Techno-environmental Assessment of Two Biorefinery Systems to Valorize the Residual Lignocellulosic Biomass of the Basilicata Region

This section reports the process configurations investigated in this study and the methodology used to assess and analyze each case. The commercial software Aspen Plus^TM V.10 was used for process simulation. Straw was considered in the analysis because this biomass is the most abundant in the Basilicata Region (about 400 kt/y). Table 1 shows the biomass composition considered in this work. For the case of bioethanol production available straw was obtained through the ENEA’s national Atlas of biomass. The biomass availability referred to 2017 and was calculated from the annuals grains productivity in the ISTAT census. The overall biomass potential was then multiplied by 0.4 accounting for the need to leave roughly 60% of biomass on the fields [16]. For the case of biomethane production, 20 biogas plants already existing in Basilicata were considered (Table 2). Table 3 shows the OFMSW composition considered in this work. In this case, available straw was fed to the plants after a chemical-physical pretreatment facilitating the microbial conversion to biomethane the second-generation biogas is produced from lignocellulosic biomass derived from agricultural waste, forest waste, municipal and industrial waste, and non-edible plants such as grass and aquatic plants. Moreover, all these resources share the characteristic that they are formed from non-food resources [17].

Several lignocellulosic biomass pretreatments can improve the methane yield by anaerobic fermentation [17]. Steam Explosion of corn stover or straw was studied by Lizasoain et al. [18] and Siddhu et al. [19]. The steam explosion technology was selected based on some previous publications [20, 21].

Table 1. Straw composition [5]

Table 2. Biogas plants in the Basilicata Region by Atlaimpianti [22]

Table 3. Organic Fraction of Municipal Solid Waste composition

Process simulation suites can be successfully used to perform the process design of lignocellulosic biorefineries [23]. In the present study, Aspen Plus^TM v10 was used to simulate a biorefinery producing bioethanol from lignocellulosic biomass using. Plant capacity of 158,000 t/y of dry feedstock, corresponding to a total of 7200 h per year, was assumed based on common figures reported in the literature [24-26]. Components from Aspen Plus^TM databank were used whenever possible with NREL components and properties being added to complement the missing data from Aspen. Non-random two liquid (NRTL) property method was selected for the simulations. Straw is fed to the pretreatment section based on the Steam Explosion (SE) technology. The steam explosion route was modeled assuming a steam explosion unit using medium pressure steam at 200°C with a residence time of 3 min [27]. After the steam explosion reactor, a flash operation is performed in which the volatile fraction is removed from the product stream, also containing water and biomass degradation products that can act as fermentation inhibitors. A filter separates the solid and liquid fractions from the bottom product of the flash and the stream is then sent to the fermentation section. Figure 1 shows the block diagram of the lignocellulosic biorefinery considered in this work. Separate Hydrolysis and Fermentation (SHF) were employed to convert the biomass into sugar monomers and latter to ethanol. Cellulase enzymes were produced in an in situ plant using a small amount of cellulose as substrate [28]. Hydrolysis was carried at 48°C and the reactions rate was obtained from the published articles referenced in each pretreatment section. The hydrolysis reactor residence time was defined as 84h and afterward, the slurry was cooled to 35°C and sent to the fermentors. Zymomonas mobilis was responsible for the fermentation reactions with a total conversion rate of 95% for the glucose and 85% of the xylose [25]. The fermentation process required a total of 36 h of residence time and the fermentation gases were sent to the vent scrubber in the product purification section. The fermentation broth was initially sent to a beer well and from there it was sent to the beer column. The beer column separates the fermentation broth in three streams. The overhead product is mainly composed of carbon dioxide and a diluted ethanol beer (around 3% w/w). The overhead stream is mixed with the carbon dioxide produced in the fermentation tanks and sent to a vent scrubber where water in a countercurrent flow recovers most of the ethanol carried in the gas stream. The carbon dioxide from the scrubber is vented whereas the water-ethanol stream is sent back to the beer well. The bottom product of the beer column contains unconverted biomass, inorganic impurities and water. The bottom stream is filtered, and the solids are sent for combustion and co-generation of heat and power, whereas the water is sent for wastewater treatment and recycled back to the process. From the beer column, a side stream is drawn with an ethanol concentration of 40% (w/w). This stream is sent to the rectifying column where ethanol is concentrated near to its azeotropic point. The overhead from the rectifying column containing ethanol is fed to the molecular sieves where it is further dehydrated to reach the fuel grade category. The bottom stream from the rectifying column contains water pure enough to be used in the process without further purification. The unconverted biomass and lignin can be burned to produce high-pressure steam and electricity. The lignin stream, coming from the separation section, is sent to the lignin combustion coupled with a Rankine cycle. High-pressure steam is produced in the boiler and is sent to the steam turbine train at three pressure levels [15]. A heat exchanger transfers the heat of the flue-gas produced into the equilibrium reactor to the water, in such quantities to produce steam at 500 °C and 96 bar. At the end of the expansion (5 bar), the steam is still overheated, avoiding problems caused by the presence of water droplets in the latest expansion stages.

A backpressure configuration of the co-generation section was chosen considering the high vapor requirement of the process. The heart of the system is the multistage turbine, connected to an alternator to generate electricity. During the steam explosion, two extractions are performed. The first one at 40 bar and the second one at 16 bar available for any utilities, from which the various steam users in the plant (distillation columns, etc.) are fed. The turbine discharges directly into the low-pressure steam line of the plant (about 5 bar). Since there is no vapor condensation section, the corresponding high cooling water requirements and the high costs and consumptions associated with an air cooling system (towers, fans, etc.) are avoided. The expected expansion rates are those that are usually obtainable in steam turbines, between 82 and 85% (higher for intermediate stages, where the expansion curve is less elongated). Finally, there is a make-up stream for the fluid that performs the thermodynamic cycle. In the water treatment section, the residues of ethanol, glucose, xylose and lignin are transformed into biogas and sent to the cogeneration section and the water is purified or treated for process recycling use [15]. The liquid stream from the beer column containing suspended cellulose is initially hydrolyzed and the resulting stream is sent to an anaerobic digester, where biogas is obtained. The product from the digestor is directed to the combustion area, whereas the water is recycled back to the process.

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Figure 1. Ethanol biorefinery block diagram

Biorefinery processes contain several streams with different temperature targets spread along with the plant. A heat integration together with a heat exchanger network can be employed to reduce the utility consumption and the environmental impacts associated with them, and at the same time increase the profitability of the setups. In all the setups, a minimum temperature difference of 10°C was used to calculate the final utility target. The process heat integration also allows the identification of the quality and the amount of steam necessary to supply the process and attune the co-generation stage for the process requirements.

2.2 Process simulation description for biomethane production

The anaerobic digestion (AD) system considered in this work was based on the works of Migliori et al. [29] and Li et al. [30] and it was not included in the process simulation but was considered for environmental aspects. The base of calculation were 20 biogas plants operative in Basilicata. Biogas plants can use as feedstock the organic fraction of the municipal solid waste (OFMSW) or other traditional fermentable material. Feedstock flowrates are summarized in Table 4. OFMSW (about 27 wt% of solid content) that are continuously pumped in a horizontal reactor keeping a residence time of about one month under mesophilic conditions.