The overall methodology used for technical and economic assessment can be divided in three main steps: flowsheet design and simulations, capital cost evaluation and investment cost analysis.
First, the flowsheets for the various process configurations were designed and simulated using the commercial software AspenPlus v7.1 (AspenTechnology Inc.) to perform rigorous thermodynamic calculations for mass and energy balances. Most of the process data implemented in AspenPlus were obtained experimentally within the project, such as bagasse and leaves composition [39
], the recovery of sugars in the liquid and solid fractions after pretreatment, sugar yields from enzymatic hydrolysis at various process conditions, ethanol yield from fermentation and data for the combustion of solids residues [40
]. Other data were estimated based on the authors' and partners' experience (running plant conditions).
The AspenPlus physical properties database contains most of the data needed for simulation of the entire process; but data for complex substances, for instance yeasts, enzymes and biomass components, such as lignin, were taken from the NREL physical properties database [41
]. For specific process intermediates, such as bagasse and enzymatic hydrolysis (EH) residues, the overall higher heating value (HHV) obtained from the weighted sum of each structural component (e.g. lignin, cellulose) HHV embedded in the databases was in accordance with the experimentally determined values.
The output data from AspenPlus simulations were then used as input for the Aspen Process Economic Analyzer (AspenTechnology Inc.) for equipment sizing and for the estimates of both direct and indirect capital costs. For some specific equipment the cost was obtained from vendor quotations.
Finally, the overall ethanol production cost was calculated in a Microsoft® Excel spreadsheet starting from the capital cost and the operating costs. The profitability of the various process configurations investigated was expressed as the minimum ethanol selling price for the 2G (cellulosic) ethanol (MESP-2G).
Second-generation ethanol production from sugar cane residues can be positively influenced by integration with the 1G ethanol process and the benefits obtained depend on the level of process integration. Moreover, the operating conditions for the 2G process can also improve the overall economic feasibility. The aim of this study was to compare different operating conditions, process options and levels of integration for ethanol production from whole sugar cane in terms of energy efficiency and capital cost, in order to identify the lowest MESP.
A full-scale plant was modelled and the following four main processes were simulated: 1G ethanol production from sugar cane juice, 2G ethanol production from lignocellulosic residues of the sugar cane (bagasse and leaves), combined heat and power (CHP) plant and waste water treatment by anaerobic digestion (AD). The 1G process (autonomous distillery) comprises the traditional Melle-Boinot steps, well-experienced so far in the sugar cane-to-ethanol industry, while the 2G ethanol production is based on SHF of H3PO4-catalysed steam-pretreated bagasse and leaves. The CHP plant provides heat and power either to the 1G plant or to the combined 1G and 2G processes by burning bagasse, EH residues and biogas produced by AD.
Besides the process configurations, process variables found to have significant impact on the plant design and capital cost, based on experimental results obtained within the project, were investigated. These were mainly parameters associated with the enzymatic hydrolysis step, i.e. the residence time, enzyme dosage, concentration of water-insoluble solid material (WIS) in the pretreated material, as well as the addition of sugar cane leaves to raw material for 2G ethanol production. Among the pretreatment conditions tested experimentally for bagasse and leaves in the project, those resulting in the best EH yield were selected for model simulation.
Autonomous distillery using sugar juice
The autonomous distillery modelled as it is today was used to represent the actual state of the art of an ethanol distillery using sugar cane juice in Brazil (1G), and was also considered as the initial flowsheet to which the 2G process was added (Figure ). Data for this model were provided by Centro de Tecnologia Canavieira (CTC) - Piracicaba, SP, Brazil - for a conventional plant with an input flow rate of sugar cane equivalent to 540 tons sugar cane per hour (ton SC/h), or 165 tons dry sugar cane per hour (ton-dSC/h).
Sugar cane shredding and milling are the first processing stages, in which the juice used for fermentation in the 1G plant is separated from the bagasse, which is combusted in the CHP plant. The latter is used for the production of steam and electricity allowing the mill to be self-sufficient in energy, and providing excess electricity that can be sold. The sugar cane juice is purified by adding CaO and a flocculant polymer, followed by clarification. Since the sucrose concentration is too low (13.7 wt-%) to reach the desired ethanol titer in the fermentation stage, an evaporation unit is used to increase the concentration to 19 wt-%. The unit is composed of two evaporators in parallel, each with an area of 3500 m2, requiring 30 kg/h/m2 live steam at 144°C and 2.5 bar from the CHP plant. The theoretical ethanol yield from fermentation based on the available sugars is 94%, and the ethanol concentration in the stream sent for distillation is 70 g/L. The secondary steam recovered after the evaporation flash is used to provide energy for distillation, using two strippers (34 trays each) and two rectifiers (56 trays each) arranged in parallel.
Figure shows the schematic flowsheet for the CHP plant. The boiler, having an efficiency of 0.80 based on the lower heating value, generates superheated steam at 500°C and 65 bar, which is used in turbines to supply electricity (77 MWel) and steam to the whole plant. The isentropic efficiency of the turbines was assumed to be 0.80. The steam for the plant is withdrawn at 144°C and 2.5 bar. In the autonomous distillery the material used in the CHP plant is the sugar cane bagasse at a dry matter (DM) content of 50% from the mill section. When bagasse is used in the 2G ethanol production process, the material combusted in the CHP plant consists predominantly of the solid residues after EH, mainly lignin, with the addition of some bagasse when the solid residues are not sufficient to meet the demand for steam from the 1G and 2G processes. When leaves are also used for ethanol production the material used in the CHP plant consists only of the solid residues after EH of bagasse and leaves. In the simulated scenarios, the boiler size was based on the mass flow feed rate of solids to the boiler, and the turbine system was modified to the steam requirement as the 2G process. Several turbine withdrawals were designed to supply superheated 20 bar steam to the dryer, saturated 10 bar steam for pretreatment, and 2.5 bar steam for evaporation and preheating of the feed to EH and distillation. A condensing turbine operating at 0.11 bar was also added when the steam requirement for the plant was lower than the total amount of steam produced.
Schematic flowsheet of the CHP plant. Stream types are differentiated by colours: steam (red), condensate (blue), air for combustion (green), flue gases (orange).
2G ethanol from bagasse and leaves
Figure shows the main steps involved in the 2G process investigated, consisting of steam pretreatment, solid-liquid separation, SHF and distillation, together with the 1G layout. The feedstock for the Base case consists of sugarcane bagasse from the mill section of the 1G plant at 50% DM, and is also the raw material in most of the other scenarios, except when leaves were added to the feedstock, in which case the amount of leaves is equivalent to 50% of bagasse on dry basis.
Bagasse (and leaves when added) are impregnated with H3PO4 (9.5 mg acid/g dry material) and then preheated to 95°C by direct injection of low-pressure secondary steam, before being fed to the continuous steam pretreatment reactor. Due to the high flow rate of feedstock two pretreatment reactors are needed for the bagasse. In the scenarios where leaves are also used a third pretreatment reactor is needed. The temperature in the reactor is maintained at 180°C by injecting high-pressure saturated steam at 10 bar, and the residence time is 10 minutes for both bagasse and leaves. Heat losses are assumed to be 10% of the adiabatic heat demand. The pretreated material is then flashed in three pressure reduction steps (7, 4, 1 bar), and the flash vapours obtained are condensed to heat other streams in the plant. After flashing part of the water in the slurry, the WIS concentration is 16%. The pretreatment step is the same in all scenarios.
The slurry from the last flash step in the pretreatment is neutralized to pH 5 with NH3 and washed in a filter press to recover the solubilized sugars, mainly pentoses, in the liquid fraction. The filter cake of solids content (35 wt-% DM) is diluted to obtain the desired WIS content, and enzymes are added for the EH process, which is performed at a temperature of 50°C. To investigate the effect of integrating the 1G and 2G processes, various residence times, enzyme loadings and WIS concentrations in the EH step were used. Yields of released glucose as a function of the enzyme dosage and residence time were based on experimental data, see Table . The residence time was set to 48 h or 72 h, depending on the scenario, and the size of the EH tanks was set to 1500 m3, resulting in 32 to 70 tanks. After EH, a filter press is used to separate the fermentable broth from the partially hydrolysed solid residues, which are sent to the CHP plant.
Glucose released from enzymatic hydrolysis of bagasse and leaves at 7 % WIS a
The liquid is then fermented using an industrial yeast strain from CTC, giving 94% of the theoretical ethanol yield from hexose sugars. The complete cycle, which includes filling, fermentation, draining and cleaning, lasts 8.2 hours, and the size of the fermentors was set to 1200 m3, resulting in between 8 and 12 fermentors. The yeast is recovered by centrifugation prior to the liquid fraction entering the distillation unit, and low-pH washing of the yeast prevents any possible infections. The ethanol from the fermentation step is concentrated by distillation to 92.8 wt-%. The distillation section consists of two stripper columns (24 trays each), and a rectifier (36 trays), which are heat-integrated by operating at different pressures. The feed from the fermentation stage is divided between the two strippers. The first stripper is heated with live steam while the second stripper is heated by the overhead vapour from the first stripper. The rectifier is heated by the overhead vapour from the second stripper. Ethanol recovery was assumed to be 99.5% in each column.
The integrated 1G and 2G plant has many waste streams with a high COD content that can be treated by AD to produce biogas, which is burnt in the boiler for steam and electricity production. These streams include the bottom streams from distillation (stillage, also called vinasse), the condensates obtained during pretreatment, evaporation and drying, and the pentose-rich liquid fraction from filtration of the pretreated material prior to enzymatic hydrolysis. Experimental trials were performed to obtain data for the calculation of the biogas potential from these streams. These data are required as input to the flowsheet calculations and are fundamental in the energy balance of the integrated 1G and 2G system, since the amount of biogas can be important in improving the energy supply and reducing the final ethanol production cost. The biogas potential was measured in batch trials for several stillage streams and for the pentose-rich stream after filtration of the pretreated material (data not shown).
The microorganism consortium was taken from the Domsjö Fabriker AB spent sulphite liquor plant (Domsjö, Sweden). The stillage streams required at least 10 days residence time in AD, whereas the pentose-rich samples reached maximum production after about 5 days. Continuous AD could probably shorten the residence time, but the results for the batch system were used to be on the conservative side. The final methane yield was 0.112 and 0.127 gCH4/gCOD for the stillage and pentose-rich streams from bagasse, respectively. Table presents a summary of the streams that can be used in AD in the combined 1G + 2G plant. All the streams are not suitable for anaerobic digestion, due to high flow rate or high dilution of COD, which means a high capital cost compared with the methane production capacity. Indeed, if all the streams were subjected to AD, 151 stirred tank reactors with a volume of 2500 m3 each would be required, and the investment cost for the biogas facility alone would be about 68 million US$. For this reason, only AD of the pentose-rich stream, which has a higher content of COD and requires 98 hours residence time to reach 96% of the maximum biogas production with a yield of 0.116 gCH4/gCOD, was included in the final simulations.
Origin and composition of streams used in anaerobic digestion
The integrated 1G + 2G plant
When the 2G plant is simply annexed to the existing 1G facility, there is no integration except the sharing of heat and power produced in the CHP plant. Thus the synergies arising from the combination of the two processes cannot be exploited to minimize the energy demand and the capital cost. Alternative configurations for the combined 1G + 2G facility rely on the strategy adopted for integrating the 1G and 2G processes in a new plant. For this reason, energy reduction and recovery, as well as process stream mixing are the integration options that were investigated to find the configuration with the lowest MESP-2G. Energy integration is achieved by further increasing the use of new and more efficient equipment, suitable for recovery and/or in a different configuration. However, new equipment can significantly increase the capital investment cost, thus the economic feasibility of better energy use was also evaluated. Process energy efficiency was defined as the energy output in the products (ethanol and excess electricity) divided by the energy input, i.e. the energy contained in the raw materials (sugar cane, and leaves in the scenarios were these were included). Calculations were based on the HHVs of the raw materials and ethanol. The electric power was recalculated in terms of the fuel necessary to produce this electricity, assuming an electricity-to-fuel ratio of 0.33.
Regarding stream integration, mixing of material flows can have beneficial effects on the energy balance, especially in the distillation and evaporation stages. Two mixing options were considered in an attempt to reduce the energy demand in these two steps: mixing of the 1G ethanol-rich stream and the 2G ethanol-poor stream prior to the distillation unit (see Figure ); and mixing of the 2G glucose stream after EH with the 1G sugar juice prior to and after evaporation (see Figure ).
The results from AspenPlus simulations (mass and energy balances) were imported into Aspen Process Economic Analyzer v.7.1 (Aspen Technology, Inc.) where equipment was sized and direct/indirect costs were determined. Fixed capital investment costs were estimated based on costs for the first quarter of 2008, except for the cost of the pretreatment unit, which was obtained from a vendor quotation based on pretreatment using SO2 as acid catalyst (in 2010). The fixed capital investment cost was recalculated from 2008 to 2010 using cost indices for Houston, Texas, USA. A factor of 0.82 was then applied to scale fixed capital investment costs from the US gulf coast to Brazilian conditions. These data, together with stream data, were exported from Aspen Process Economic Analyzer and used in the Excel model developed for the final cost calculations.
The Excel model comprises investment costs and data on biomass, chemicals, enzymes, electricity price and all other running costs. The main premises for the production cost estimate are that it is a stand-alone ethanol production plant processing 165 dry metric ton of sugar cane per hour and is operated 200 days per year. The plant is equipped with a CHP plant for production from residue and/or sugar cane leaves. Excess electricity is exported.
It is important to point out that electricity is accounted for as an opportunity cost; negative for the 1G process (revenue from selling electricity produced by burning bagasse) and positive for the 2G plant (cost due to the increased energy demand compared with the 1G process alone). The plant is assumed to be the Nth plant, meaning that it is considered to be based on known technology at the time of construction (N could be 3-6 depending on the size and complexity of the demonstration plant). The Minimum Ethanol Selling Prices (MESPs) for the integrated 1G + 2G process and the separate processes are calculated as the sum of each single production cost item (see Table and Table ). The marginal cost items accounted for the 2G process are derived from the following expression in respect to the integrated 1G + 2G process and 1G process.
where Pi 2G is the 2G cellulosic ethanol production cost for the item i given by a weighted ratio between the difference in the cost of item i for 1G + 2G and 1G ethanol and the volume of 2G ethanol produced. The data for 1G ethanol production, indicated by the index 1G, were derived for the autonomous distillery (Reference scenario). Pi denotes the production cost for item i and F the amount of ethanol produced in the specific process. The MESP is not only a measure of total production cost but also includes the investment return, accounted for as capital cost, at the desired internal rate of return (IRR). The MESP was varied in the spreadsheet model until the net present value (NPV) equalled to zero at the selected IRR (10%). The NPV was calculated based on the values given in Table . The prices of all raw materials and products were assumed to be subject to the same rate of inflation. Tax-deductible depreciation was assumed to be 20% p.a. for fixtures, cars and IT, 10% for process equipment and 4% for buildings. Since the largest cost is that of process equipment, 10% was chosen as an average for simplicity, i.e. linear full depreciation in ten years. The cost of sugarcane was assumed to be 65 US$/dry ton delivered to the plant, based on whole unwashed stalks and the DM content was assumed to be 30%. The cost of sugar cane leaves was assumed to be 26 US$/dry ton delivered to the plant, unground and unwashed and the DM content was assumed to be 50%. Costs for chemicals are given in Table . The enzyme cost is based on budget price for the cutting-edge proteins in 2009. The electricity selling price (daily and annual average) to the local grid was assumed to be 87 US$/MWh. The value of the treated water was set to 0.03 US$/ton, and the stillage stream, which is recycled to fields, was assumed to have a value of 1.6 US$/ton based on the N, P and K in the stream. Labour costs were assumed to be 3.5 million US$ per year.
Main assumptions for economic calculations