Semester

Fall

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Chemical and Biomedical Engineering

Committee Chair

Jianli Hu

Committee Co-Chair

Xingbo Liu

Committee Member

Cosmin Dumitrescu

Committee Member

Debangsu Bhattacharyya

Committee Member

Jeremy Hardinger

Abstract

Novel biomass reforming strategy through synergistic co-processing with flare gas (methane and carbon dioxide) was developed at West Virginia University. Hardwood biomass comprised of lignin, hemicellulose, and cellulose is abundant in the US and potentially a sustainable source of hydrogen through extensive reforming and gasification. Ever-increasing shale gas production in the US occasionally leads to flaring owing to stranded production across the US. Achieving biomass co-processing with natural gas about to be flared in such stranded shale plays is the underlying motivation here. A novel reaction pathway was discovered wherein methane and carbon dioxide assisted reforming of biomass could be performed in a modular fashion to obtain hydrogen-rich syngas. Laboratory scale fixed bed reactor setup and bench scale bubbling fluidized bed setup were developed for showcasing the synergy in catalytic co-processing of ligno-cellulosic biomass with flare gas. Highly active metal-metal carbide dispersed catalyst supported on graphene (GNS) / carbon nanofibers (CNF) was developed and evaluated. Transition metal (Fe, Ni, Pd) doped Mo2C nanoclusters dispersed on GNS / CNF support showed considerably better activity than traditional ZSM-5 supported catalysts and showed excellent resistance to deactivation over multiple reaction cycles through self-regeneration. This laboratory-scale synergistic reforming approach was scaled up to a bench scale semi-continuous bubbling fluidized bed reactor. With a reactor bed ID of 1.5 inch, a fluidized bed reactor would allow for higher throughput of biomass with CH4 / CH4 – CO2 in the fluidizing gas. Lignin and ligno-cellulosic hardwood biomass were the two feedstocks chosen for reforming based on their higher oxygen content.

Reforming of lignin and hardwood biomass was performed through extensive hydrodeoxygenation (HDO) using methane as source of atomic hydrogen. In the initial investigation of methane (primary natural gas component) assisted synergistic biomass gasification was performed on iron-molybdenum (Fe-Mo) supported on ZSM-5 (zeolite) support. Non-oxidative gasification of biomass on the FeMo-ZSM-5 catalyst at 850oC and 950oC produced syngas with CH4 concentration >55%. In the absence of external oxygen source, thermal cracking of ligno-cellulosic components and reverse steam methane reforming (SMR) dominates reaction sequence. With the addition of 5 to 15 vol.% CH4 balanced with N2, high hydrogen concentration (>70%) in the syngas and high H2:CO ratio of 4 to 10 was obtained at 850oC and 950oC on FeMo-ZSM-5. However, the molar yield of syngas was low accompanied by low biomass conversion and higher coke deposition on the catalyst. Coke deposition increased with increasing CH4 concentration from 5 to 15 vol.% whereas H2:CO ratio decreased by almost 50% from 7.5 to 3.7 for reaction at 950oC.

Although higher H2:CO ratio in syngas is desired from biomass reforming, valuable carbon and oxygen in ligno-cellulosic biomass has to be obtained in the syngas to minimize the tar yield, coke formation, char residue, and unconverted carbon lost in the reactor. Methane assisted reforming was thus promoted with the addition of a small amount of CO2 (1 vol.%) in the reacting mixture. CO2 activation on Fe active sites in the hydrogen-dominated neighborhood was achieved to cause dry reforming of hydrogen to yield equimolar H2 and CO. The addition of CO2 along with 5% CH4 leads to synergistic bi-reforming of biomass which produces syngas with balanced H2:CO of 2 to 2.5. Novel catalyst development was performed in this study by replacing the zeolite ZSM-5 support with GNS / CNF support as mentioned above. This novel support was generated by pyrolysis of Fe, Mo impregnated biomass. Contrary to FeMo-ZSM-5, Mo existed as β-Mo2C and Fe as a mixture of Fe / Fe3C on the new support. This naturally synthesized metal carbide catalyst showed excellent resistance to deactivation by actively altering the coke deposition mechanism on the catalyst. Catalyst performance was thoroughly evaluated by performing 15 reaction cycles at temperature ranging from 800oC to 900oC for time-on-stream isothermal reaction for 2 hours. Fe loading was changed from 0.5 wt.% to 5 wt.% which predominantly lead to synthesis of graphene support with low Fe concentration and carbon nanofiber (CNF) support with higher Fe loading (2.5, 5 wt.%). Over 15 cycles, Fe / Fe3C – Mo2C – GNS / CNF support was discovered to regenerate after 10 to 15 cycles.

Fundamental investigation of methane assisted ligno-cellulosic biomass reforming was performed for molecular understanding of the underlying reaction mechanism and catalyst evaluation by replacing transition metal (Fe) dopant with Ni. Fe and Ni doped Mo2C-GNS catalysts are being on evaluated for p-cresol as a model feedstock representing the reformed oxygenated components in ligno-cellulosic biomass. CH4 assisted p-cresol reforming is being scaled up to alkali lignin and hardwood biomass. Experimental and density functional theory (DFT) based investigation is being used for micro-kinetic modelling of the reforming reactions on Fe-Mo2C-GNS and Ni-Mo2C-GNS. Lignin biomass is a major waste by-product from paper-pulp processing and constitutes a major component of the natural feedstock. Lignin reforming through flare gas co-processing is planned to be studied and scaled-up to the bench scale fluidized bed reactor setup along with biomass.

Process scale-up to bubbling fluidized bed reactor setup has been planned to showcase biomass – flare gas synergistic co-processing. A 1.5-inch ID bubbling fluidized bed reactor was designed and developed in collaboration with The Department of MAE department in WVU and National Energy Technology Laboratory (DOE), Morgantown. Preliminary experimental tests for investigation of high-temperature non-reactive fluidization hydrodynamics were performed. Important operation parameters for fluidization like minimum fluidization flowrate and optimum fluidizing gas flow rate were calculated by incorporating the effect of temperature. Minimum fluidization flowrate decreased with an increase in the reactor temperature from 200oC to 800oC by 60 to 70% of that at ambient conditions. Preliminary biomass and coal gasification tests were also performed on the BFB reactor to evaluate the performance of the fluidized bed against a fixed bed. The fluidized bed can be operated at higher efficiency over a fixed bed reactor if stable uniform fluidization is obtained. Stable fluidization was observed through pressure sensing at different locations in the BFB reactor bed along with overall bed pressure drop. Preliminary test results have provided the required groundwork for rapid scale-up of the CH4 / CH4 – CO2 assisted catalytic biomass reforming from lab scale fixed bed to bench scale fluidized bed reactor.

Lab-scale experimental investigation was also performed for non-catalytic and catalytic co-processing of waste plastics, southern pine biomass, and Illinois #6 coal. Biomass-coal-waste plastics gasification to syngas was performed over Fe-Mo2C-GNS / CNF catalyst and FeOOH-SO4 catalysts. Co-gasification of biomass, coal, and plastics has recently generated commercial interest for development of novel hydrogen production technologies with net negative CO2 emissions. Specific mix of waste plastics was selected for co-gasification with coal and biomass. High H2:CO ratio between 2 to 3 was obtained on the 0.5% Fe-4% Mo2C-GNS, 5% Fe-4% Mo2C-CNF, and FeOOH-SO4 catalysts.

Embargo Reason

Patent Pending

Available for download on Friday, December 03, 2021

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