TRI- REFORMING OF METHANE FOR THE PRODUCTION OF SYNTHESIS GAS

Abstract

The conversion of the two most problematic gases, i.e., CO2 and CH4, to the valuable synthesis gas via the tri-reforming (TRM) is a very promising route. The TRM with CO2, H2O, and O2 to produce synthesis gas (syngas) with an H2/CO molal ratio of 1.5-2 is highly desirable for the production of important chemicals and fuel additives, including methanol, dimethyl ether (DME), and substitute natural gas (SNG), etc., via the Fischer-Tropsch (F-T) process. In this thesis, a novel nano-nickel metal catalyst dispersed on mesoporous-zirconia is developed for the controlled production of the synthesis gas with an H2/CO mol ratio of 1.5-2 via the TRM. The catalysts were tested in a downflow-fixedbed reactor at 600-850 oC and 1 atm. At the optimum feed (CH4:CO2:O2:H2O:N2) ratio of 1: 0.5: 0.1: 0.0125:1, the maximum CO2 and CH4 conversion was ~28% and ~86%, respectively, over the 5 wt.% Ni/ZrO2. At this condition, the syngas with an H2/CO ratio of ~1.5 was obtained at a lower reaction temperature of 700 oC. The superior activity of this catalyst was due to the presence of highly dispersed and reduced nickel particles over the combined tetragonal and monoclinic phases of mesoporous ZrO2. The basic strength of the catalyst, the nickel particle size, and metal dispersion played a important role in controlling the TRM activity as well as the H2/CO mole ratio. The time-on-stream (TOS) study and the used catalyst characterization results established that the nanosized nickel metal particles dispersed on mesoporous-zirconia were thermally stable and coke-resistant. Further, a series of Zr-MOF were synthesized via the solvothermal method, and an impregnation technique was used to synthesize the nickel impregnated on a MOF-derived ZrO2 catalyst. The catalyst was characterized by various methods, including N2-porosimetry, X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), temperature programmed reduction (TPR), H2-chemisorption, CO2-temperature programmed desorption (CO2-TPD), high-resolution transmission electron microscopy (HR-TEM), field-emission scanning electron microscopy (FE SEM), etc. Characterization results supported the formation of the Zr-MOF and nickel metal dispersed on MOF-derived ZrO2. Further, the TRM activity of the catalyst developed were tested in a downflow-fixed bed reactor. The various catalysts were screened for TRM activity at different temperatures (600-850 oC). Results demonstrated that TRM was highly favorable over the NZ-1000 catalyst due to its desirable physicochemical properties, including nickel metal surface area (2.3 i m2.gcat-1), metal dispersion (7.1%), and nickel metal reducibility (45%), respectively. Over the NZ 1000 catalyst, an optimum H2/CO ratio of ~1.6-2 was achieved at 750 °C, and it was stable for a longer period of time. To understand the kinetic behavior of TRM reaction, the kinetic study was performed over 5 wt.% Ni/ZrO2 catalyst under steady-state condition. The reaction temperature and W/FAo were varied from 650-800 oC and 0.61-3.06 g.min.mol-1 at 1 atm. pressure. The Power law model and Langmuir Hinshelwood-Hougen-Watson (LHHW) model for heterogeneous reaction was proposed. The model equations were solved by using MATLAB. For the Power law model, the calculated activation energy with respect to CH4 and CO2 was 42.31 and 83.31 kJ.mol-1, respectively. For the LHHW model, the POM reaction was not considered due to the complete consumption of O2 during the reaction. Therefore, DRM, SRM, and WGS reaction were used for kinetic study. For deriving the rate expression, the surface reaction was considered as the rate-controlling and irreversible in nature. The rate expressions were solved by an ode23 solver in MATLAB in combination with the genetic algorithm optimization tools for the estimation of kinetic parameters. The obtained activation energy for DRM, SRM, and WGS reaction were 76.29, 117.16, and 67.33 kJ.mol-1, respectively.