Experimental Studies and Modeling of Synthesis Gas Production and Fischer-Tropsch Synthesis

Abstract

Indirect route can be used to convert abundant natural resources such as natural gas (NG), coal and biomass to synthetic fuels (referred to as gas-to-liquid (GTL), coalto- liquid (CTL) and biomass-to-liquid (BTL)). It is currently one of the most effective solutions to the problem of finding suitable substitutes for liquid clean fuels. In this work, an investigation on the production of synthetic fuel from gaseous hydrocarbons (HCs)/bio-HCs and liquid bio-HCs on a small-scale unit has been carried out. The research project consists of two major parts, a modified version of a plasma-assisted catalytic partial oxidation (CPO) gliding arc (GlidArc) reactor and a thermally stable single-tube fixed-bed Fischer−Tropsch (FT) reactor.

The potential for the CPO of methane to produce synthesis gas (syngas) was studied both experimentally and thermodynamically at a fixed pressure (1 bar) and electric power (0.3 kW). The investigations were performed in a partially adiabatic plasma-assisted (nonthermal) GlidArc reactor, using a Ni-based catalyst. Two cases were studied: in the first, normal air (molar ratio of O2/N2=21/79) was used, whereas enriched air (O2/N2=40/60) was utilized in the second. The individual effect of the O2/CH4 molar ratio, gas hour space velocity (GHSV) and bed exiting temperature (Texit) was studied for both cases. The main trends of the CH4 conversion, the syngas (H2 and CO) yield and the thermal efficiency of the reactor based on the lower heating value (LHV) were analyzed and compared.

A numerical investigation of the CPO of methane to syngas using a GlidArc reactor was also studied. A 2D heterogeneous plug-flow model with radial dispersion and no gradients inside the catalyst pellet are used, including the transport equations for the gas and solid phase and reaction rate equations. The governing equations of this model formed a set of stationary differential algebraic equations coupled with the non-linear algebraic equations, and were solved numerically using in-house MATLAB code. Model results of CPO of methane were compared to previous experimental data with the GlidArc reactor found in the literature. A close match between the calculated and experimental results for temperature, reactant (CH4 and O2) conversion, H2 and CO yields and species molefraction were obtained. The developed model was extended to predict and quantify the influence of the GHSV as well as determine the influence of the reactor energy density (RED), the O2/CH4 molar ratio and the O2/N2 molar ratio. The predicted behaviors for the species mole-fraction, reactants conversion, H2 and CO yields and temperature along the length of the reactor have been analyzed.

Furthermore, FT synthesis of a model biosyngas (33% H2, 17% CO and 50% N2) in a single tube fixed-bed FT reactor was investigated. The FT reactor consisted of a shell and tube with high-pressure boiling water circulating throughout the shell. A spherical unpromoted cobalt catalyst was used with the following reaction conditions: a wall temperature of 473 K, a pressure of 20 bars and a GHSV of 37 to 180 NmL/(gcat.h). The performance of the FT reactor was also validated by developing a 2D pseudo-homogeneous model that includes transport equations and reaction rate equations. Good agreement between the model predictions and experimental results were obtained. This developed model was extended to predict and quantify the influence of the FT kinetics as well as determine the influence of the tube diameter and the wall temperature. The predicted behaviors for CO and H2 conversion, productivity of HCs (mainly CH4 and C5 +) and fluid temperature along the axis of the reactor have been analyzed.

In addition, the initial tests results are presented for the conversion of waste cookingoil (WCO) to biosyngas by CPO over a granular Ni-based catalyst. Additionally, autothermalreforming (ATR) of propane with water and normal air was also carried out.The investigations were performed in a partially adiabatic plasma-assisted (non-thermal)GlidArc reactor at fixed pressure (1 bar) and electric power (0.3 kW). Detailed axial temperaturedistributions, product concentrations, reactant conversions, H2 and CO yield,H2/CO ratio and thermal efficiency, as a function of the cold and hot WCO flow rate, thewater flow rate and the time on stream were studied. Propane and normal air were usedas oxidizing components to maintain autothermal operation.

Finally, an investigation of the influence of process conditions on the production ofsyngas from model biogas (molar ratio of CH4/CO2=60/40) through partial oxidationover a granular Ni-based catalyst was explored. The investigations were performed in apartially adiabatic plasma-assisted (non-thermal) GlidArc reactor in a transitional flowregime at a fixed pressure (1 bar) and electric power (0.3 kW). The emphasis of this investigationwas on an experimental study and a comparative thermodynamic analysis. Theequilibrium compositions were calculated using a Lagrange multiplier and resulted in thedevelopment of systems of non-linear algebraic equations, which were solved numericallyusing the MATLAB function “fmincon”. Two cases were studied: normal air (molar ratioof O2/N2=21/79) and enriched air (O2/N2=40/60). The individual effects of the O2/CH4molar ratio and the Texit were studied in both cases. The main trends of the CH4 conversion,the syngas yield, the H2/CO ratio and the thermal efficiency of the reactor wereanalyzed.