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A Novel Combined Ethanol and Power Model of Microgrid Driven by Sweet Sorghum Stalks Using ASSF
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 103 (2016) 244 – 249
Applied Energy Symposium and Forum, REM2016: Renewable Energy Integration with Mini/Microgrid, 19-21 April 2016, Maldives
A novel combined ethanol and power model of microgrid driven by sweet sorghum stalks using ASSF Lei Zhanga, Xu Zua, Jun Fub, Jihong Lia, Shizhong Lia* a
b
Beijing Engineering Research Center for Biofuels, Tsinghua University, Beijing, 100084, China Shandong Electric Power Engineering Consulting Institute Co. ,LTD, No. 106, Minziqian Road, Jinan, Shangdong, 250013, China
Abstract In this study, we proposed a novel combined ethanol and power (CEP) model of microgrid driven by sweet sorghum ethanol using advanced solid-state fermentation (ASSF), during which a great deal of solid vinasse will be generated. In the model, a 10,000-tonne scale ethanol plant with a 2.5 MW biopower plant driving by ASSF technology can support ~4,000 households of remote area for one-year regular electricity use, denoting that economically viable ethanol production is able to provide a steady biomass supply for combined heat and power generation. Accordingly, the novel CEP microgrid model exhibits much more robust and stable traits compared with conventional microgrids. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-reviewofunder responsibility of REM2016 Peer-review under responsibility the scientific committee of the Applied Energy Symposium and Forum, REM2016: Renewable Energy Integration with Mini/Microgrid. Keywords: Microgrid, sweet sorghum, advanced solid-state fermentation, ethanol, biomass power generation
1. Introduction Microgrids, a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that act as small-version and controllable local grids of centralized electricity systems has attracted substantial attention due to the achievements of the specific local goals, such as reliability, resilience to grid outages, interoperable relationship with existing grid, and carbon emission reduction [1]. The most pivotal trait of microgrids is the diversification of energy sources, which is an ideal way to integrate renewable energies on the community level [2].
* Corresponding author. Tel.:+86 10 89796023; fax: +86 10 89794050. E-mail address:[email protected]
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the Applied Energy Symposium and Forum, REM2016: Renewable Energy Integration with Mini/Microgrid. doi:10.1016/j.egypro.2016.11.280
Lei Zhang et al. / Energy Procedia 103 (2016) 244 – 249
Nomenclature CEP CHP ASSF SSE LSF SSF
combined ethanol and power combined heat and power advanced solid-state fermentation sweet sorghum ethanol liquid-state fermentation solid-state fermentation
Generally, renewable energies powering microgrids come mainly from solar, wind, and biomass power. Solar and wind powers are collected for electricity generation with low maintenance, pollution, and fuel costs, therefore becoming attractive renewable energy sources for power generation. However, both of solar and wind powers determined by climate, season, and geographic location are variable and nondispatchable, leading to a limited application scope of them [3, 4]. Meanwhile, combined heat and power (CHP) from biomass is recognized to be the most energy efficient strategy of transforming energy from biomass into electric power [4, 5]. Unfortunately, few cases adopting CHP demonstrate economically viable so far due to lack of a steady supply of available biomass feedstock, and high cost of collection, transportation, and transportation fee of biomass feedstock [2]. Recently, we have developed a novel process for ethanol production, advanced solid-state fermentation (ASSF), which exhibits a higher cost-effective and lower overall environmental footprint compared with conventional liquid-state fermentation (LSF), due to its simpler process configuration and fewer sewage disposal [6]. Over the past several years, ASSF has been successfully scaled up from bench scale to industrial scale [7, 8]. In 2015 an ASSF-driven ethanol plant with two sets of 555 m3 rotary drum fermenter has been constructed, which is the largest continuous solid-state fermenter used for ethanol production in the world. The largest amount of byproduct of ASSF sweet sorghum ethanol (SSE) is solid vinasse, which has been proven to be a type of excellent solid fuel due to relatively high content of component. In this study, we proposed a novel combined ethanol and power (CEP) contributing model of microgrid driven by SSE using ASSF. In the model, economically viable ethanol production is able to provide a steady biomass supply for CHP generation. The excess heat and power can be divided into two portions, one for community supply, and the other for ethanol plant running. Most importantly, ASSF driven SSE plant should be deployed as the core component of the microgrid, which decreased the costs of biomass handling and transportation significantly compared to the regular biomass powered community. Basically, the model presents a microgrid that ASSF driven SSE plant as a dominant power pivot provides fuel ethanol and solid fuel for CHP generation, and meanwhile, solar and wind power are introduced into the system as auxiliary renewable energy. Therefore, the novel microgrid model would exhibit much more robust and stable traits compared with conventional microgrids. The paper is organized as follows. In Section 2 the great potential of SSE is presented, in which the characteristics of sweet sorghum and ASSF is described, and the economic analysis of CEP model for SSE using ASSF is analyzed. In Section 3 and 4 the CEP contributing model of microgrid driven by ASSF SSE is proposed and defined. At the end of the paper, the conclusion is presented in Section 5. 2. ASSF-driven SSE, a promising biofuel model 2.1 Sweet sorghum: an energy crop holding unmatched versatility for bioenergy applications Sweet sorghum has received increasing attention worldwide because it holds unmatched versatility for bioenergy applications, which is the only energy crop platform that can provide starch, sugar and lignocelluloses [9]. As a sugarcane-like energy crop, sweet sorghum can accumulate high levels of fermentable sugars in its stalk up to 18%, which could be directly fermented into ethanol by yeast [10]. Moreover, grain from sweet sorghum can also be used as a source of starch for ethanol production, and several commercial ethanol plants located in the sorghum production regions of the USA rely on sorghum as their primary starch source [11]. Sweet sorghum bagasse has the potential to be cellulosic feedstock as well. Most importantly, compared to the other sugar based feedstocks, sweet sorghum is capable of growing across a wide range of climates from the tropics to cool temperate zones, exhibiting strong tolerance to drought, waterlogging, salinity, and alkalinity [12], while sugarcane can only survive from an
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optimal temperature of 30-34 oC and 1000-1500 mm of rainfall a season on the loamy arable land, with 10-13 months of growth period. As a result, the Brazilian government is starting to encourage growers to adopt sweet sorghum [13], though its sugarcane ethanol is considered as the most successful alternative fuel model to date. Moreover, Therefore, sweet sorghum is considered as the priority energy crop for development to diversify the source of feedstocks for biofuels production [14, 15]. 2.2 ASSF industrializes SSE The conventional sugarcane ethanol process is based on LSF, which requires an energy-intensive process for juice squeezing, consequently resulting in significant issue of wastewater disposal and tremendous energy input. Unlike sugarcane, there is spongy-like pith existing within the stalk of sweet sorghum [16], thereby leading to much more energy consumption for the juice squeezing process [17]. Accordingly, SSE cannot benefit largely from traditional sugarcane ethanol industry. Compared with LSF, SSF has the advantage that can convert fermentable sugars directly to the target products without any pressing process. Therefore, a large portion of the energy and water is saved, wastewater is reduced and sugar utilization is increased [18]. However, during the SSF process, the absence of free water leads to poor heat removal, and it is not easy to mix solid particles well. The control of mass and heat transfer is a major challenge in the design and operation of large-scale solid-state fermenters. Other disadvantages of SSF involved difficulty in agitation of the high-viscosity substrate, difficulty in efficient solid handling especially for continuous operations and limited types of microorganisms that can grow in low moisture level [18]. Due to lack of engineering data and knowledges about the design and scale-up of solid-state fermenter, the SSF has not yet been proved to be feasible in large-scale production up to the present. To address these challenges, we developed the ASSF process using non-food and high potential sugarbased sweet sorghum for fuel ethanol production [12], which largely circumvents fundamental constraints of SSF, such as poor mass and heat transfer and low fermentation rate, with a redesigned rotary drum fermenter and a proprietary yeast strain [6, 8, 19]. We first corroborated the feasibility of the application of SSF and economically viable utilization of sweet sorghum at industrial scale in the world [7], making ASSF-driven SSE be a promising biofuel model worldwide. 3. Economic analysis of CEP model for SSE Full utilization of the products of sweet sorghum can make SSE more cost-effective. Therefore, to further reduce the cost and meet different demands, we devised two models for SSE production using the ASSF technology. For the areas where power is not in urgent demand, such as the United States and the European Union, Model 1 should be adopted, in which the vinasse would be processed for animal feeding. For the areas where lack fuel or power, such as Indonesia and Malaysia, the vinasse would be processed for power and heat generation using biomass boiler, named model 2. In this paper, the CEP model, model 2, is described in detail. It should be noted that the following investment and financial data based upon one crop per year is from the Chinese market and labor cost. In this model, 2000 hectares of sweet sorghum can produce 10,000 tonnes of ethanol and the residue of the distillation unit can supply 9 million kWh to the national grid from a 2.5 MW biopower plant. The minimum ethanol sell price (MESP) of the CEP model is estimated at US$652/tonne ethanol (US$1.98/gallon) at the sorghum stalk cost of US$25/tonne; the power generation cost is 6 cents/kwh; the capital cost is around US$22-24 million for the ethanol plant with a capacity of 10,000 tonnes/year affiliated with a 2.5 MW biopower plant. On the other hand, due to the short crop duration of only 90-110 days, sweet sorghum can grow 3 seasons a year in tropical area, especially for the countries like Indonesia, of which two thirds are tropical rainforest climate [20]. Accordingly, high biomass and short crop duration of sweet sorghum ensure year-round ethanol production, thereby providing a stable feedstock supply for ethanol and power generation. Moreover, SSE driven by ASSF showed obviously higher energy balance at 2.5 than that of the other feedstocks but sugarcane. Nonetheless, taking into account the highest productivity of sweet sorghum due to more crops per year compared with sugarcane, combined with much less water requirement and tolerance to drought, sweet sorghum has great potential to be the principal energy crop worldwide. It should be noted that the energy balance of sweet sorghum ethanol would be increased up to 22 if the vinasse was made into pelletizing fuel for energy recovery.
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4. Evaluation of the microgrid contributed by CEP model of ASSF SSE for Indonesia 4.1 Electricity demand in Indonesia Currently, more than 130 million people in Southeast Asia, or over one-fifth of the population, still lack access to electricity. And almost half of the region’s population still relies on traditional use of biomass for cooking, which poses a serious risk of premature deaths from indoor air pollution [21]. Indonesia is the largest energy consumer in Southeast Asia. As the largest and most populous archipelago in the world, however, it is a huge challenge for Indonesia providing full modern energy access. Currently, the 27% of Indonesians lack access to electricity, which partly explains its low level of per-capita energy consumption [22]. To overcome the electricity problem, the Indonesian government allowed independent power producers to produce and sell electricity to state-owned company since 2009 [23]. As mentioned above, SSE using ASSF manifests a great potential for economically viable and much broader application worldwide compared to any other biofuel model, due to the supreme agronomic traits of sweet sorghum and the advanced ASSF technology. For the countries with tropical/tropical rainforest climate, like Southeast Asia, sweet sorghum can grows three seasons [20], which denotes year-round ethanol production would entail an adequate supply of the solid residues for power generation. 4.2 Microgrid configuration Table 1. The households electricity usage supported by the vinasse fuel for power generation based upon a 10,000-tonne SSE plant Vinasse (tonnes/yr, dry base)
Heat value (kcal/kg, dry base)
Electricity (kWh/yr)
Household electricity usage (kWh/day)
Households support/yr
27,520 3,900a 37,440,000b,c 15d 3,829 The heat value of sweet sorghum vinasse is 3900 kcal/kg, which is higher than 3500 kcal/kg of corn stover. b The conversion ratio of biomass heat value to electricity is 0.3. c The 56% of total electricity from solid vinasse is used to support households of remote area, and the rest is for self-sufficiency of ethanol and power production. d The data source is adopted from previous report [24], which is based on Jogjakarta, Indonesia. a
Rated power MW 2.5
Table 2. Basic parameters of the biomass power unit Voltage class Rated current kV A 0.4 2706
Power factor 0.8
In this study, the CEP model is presumed to be applied in remote area in Indonesia based upon a 10,000-tonne scale ethanol plant driving by ASSF technology, which can theoretically supply ~30,000 tonne of dry-base solid vinasse per year for power generation (table 1). Therefore, a 2.5 MW biomass boiler will be equipped for power generation. In the model, the biomass power generation is used as the main power supply. Based on the ASSF ethanol production line with sweet sorghum as the feedstocks, the vinasse is used as the main fuels for power generation, while other agricultural wastes, such as corn stalk and wheat straw, are used as the auxiliary fuels. During the power generation, water is heated by a 1500 kW biomass boiler unit (table 2) to generate steam based on a steam turbine which drives a generator unit to generate power. The model is expected to supply a portion of electric energy for self-sufficiency of ethanol and power production, while the rest can be provided to ~4,000 households of the island or remote area for one-year regular use. It should be noted that 15 kWh of daily electricity household usage is adopted from the previous report [24] in the model, which bases upon the city of Jogjakarta, a city of southern Java, the major cultural center of Indonesia. If reside in remote area, the daily electricity household usage should be 5-8 kWh, which means the CEP model could support ~7,000-11,000 households for one-year regular electricity use. According to the study results of the CEP technology, the service power of a biomass unit accounts for about 15% of the unit capacity, the power consumption of the ethanol production line accounts for about 29%, and the remained electric energy is about 56%. The remained electric energy will be supplied to the local residents as their living electricity consumption through the transmission line. Before starting the
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biomass generator unit to operate, start the diesel generator so as to supply power to the CEP production. When the system operates properly, the system starts the biomass generator unit, and then uses the synchronizing device to check the synchronizing signals. After the grid-connection conditions are met, achieve the grid-connection operation of the generator unit and the micro-grid. The details of the main connection are presented in fig. 1.
Fig. 1. The schematic diagram of the details of the main connection of CEP
4.3 Control system of microgrid In this study, the model is presumed to be applied on an islet and its main operation mode is independent operation. It is mainly used for regulating the output of the distributed power supply and the power consumption of each load. The centralized controller of microgrid is a set of embedded microgrid host, and also used as the operator station. It can comprehensively monitor the operation of all primary equipment of the whole microgrid, achieve the switchover between the grid-connection mode and independent mode, analyze the operation of the microgrid in real time, optimize the whole microgrid, properly adjust strategies, and rapidly implement such strategies automatically. Meanwhile, it can also be used as the database server. Therefore, it is a key component for the energy management system of the microgrid. 5. Conclusions The ASSF technology first demonstrates that SSF can be used for mass production of fuel ethanol, entailing sweet sorghum much more competitive as a supreme energy crop. ASSF driven SSE has much broader application scope worldwide compared to any other biofuel model. In the country like Indonesia with tropical/tropical rain forest climate, sweet sorghum can grow 3 seasons a year, which ensure the stable generation of ethanol and solid vinasse. Consequently, the CEP model driven microgrid would be fully supported by the adequate feedstock mainly from SSE production. In this study, ethanol plant is presumed to be deployed as the core component of the microgrid, thereby decreasing the costs of biomass handling and transportation significantly compared to the regular biomass powered community. Therefore, the CEP model is a promising solution of electricity access for the residences of remote area. Furthermore, the CEP model can create employments and develop rural economy with a low environmental footprint, especially for the archipelago country, Indonesia, with lack access to electricity. References [1] Fathima AH, Palanisamy K. Optimization in microgrids with hybrid energy systems – A review. Renewable and Sustainable Energy Reviews. 2015;45:431-46. [2] Gregg Tomberlin GM. Feasibility Study of Economics and Performance of Biomass Power Generation at the Former Farmland Industries Site in Lawrence, Kansas. National Renewable Energy Laboratory; 2013. [3] Halamay DA, Brekken TK, Simmons A, McArthur S. Reserve requirement impacts of large-scale integration of wind, solar, and ocean wave power generation. Sustainable Energy, IEEE Transactions on. 2011;2:321-8. [4] IRENA. Biomass for power generation. 2012. [5] McKendry P. Energy production from biomass (part 1): overview of biomass. Bioresource technology. 2002;83:37-46. [6] Wang E-Q, Li S-Z, Tao L, Geng X, Li T-C. Modeling of rotating drum bioreactor for anaerobic solid-state fermentation. Applied Energy. 2010;87:2839-45.
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[7] Li S, Li G, Zhang L, Zhou Z, Han B, Hou W, et al. A demonstration study of ethanol production from sweet sorghum stems with advanced solid state fermentation technology. Applied Energy. 2013;102:260-5. [8] Du R, Yan J, Feng Q, Li P, Zhang L, Chang S, et al. A novel wild-type Saccharomyces cerevisiae strain TSH1 in scaling-up of solid-state fermentation of ethanol from sweet sorghum stalks. PloS one. 2014;9:e94480. [9] Zegada-Lizarazu W, Monti A. Are we ready to cultivate sweet sorghum as a bioenergy feedstock? A review on field management practices. Biomass and Bioenergy. 2012. [10] Wu X, Staggenborg S, Propheter JL, Rooney WL, Yu J, Wang D. Features of sweet sorghum juice and their performance in ethanol fermentation. Industrial Crops and Products. 2010;31:164-70. [11] RFA. Changing the climate, ethanol industry outlook 2008. ETHANOL INDUSTRY OUTLOOK. Washington, DC: Renewable fuel association; 2008. [12] Li S-Z, Chan-Halbrendt C. Ethanol production in (the) People’s Republic of China: Potential and technologies Applied Energy. 2009;86:162-9. [13] Moreno A. Sorghum ethanol production could reach 300 million liters in 2013. 2012. [14] Zhang C, Xie G, Li S, Ge L, He T. The productive potentials of sweet sorghum ethanol in China. Applied Energy. 2010;87:2360-8. [15] Nasidi M, Akunna J, Deeni Y, Blackwood D, Walker G. Bioethanol in Nigeria: comparative analysis of sugarcane and sweet sorghum as feedstock sources. Energy Environ Sci. 2010;3:1447-57. [16] Billa E, Koullas DP, Monties B, Koukios EG. Structure and composition of sweet sorghum stalk components. Industrial Crops and Products. 1997;6:297-302. [17] O'Hara IM, Research ARI, Corporation D, Research RI, Corporation D. Sweet Sorghum: Opportunities for a New, Renewable Fuel and Food Industry in Australia. Rural Industries Research and Development Corporation.; 2013. [18] Krishna C. Solid-state fermentation systems-an overview. Critical Reviews in Biotechnology. 2005;25:1-30. [19] Zhou Z, Li J, Zhou J, Li S, Feng J. Enhancing mixing of cohesive particles by baffles in a rotary drum. Particuology. 2015. [20] Aken JPv. Pilot project bio-ethanol from sweet sorghum. Netherlands: Widjajatunggal Sejahtera; 2014. [21] Southeast Asia energy outlook. World Energy Outlook Special Report: IEA; 2013. [22] The Indonesian electricity system - a brief overview. DIFFER GROUP; 2012. [23] Indonesia-Investments. Electricity in Indonesia: Plenty Natural Resources but Shortage of Electricity. Indonesia-Investments; 2014. [24] Wijaya ME, Tezuka T. A comparative study of households' electricity consumption characteristics in Indonesia: A technosocioeconomic analysis. Energy Sustain Dev. 2013;17:596-604.
Biography Dr. Shizhong Li is a full professor and deputy director of the Institute of New Energy Technology, Tsinghua University, Adjunct Professor of Hong Kong University of Science & Technology. He is also the executive director of MOSTUSDA Joint Research Center for Biofuels, the director of Beijing Engineering Research Center for Biofuels
249
ScienceDirect Energy Procedia 103 (2016) 244 – 249
Applied Energy Symposium and Forum, REM2016: Renewable Energy Integration with Mini/Microgrid, 19-21 April 2016, Maldives
A novel combined ethanol and power model of microgrid driven by sweet sorghum stalks using ASSF Lei Zhanga, Xu Zua, Jun Fub, Jihong Lia, Shizhong Lia* a
b
Beijing Engineering Research Center for Biofuels, Tsinghua University, Beijing, 100084, China Shandong Electric Power Engineering Consulting Institute Co. ,LTD, No. 106, Minziqian Road, Jinan, Shangdong, 250013, China
Abstract In this study, we proposed a novel combined ethanol and power (CEP) model of microgrid driven by sweet sorghum ethanol using advanced solid-state fermentation (ASSF), during which a great deal of solid vinasse will be generated. In the model, a 10,000-tonne scale ethanol plant with a 2.5 MW biopower plant driving by ASSF technology can support ~4,000 households of remote area for one-year regular electricity use, denoting that economically viable ethanol production is able to provide a steady biomass supply for combined heat and power generation. Accordingly, the novel CEP microgrid model exhibits much more robust and stable traits compared with conventional microgrids. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-reviewofunder responsibility of REM2016 Peer-review under responsibility the scientific committee of the Applied Energy Symposium and Forum, REM2016: Renewable Energy Integration with Mini/Microgrid. Keywords: Microgrid, sweet sorghum, advanced solid-state fermentation, ethanol, biomass power generation
1. Introduction Microgrids, a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that act as small-version and controllable local grids of centralized electricity systems has attracted substantial attention due to the achievements of the specific local goals, such as reliability, resilience to grid outages, interoperable relationship with existing grid, and carbon emission reduction [1]. The most pivotal trait of microgrids is the diversification of energy sources, which is an ideal way to integrate renewable energies on the community level [2].
* Corresponding author. Tel.:+86 10 89796023; fax: +86 10 89794050. E-mail address:[email protected]
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the Applied Energy Symposium and Forum, REM2016: Renewable Energy Integration with Mini/Microgrid. doi:10.1016/j.egypro.2016.11.280
Lei Zhang et al. / Energy Procedia 103 (2016) 244 – 249
Nomenclature CEP CHP ASSF SSE LSF SSF
combined ethanol and power combined heat and power advanced solid-state fermentation sweet sorghum ethanol liquid-state fermentation solid-state fermentation
Generally, renewable energies powering microgrids come mainly from solar, wind, and biomass power. Solar and wind powers are collected for electricity generation with low maintenance, pollution, and fuel costs, therefore becoming attractive renewable energy sources for power generation. However, both of solar and wind powers determined by climate, season, and geographic location are variable and nondispatchable, leading to a limited application scope of them [3, 4]. Meanwhile, combined heat and power (CHP) from biomass is recognized to be the most energy efficient strategy of transforming energy from biomass into electric power [4, 5]. Unfortunately, few cases adopting CHP demonstrate economically viable so far due to lack of a steady supply of available biomass feedstock, and high cost of collection, transportation, and transportation fee of biomass feedstock [2]. Recently, we have developed a novel process for ethanol production, advanced solid-state fermentation (ASSF), which exhibits a higher cost-effective and lower overall environmental footprint compared with conventional liquid-state fermentation (LSF), due to its simpler process configuration and fewer sewage disposal [6]. Over the past several years, ASSF has been successfully scaled up from bench scale to industrial scale [7, 8]. In 2015 an ASSF-driven ethanol plant with two sets of 555 m3 rotary drum fermenter has been constructed, which is the largest continuous solid-state fermenter used for ethanol production in the world. The largest amount of byproduct of ASSF sweet sorghum ethanol (SSE) is solid vinasse, which has been proven to be a type of excellent solid fuel due to relatively high content of component. In this study, we proposed a novel combined ethanol and power (CEP) contributing model of microgrid driven by SSE using ASSF. In the model, economically viable ethanol production is able to provide a steady biomass supply for CHP generation. The excess heat and power can be divided into two portions, one for community supply, and the other for ethanol plant running. Most importantly, ASSF driven SSE plant should be deployed as the core component of the microgrid, which decreased the costs of biomass handling and transportation significantly compared to the regular biomass powered community. Basically, the model presents a microgrid that ASSF driven SSE plant as a dominant power pivot provides fuel ethanol and solid fuel for CHP generation, and meanwhile, solar and wind power are introduced into the system as auxiliary renewable energy. Therefore, the novel microgrid model would exhibit much more robust and stable traits compared with conventional microgrids. The paper is organized as follows. In Section 2 the great potential of SSE is presented, in which the characteristics of sweet sorghum and ASSF is described, and the economic analysis of CEP model for SSE using ASSF is analyzed. In Section 3 and 4 the CEP contributing model of microgrid driven by ASSF SSE is proposed and defined. At the end of the paper, the conclusion is presented in Section 5. 2. ASSF-driven SSE, a promising biofuel model 2.1 Sweet sorghum: an energy crop holding unmatched versatility for bioenergy applications Sweet sorghum has received increasing attention worldwide because it holds unmatched versatility for bioenergy applications, which is the only energy crop platform that can provide starch, sugar and lignocelluloses [9]. As a sugarcane-like energy crop, sweet sorghum can accumulate high levels of fermentable sugars in its stalk up to 18%, which could be directly fermented into ethanol by yeast [10]. Moreover, grain from sweet sorghum can also be used as a source of starch for ethanol production, and several commercial ethanol plants located in the sorghum production regions of the USA rely on sorghum as their primary starch source [11]. Sweet sorghum bagasse has the potential to be cellulosic feedstock as well. Most importantly, compared to the other sugar based feedstocks, sweet sorghum is capable of growing across a wide range of climates from the tropics to cool temperate zones, exhibiting strong tolerance to drought, waterlogging, salinity, and alkalinity [12], while sugarcane can only survive from an
245
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Lei Zhang et al. / Energy Procedia 103 (2016) 244 – 249
optimal temperature of 30-34 oC and 1000-1500 mm of rainfall a season on the loamy arable land, with 10-13 months of growth period. As a result, the Brazilian government is starting to encourage growers to adopt sweet sorghum [13], though its sugarcane ethanol is considered as the most successful alternative fuel model to date. Moreover, Therefore, sweet sorghum is considered as the priority energy crop for development to diversify the source of feedstocks for biofuels production [14, 15]. 2.2 ASSF industrializes SSE The conventional sugarcane ethanol process is based on LSF, which requires an energy-intensive process for juice squeezing, consequently resulting in significant issue of wastewater disposal and tremendous energy input. Unlike sugarcane, there is spongy-like pith existing within the stalk of sweet sorghum [16], thereby leading to much more energy consumption for the juice squeezing process [17]. Accordingly, SSE cannot benefit largely from traditional sugarcane ethanol industry. Compared with LSF, SSF has the advantage that can convert fermentable sugars directly to the target products without any pressing process. Therefore, a large portion of the energy and water is saved, wastewater is reduced and sugar utilization is increased [18]. However, during the SSF process, the absence of free water leads to poor heat removal, and it is not easy to mix solid particles well. The control of mass and heat transfer is a major challenge in the design and operation of large-scale solid-state fermenters. Other disadvantages of SSF involved difficulty in agitation of the high-viscosity substrate, difficulty in efficient solid handling especially for continuous operations and limited types of microorganisms that can grow in low moisture level [18]. Due to lack of engineering data and knowledges about the design and scale-up of solid-state fermenter, the SSF has not yet been proved to be feasible in large-scale production up to the present. To address these challenges, we developed the ASSF process using non-food and high potential sugarbased sweet sorghum for fuel ethanol production [12], which largely circumvents fundamental constraints of SSF, such as poor mass and heat transfer and low fermentation rate, with a redesigned rotary drum fermenter and a proprietary yeast strain [6, 8, 19]. We first corroborated the feasibility of the application of SSF and economically viable utilization of sweet sorghum at industrial scale in the world [7], making ASSF-driven SSE be a promising biofuel model worldwide. 3. Economic analysis of CEP model for SSE Full utilization of the products of sweet sorghum can make SSE more cost-effective. Therefore, to further reduce the cost and meet different demands, we devised two models for SSE production using the ASSF technology. For the areas where power is not in urgent demand, such as the United States and the European Union, Model 1 should be adopted, in which the vinasse would be processed for animal feeding. For the areas where lack fuel or power, such as Indonesia and Malaysia, the vinasse would be processed for power and heat generation using biomass boiler, named model 2. In this paper, the CEP model, model 2, is described in detail. It should be noted that the following investment and financial data based upon one crop per year is from the Chinese market and labor cost. In this model, 2000 hectares of sweet sorghum can produce 10,000 tonnes of ethanol and the residue of the distillation unit can supply 9 million kWh to the national grid from a 2.5 MW biopower plant. The minimum ethanol sell price (MESP) of the CEP model is estimated at US$652/tonne ethanol (US$1.98/gallon) at the sorghum stalk cost of US$25/tonne; the power generation cost is 6 cents/kwh; the capital cost is around US$22-24 million for the ethanol plant with a capacity of 10,000 tonnes/year affiliated with a 2.5 MW biopower plant. On the other hand, due to the short crop duration of only 90-110 days, sweet sorghum can grow 3 seasons a year in tropical area, especially for the countries like Indonesia, of which two thirds are tropical rainforest climate [20]. Accordingly, high biomass and short crop duration of sweet sorghum ensure year-round ethanol production, thereby providing a stable feedstock supply for ethanol and power generation. Moreover, SSE driven by ASSF showed obviously higher energy balance at 2.5 than that of the other feedstocks but sugarcane. Nonetheless, taking into account the highest productivity of sweet sorghum due to more crops per year compared with sugarcane, combined with much less water requirement and tolerance to drought, sweet sorghum has great potential to be the principal energy crop worldwide. It should be noted that the energy balance of sweet sorghum ethanol would be increased up to 22 if the vinasse was made into pelletizing fuel for energy recovery.
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4. Evaluation of the microgrid contributed by CEP model of ASSF SSE for Indonesia 4.1 Electricity demand in Indonesia Currently, more than 130 million people in Southeast Asia, or over one-fifth of the population, still lack access to electricity. And almost half of the region’s population still relies on traditional use of biomass for cooking, which poses a serious risk of premature deaths from indoor air pollution [21]. Indonesia is the largest energy consumer in Southeast Asia. As the largest and most populous archipelago in the world, however, it is a huge challenge for Indonesia providing full modern energy access. Currently, the 27% of Indonesians lack access to electricity, which partly explains its low level of per-capita energy consumption [22]. To overcome the electricity problem, the Indonesian government allowed independent power producers to produce and sell electricity to state-owned company since 2009 [23]. As mentioned above, SSE using ASSF manifests a great potential for economically viable and much broader application worldwide compared to any other biofuel model, due to the supreme agronomic traits of sweet sorghum and the advanced ASSF technology. For the countries with tropical/tropical rainforest climate, like Southeast Asia, sweet sorghum can grows three seasons [20], which denotes year-round ethanol production would entail an adequate supply of the solid residues for power generation. 4.2 Microgrid configuration Table 1. The households electricity usage supported by the vinasse fuel for power generation based upon a 10,000-tonne SSE plant Vinasse (tonnes/yr, dry base)
Heat value (kcal/kg, dry base)
Electricity (kWh/yr)
Household electricity usage (kWh/day)
Households support/yr
27,520 3,900a 37,440,000b,c 15d 3,829 The heat value of sweet sorghum vinasse is 3900 kcal/kg, which is higher than 3500 kcal/kg of corn stover. b The conversion ratio of biomass heat value to electricity is 0.3. c The 56% of total electricity from solid vinasse is used to support households of remote area, and the rest is for self-sufficiency of ethanol and power production. d The data source is adopted from previous report [24], which is based on Jogjakarta, Indonesia. a
Rated power MW 2.5
Table 2. Basic parameters of the biomass power unit Voltage class Rated current kV A 0.4 2706
Power factor 0.8
In this study, the CEP model is presumed to be applied in remote area in Indonesia based upon a 10,000-tonne scale ethanol plant driving by ASSF technology, which can theoretically supply ~30,000 tonne of dry-base solid vinasse per year for power generation (table 1). Therefore, a 2.5 MW biomass boiler will be equipped for power generation. In the model, the biomass power generation is used as the main power supply. Based on the ASSF ethanol production line with sweet sorghum as the feedstocks, the vinasse is used as the main fuels for power generation, while other agricultural wastes, such as corn stalk and wheat straw, are used as the auxiliary fuels. During the power generation, water is heated by a 1500 kW biomass boiler unit (table 2) to generate steam based on a steam turbine which drives a generator unit to generate power. The model is expected to supply a portion of electric energy for self-sufficiency of ethanol and power production, while the rest can be provided to ~4,000 households of the island or remote area for one-year regular use. It should be noted that 15 kWh of daily electricity household usage is adopted from the previous report [24] in the model, which bases upon the city of Jogjakarta, a city of southern Java, the major cultural center of Indonesia. If reside in remote area, the daily electricity household usage should be 5-8 kWh, which means the CEP model could support ~7,000-11,000 households for one-year regular electricity use. According to the study results of the CEP technology, the service power of a biomass unit accounts for about 15% of the unit capacity, the power consumption of the ethanol production line accounts for about 29%, and the remained electric energy is about 56%. The remained electric energy will be supplied to the local residents as their living electricity consumption through the transmission line. Before starting the
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biomass generator unit to operate, start the diesel generator so as to supply power to the CEP production. When the system operates properly, the system starts the biomass generator unit, and then uses the synchronizing device to check the synchronizing signals. After the grid-connection conditions are met, achieve the grid-connection operation of the generator unit and the micro-grid. The details of the main connection are presented in fig. 1.
Fig. 1. The schematic diagram of the details of the main connection of CEP
4.3 Control system of microgrid In this study, the model is presumed to be applied on an islet and its main operation mode is independent operation. It is mainly used for regulating the output of the distributed power supply and the power consumption of each load. The centralized controller of microgrid is a set of embedded microgrid host, and also used as the operator station. It can comprehensively monitor the operation of all primary equipment of the whole microgrid, achieve the switchover between the grid-connection mode and independent mode, analyze the operation of the microgrid in real time, optimize the whole microgrid, properly adjust strategies, and rapidly implement such strategies automatically. Meanwhile, it can also be used as the database server. Therefore, it is a key component for the energy management system of the microgrid. 5. Conclusions The ASSF technology first demonstrates that SSF can be used for mass production of fuel ethanol, entailing sweet sorghum much more competitive as a supreme energy crop. ASSF driven SSE has much broader application scope worldwide compared to any other biofuel model. In the country like Indonesia with tropical/tropical rain forest climate, sweet sorghum can grow 3 seasons a year, which ensure the stable generation of ethanol and solid vinasse. Consequently, the CEP model driven microgrid would be fully supported by the adequate feedstock mainly from SSE production. In this study, ethanol plant is presumed to be deployed as the core component of the microgrid, thereby decreasing the costs of biomass handling and transportation significantly compared to the regular biomass powered community. Therefore, the CEP model is a promising solution of electricity access for the residences of remote area. Furthermore, the CEP model can create employments and develop rural economy with a low environmental footprint, especially for the archipelago country, Indonesia, with lack access to electricity. References [1] Fathima AH, Palanisamy K. Optimization in microgrids with hybrid energy systems – A review. Renewable and Sustainable Energy Reviews. 2015;45:431-46. [2] Gregg Tomberlin GM. Feasibility Study of Economics and Performance of Biomass Power Generation at the Former Farmland Industries Site in Lawrence, Kansas. National Renewable Energy Laboratory; 2013. [3] Halamay DA, Brekken TK, Simmons A, McArthur S. Reserve requirement impacts of large-scale integration of wind, solar, and ocean wave power generation. Sustainable Energy, IEEE Transactions on. 2011;2:321-8. [4] IRENA. Biomass for power generation. 2012. [5] McKendry P. Energy production from biomass (part 1): overview of biomass. Bioresource technology. 2002;83:37-46. [6] Wang E-Q, Li S-Z, Tao L, Geng X, Li T-C. Modeling of rotating drum bioreactor for anaerobic solid-state fermentation. Applied Energy. 2010;87:2839-45.
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[7] Li S, Li G, Zhang L, Zhou Z, Han B, Hou W, et al. A demonstration study of ethanol production from sweet sorghum stems with advanced solid state fermentation technology. Applied Energy. 2013;102:260-5. [8] Du R, Yan J, Feng Q, Li P, Zhang L, Chang S, et al. A novel wild-type Saccharomyces cerevisiae strain TSH1 in scaling-up of solid-state fermentation of ethanol from sweet sorghum stalks. PloS one. 2014;9:e94480. [9] Zegada-Lizarazu W, Monti A. Are we ready to cultivate sweet sorghum as a bioenergy feedstock? A review on field management practices. Biomass and Bioenergy. 2012. [10] Wu X, Staggenborg S, Propheter JL, Rooney WL, Yu J, Wang D. Features of sweet sorghum juice and their performance in ethanol fermentation. Industrial Crops and Products. 2010;31:164-70. [11] RFA. Changing the climate, ethanol industry outlook 2008. ETHANOL INDUSTRY OUTLOOK. Washington, DC: Renewable fuel association; 2008. [12] Li S-Z, Chan-Halbrendt C. Ethanol production in (the) People’s Republic of China: Potential and technologies Applied Energy. 2009;86:162-9. [13] Moreno A. Sorghum ethanol production could reach 300 million liters in 2013. 2012. [14] Zhang C, Xie G, Li S, Ge L, He T. The productive potentials of sweet sorghum ethanol in China. Applied Energy. 2010;87:2360-8. [15] Nasidi M, Akunna J, Deeni Y, Blackwood D, Walker G. Bioethanol in Nigeria: comparative analysis of sugarcane and sweet sorghum as feedstock sources. Energy Environ Sci. 2010;3:1447-57. [16] Billa E, Koullas DP, Monties B, Koukios EG. Structure and composition of sweet sorghum stalk components. Industrial Crops and Products. 1997;6:297-302. [17] O'Hara IM, Research ARI, Corporation D, Research RI, Corporation D. Sweet Sorghum: Opportunities for a New, Renewable Fuel and Food Industry in Australia. Rural Industries Research and Development Corporation.; 2013. [18] Krishna C. Solid-state fermentation systems-an overview. Critical Reviews in Biotechnology. 2005;25:1-30. [19] Zhou Z, Li J, Zhou J, Li S, Feng J. Enhancing mixing of cohesive particles by baffles in a rotary drum. Particuology. 2015. [20] Aken JPv. Pilot project bio-ethanol from sweet sorghum. Netherlands: Widjajatunggal Sejahtera; 2014. [21] Southeast Asia energy outlook. World Energy Outlook Special Report: IEA; 2013. [22] The Indonesian electricity system - a brief overview. DIFFER GROUP; 2012. [23] Indonesia-Investments. Electricity in Indonesia: Plenty Natural Resources but Shortage of Electricity. Indonesia-Investments; 2014. [24] Wijaya ME, Tezuka T. A comparative study of households' electricity consumption characteristics in Indonesia: A technosocioeconomic analysis. Energy Sustain Dev. 2013;17:596-604.
Biography Dr. Shizhong Li is a full professor and deputy director of the Institute of New Energy Technology, Tsinghua University, Adjunct Professor of Hong Kong University of Science & Technology. He is also the executive director of MOSTUSDA Joint Research Center for Biofuels, the director of Beijing Engineering Research Center for Biofuels
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