Integrated process simulation for bioethanol production: Effects of varying lignocellulosic feedstocks on technical performance

Variations in lignocellulosic feedstock composition can influence conversion performance of bioethanol production, but such effects are overlooked in several studies that rely on standard conversion factors. This study investigates the effects of seven lignocellulosic feedstocks (belonging to the categories energy crops, forest and agricultural residues) on mass, carbon, water and energy balances for biochemical bioethanol production, including a comparison of individual process step yields. We find that overall bioethanol yields vary considerably, ranging between 19.0 and 29.0%, 27.3 and 46.2%, and 19.0 and 31.0%, for energy and carbon efficiency, respectively. The highest yields are found for switchgrass, which has the largest carbohydrate content, and the lowest for forest residues (spruce). Feedstock composition also affects water and carbon balances. Overall, the type of biomass influences conversion performances, thereby calling for explicit representation of the effects of biomass types in technical, economic and environmental assessment studies of bioethanol production.


Determination of forest residues composition
The biomass of each component of the tree were estimated from BEF (Biomass Expansion Factors, in Mg·m -3 stem under bark) using Equation S.1 (Lehtonen et al., 2004), where ai and bi are parameters, t is the stand age (in years) and i is the biomass component: stem, foliage, living branches, dead branches, stump, coarse roots, small roots). Values are defined in Table S.1. One rotation period is assumed of 100 and 60 years, for Norway Spruce and Birch sp., respectively. = a i + b i e −0.01t Eq. S. 1  (Ilomäki et al., 2003) for foliage and (de Wit et al., 2006) for roots estimation.
The annual mass produced (mannual, i, ton·y -1 ) for each biomass component i, is calculated by the equation S.2, where fi is the extraction factor (percentage collected of component i) and Vannual, specie is the volume produced annually of each forestall specie. Vannual, specie considers 1 m 3 ·y -1 as base, for Norway Spruce and Birch. The forest residues are mainly composed by stem bark, branches, foliage, needles, stumps and roots. However, in this study only above-ground components, i.e. foliage and branches, are considered as available residues to be used as feedstock for ethanol production, excluding the bark and under-ground residues (stumps and roots). Other assumption is related to the extraction factor (fi) for these compounds, considering 75% of total foliage is collected for Spruce and Birch, while 75% and 50% of the branches (alive and dead) are collected for Spruce and Birch, respectively. These assumptions can be traduced as a collection of 30% and 35% of the total potential residues for Spruce and Birch, respectively. This study used foliage and branches as forest residues (detailed in Table S.2).  Table S.3 summarizes the variation in the composition, such as lignin, glucan, xylan, among others, by each tree component (see Figure S.1 for a more graphical representation).  Finally, the general composition of the biomass residues of Norway Spruce and Birch summarizes in Table S.1 (in article) are obtained multiplying the percent of each residual element (branches and foliage) in Table S.2 by the percent of every chemical compound (for foliage and branches) defined in Table S.3.

Storage and chopping
A simplified process flow diagram is shown in Figure S.2. The storage and chipping area operate at the same capacity of the biorefinery, i.e., 128,000 kg biomass·h -1 . To satisfy production requirements, the plant must receive 10 dumper trucks (13 t capacity, compliant with EURO6 emissions standards), every hour. The trucks are unloaded using the dumper, requiring ~10 min for unloading. The dumpers empty into hoppers, which send the biomass to a series of conveyors to a coverage storage section. Two storage domes (each with a 36-hour capacity) are required, so that one can be loaded while the other is empty to the conversion process. The distribution of biomass inside the storage domes is carried out by wheel loaders (2 units by storage dome), which also load the conveyors belts to transport the biomass to the chippers. Two different chippers are used in this section, chipper 1 reduces the size to 400 mm chiplength and chipper 2 mills to 40 mm chip-length size. A disc scalping screen is used to screening large and oversize biomass chips, to be returned to the chipper 2. Finally, the biomass is stored in a chip silo with a discharger to control the biomass flow delivered to the pretreatment process.  List of the technical specifications for the machinery used in the process. Two units are required for most of the machinery, so we can assume two production lines working at the same time. Table S.5 summarizes the inventory to produce 1 kg of chipped biomass or 1 MJ of bioethanol for the different biomass.  Market for lubricating oil (kg) 2.1810 -5 7.8610 -6 7.1410 -6 7.8310 -6 3.5310 -6 3.5810 -6 3.6410 -6 3.8810 -6 Market for agricultural machinery, unspecified (kg) 1.7610 -5 6.3510 -6 5.7710 -6 6.3310 -6 2.8510 -6 2.8910 -6 2.9510 -6 3.1310 -6

Enzyme production area
The enzyme production system was based on the NREL report for cellulase enzyme production (Humbird et al., 2011). This model considers the preparation of a mixture of enzymes, which are catalytic proteins capable to break down cellulose fibers into glucose, cellobiose and soluble glucooligomers. This process area produces cellulase on-site in the biorefinery. Based on the NREL model, the process design considers aerobic fermentation of Trichodema reesei, which is a filamentous fungus capable of secrets high levels of cellulase enzymes when grown in aerobic conditions and the presence of glucose and others cellulase inducers. Table S.6 summarizes the inventory to produce 1 kg of cellulose enzyme. Figure S.3 is a simplified flow diagram of this enzyme production system.

Inoculum production area
The inoculum production is conducted in a fermenter trains reactor connected in series (Figure S.4) until reaching the inoculum volume required in the fermenter reactors R-301 (assumed as 10% of total volume). The seed production system consists of a set of reactors operating in batch mode (R-302), operating at 32 C for 24 h batch time. The first train reactor is inoculated with an inoculum from laboratory and its broth is used to inoculate a larger reactor, and so on, until the cell mass is sufficient to inoculate the fermenter reactor (R-301). Table S.7 summarizes the nutrients and electricity required in the reactors R-302 connected in series. Table S.8 gives the reactions and conversion used in the seed fermenter reactors. A high-capacity pump (P-302) is used to transport the inoculum to the reactor R-302. where, 1.11 and 1.136 are the stochiometric values of the pretreatment reactions (see Table 2 in article), 180 is the molecular weight (MW, g·mol -1 ) for glucose, mannose and galactose; 162 is the MW for glucan, mannan and galactan; 150 is the MW for xylose and arabinose and 132 is the MW for xylan and arabinan.

Figure S.4. Simplified diagram of inoculum production area
The values obtained from process model and pretreatment yield for each feedstock is detailed in Table  S.9. is the theorical mass of glucose content in the glucan recovered in the pretreatment hydrolysate (defined by Eq. S.15). In that way, losses due to glucan degradation in pretreatment biomass are accounted for. Both mass values were directly obtained from the mass balance by using the results from the process simulation, detailed in Table S In order to assess the theorical amount for ethanol from glucose and xylose consider the next reactions for glucose and xylose fermentation were considered:

Net Energy Value (NEV)
The NEV for the different lignocellulosic feedstocks are presented in 3.66 MJ DM-biomassMJ-1 bioethanol for birch and spruce residues, respectively. The larger amount of biomass required in the case of woody residues is due to its low contents of carbohydrates, which make them require more sugars (and hence biomass) to produce the same quantity of bioethanol of other lignocellulosic feedstocks. Table 5 also summarizes the MJ DM-biomass by MJ-1 bioethanol reported by other studies in the literature, whose values are consistent with the results obtained in our study.
The bioethanol production requires consumption of steam for the pretreatment, bioethanol recovery and wastewater treatment. These process steps demand mid-and high-pressure steam. A highpressure steam at 320 °C and 13 atm is required in the pretreatment process to reach the temperature and pressure necessary in the dilute-acid reactor (190°C and 12 atm). The recovery area requires midpressure steams (4 atm) at 134°C and 144°C for the concentration column and rectification column, respectively. Additionally, a high-pressure steam at 226 °C and 26 atm is used in the 4-effect evaporator system in the wastewater treatment area. In order to perform the energy balance, these steam and power demands must be considered. These internal inputs are supplied by the steam and power produced in the cogeneration area.
The power not used in the processes is considered as a coproduct. The NEV for power are between -0.021 and 0.04 MJ powerMJ-1 bioethanol, which is lower than those reported from other studies, e.g., a corn stover bioethanol production plant has 0.15 MJ powerMJ-1 bioethanol of cogeneration surplus power (Luo et al., 2009), however this study no consider the power required in the biomass preprocessing through chipping. The NEV negative in power for eucalyptus and birch represent an external requirement of electricity in the bioethanol plant.