Sustainability 2022 , 14 , 4669
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The first two processes, drying, and pyrolysis of the fed stream (WP–DIS), are com- pleted in an RYield reactor (DECOMP). Such a block decomposes the non-conventional stream of WP–DIS pellet to conventional (C, H 2 , N 2 , Cl 2 , F 2 , S, and H 2 O) and non- conventional (ash) components, based on ultimate elemental analysis implemented through a FORTRAN subroutine in a calculator. The temperature of the DECOMP block is set at 400 ◦ C, which is indicated in the pertinent literature as the optimum temperature for pyrol- ysis of biomass [31]. The product exiting from DECOMP block (DECMPSS) is separated in a separator (SEP) into two sub-streams: a fraction of carbon that participates in gasification reactions (GASFED) and the remaining that forms char and ash (C-CHAR). The gasification fed stream (GASFED) is mixed in a mixer (MIXERG) with air (HOTAIR) preheated in a heat exchanger (AIRHTR) to reach the gasification temperature. RGibbs reactor (GASIFIER) is chosen to simulate the remaining two processes (gasification and combustion) involved in the gasification of WP–DIS pellets by minimizing Gibbs free energy. Each gasification reaction is restricted by assigning a specific temperature. This allows for reducing the devi- ation between predicted results and experimental values in terms of syngas composition and LHV [41,51]. The product stream (RAWSYNG) generated from WP–DIS pellets gasification is mixed with char and ash in a mixer (MIXER) to generate a unique flow (MIXFLOW). The char and ash temperature are increased in a heat exchanger (ASCHTR) to equalize that of syngas. Syngas is then cleaned to separate ash and char in an SSplit block (CYCLONE) and cooled down to the ambient temperature of 30 ◦ C in a heat exchanger (COOLER) to meet the engine specifications [43]. The simulation of the ICE is completed by connecting three consecutive blocks of the Aspen Plus library [41,43,49,50]: • a compressor (COMPR) that models the pressure increase of the incoming air through an isentropic compression; • an RGibbs reactor (BURN) that simulates the conversion of syngas internal energy to thermal energy through combustion at constant volume by minimizing Gibbs free energy; • a turbine (TURB) that converts the thermal energy of combustion exhausts (CMBST- GAS) to kinetic energy through an isentropic expansion; • a heat exchanger (UTIL) where thermal energy present in the stream exiting the turbine (EXITGAS) is extracted by cooling the exhausts to usable temperature (80 ◦ C) [41]. Such thermal energy may be employed in the paper production process or for the district or office heating purposes based on the productivity of the plant. 2.1. Modelling Assumptions The gasification model is developed by applying a non-stoichiometric equilibrium approach based on the minimization of Gibbs free energy as it gives a better agreement with the experimental outcomes than a stoichiometric and kinetic approach, in terms of syngas composition and process performance. Indeed, applying this approach reduces the deviation between numerical and experimental results, significantly increasing the accuracy of the model [52]. The thermodynamic properties of all the conventional components are estimated through the Peng–Robinson equation of state with Boston–Mathias alpha function (PR-BM) [48,53]. Enthalpy and density of non-conventional components (WP– DIS pellets and Ash) are evaluated by Aspen Plus built-in coal models HCOALGEN and DCOALIGT, respectively. Several simplifying assumptions are considered to avoid the complexity in the gasifi- cation and cogeneration model. Assumptions for gasification [46,52–54]: • model is zero-dimensional; • gasification reactions are completed with a steady-state condition; • pyrolysis is completed instantaneously;
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