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Numerical investigation of biomass fast pyrolysis in a free fall reactor

Bieniek Artur, Jerzak Wojciech, Magdziarz Aneta

Archives of Thermodynamics

2021

Vol. 42, no 3

173--196

This work presents two-dimensional numerical investigations of fast pyrolysis of red oak in a free fall reactor. The Euler–Lagrange approach of multiphase flow theory was proposed in order to describe the behaviour of solid particles in the gaseous domain. The main goal of this study was to examine the impact of the flow rate of inert gas on the pyrolysis process. Calculation domain of the reactor was made according to data found in the literature review. Volume flow rates were 3, 9, 18, and 25 l/min, respectively. Nitrogen was selected as an inert gas. Biomass pyrolysis was conducted at 550◦C with a constant mass flow rate of biomass particles equal to 1 kg/h. A parallel multistage reaction mechanism was applied for the thermal conversion of red oak particles. The composition of biomass was represented by three main pseudo-components: cellulose, hemicellulose and lignin. The received products of pyrolysis were designated into three groups: solid residue (char and unreacted particles), primary tars and noncondensable gases. In this work the impact of the volume flow rate on the heating time of solid particle, temperature distribution, yields and char mass fraction has been analysed. The numerical solutions were verified according to the literature results when the flow of nitrogen was set at 18 l/min. The calculated results showed that biomass particles could be heated for longer when the flow rate of nitrogen was reduced, allowing for a greater concentration of volatile matter.

Thermodynamic prediction of the effect of repeated recirculation of cooled flue gases on the content of major, minor, and trace compounds in oxy-coal combustion products

Jerzak Wojciech

Journal of Power Technologies

2020

Vol. 100, nr 2

102--114

This study investigates changes in the composition of oxy-coal combustion products resulting from the recirculation of cooled flue gas (FGR) at 20, 60, 100, 200, and 300°C (containing 70-95 mol% CO2). It presents the results of thermodynamic calculations describing changes in the content of the major, minor, and trace components of flue gases, ash and condensate. The results reflect a scenario of starting the oxy-coal combustion system in a fluidized bed boiler using low-purity oxygen from an air separation unit. This work demonstrates that in FGR loop the major species, i.e., Ar and N2, as well as the minor, e.g., Cl2, PbCl4, HgCl2, and CrOCl3, are accumulated. After nine FGR loops cooled to 300°C, marked increases in concentrations were observed: ZnCl2 and HCl (3-fold), as well as CrO2(OH)2 (2.5-fold). The ash that was formed contained, among others, CaSO4, SiO2, CaMgSi2O6, MgSiO3, ZnFe2O4, and MgCr2O4, whose mass changed in successive reactors asa result of the repeated FGR. Depending on the temperaturę of the cooling reactor, flue gases were subjected to recirculation and the main component of the condensate was H2O or H2SO4·6H2O. The condensate contained chloride salts, e.g.,PbCl2, KCl, and ZnCl2, as well as sulfate salts, i.e., K2SO4 and Na2SO4, in smaller amounts. A consequence of the nine-fold FGR cooled toT≤200°C was, among others, a percentage mass increase in ZnCl2in the condensate. The less cooling applied to flue gases, the more likely the occurrence of sulfates was in the condensate.