demirbas, 2011 [72] and Campbell et al., 2011 [64] agree that production cost of this oil should be lower than diesel oil. In this sense obtained biodiesel from microalgae oil covers this requirement. In the same way acid catalyzed production costs were lower than values reported by West, 2008 [68] and Zhang, 2003 [31] (USD 0.45e0.54/L). It can be explained due to the low costs considered for these feedstocks. Comparing economic evaluations among the biodiesel production processes, it can be seen how initially basic catalyzed process costs were higher than acid catalyzed and how only after including cogeneration credits this situation change positively. This situation can be explained based on difference of raw material costs (as
0.497 0.022 0.008 0.005 0.002 0.006 0.037 0.577 0.269 0.307
0.146 0.055 0.009 0.002 0.002 0.006 0.008 0.230 0.000 0.230
0.219 0.052 0.009 0.002 0.002 0.006 0.012 0.303 0.000 0.303
major cost). Vegetable oils such as palm oil, jatropha oil and microalgae oil have higher costs, mainly due to extraction process and market competition for other usages, especially in palm oil case. Conversely, waste cooking oil and tallow oil are residues from other industries with a representative content of impurities and FFA. This feature reduces its costs, but also reduces its uniformity and quality as feedstock, requiring the use of acid catalysis for its processing. Acid catalysis as modification for biodiesel production has a higher methanol consumption rate and generates more effluents. This situation drives to processes with a low production cost but potentially contaminant. The environmental analysis results using WAR algorithm revealed how all considered processes and feedstocks have a negative PEI generated per kg of product. This means from the generation point of view that all the alternatives have the potential to generate a mitigation of incoming environmental load by transforming feedstocks into added value products. However, it can be seen in Fig. 4a that the feedstocks processed using basic catalyzed routes have higher mitigation potentials (more negative PEI) compared to acid catalyzed routes. Above statement was confirmed analyzing the environmental impact by outlet PEI per kg of product (see Fig. 4b). In this analysis, it can be seen how all outlet streams contain pollutant elements, which can affect the ecosystem. Therefore, it would be desirable for these streams to have a PEI close to zero, in order to reduce the potential environmental
Fig. 4. PEI analysis for biodiesel production from different feedstocks. a) Generated PEI per mass of product. b) Outlet PEI per mass of product.
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impacts of products. In this case acid catalyzed processes have higher PEI and are less environmentally friendly than those catalyzed with bases. This effect is explained by the fact that sulfuric acid is used as catalyst, increasing the acidification potential (AP) and photochemical oxidation potential (PCOP). Thus, the emitted sulfur materials might acidify surrounding lands and rivers, and increase the formation of SOx, contributing to acid rain formation and global warming. Additionally, palm and jatropha oils had comparatively lower PEIs, with reduced impacts on human toxicity potential by ingestion (HTPI), Terrestrial toxicity potential (TTP), Aquatic toxicity potential (ATP). Consequently, basic catalyzed processes arise as a comparatively cleaner technology, where feedstocks are processed to obtain high-value products in an environmentally friendly way. 7. Conclusions Based on the results, the most convenient configuration for biodiesel production in tropical countries (in the framework of this analysis) employs jatropha oil in a basic catalyzed scheme, integrated to a cogeneration plant where jatropha oil cake is fired. This configuration can produce high rates of biodiesel with the lower production costs, improved by electricity selling. Furthermore, this configuration proves, to be the most environmentally friendly with lesser potential emissions and climate-change effect, as well as reduced land use given its ability to be growth in marginal lands. Microalgae oil is called to be the future biodiesel feedstock, due to its low land requirements and high potential to capture CO2; nevertheless, this feedstock still requires improvements on its production technologies, in order to reduce production costs and increase its economic sustainability. Acknowledgments To the Colombian Institute for Development of Science and Technology (Colciencias) and Universidad Nacional de Colombia at Manizales, at Amazonas and at Orinoquia at for the financial support of this work.
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