| dc.description.abstract | Climate change has become one of the significant challenges of the 21st century resulting in increased demand for low-carbon energy carriers to achieve a sustainable future. Thermo-chemical conversion of environmentally benign biogenic sources to syngas (containing H2, CH4, CO, and CO2) and its subsequent adsorption-based separation offers a potential route to generate hydrogen and methanol, two carriers having multiple uses spanning across various sectors. The current work reports the results of experimental investigations and analysis adopted to separate low-pressure (near-ambient) multi-species bio-syngas for two specific end gas requirements: a) proton exchange membrane fuel cell (PEMFC) compliant hydrogen and b) module [M= (H2-CO2)/(CO+CO2)molar ~ 2.05] adjusted gas mixture for catalytic methanol synthesis.
The research has been carried out in three stages, with the first involving fundamental studies conducted in a practical near-adiabatic environment to measure column breakthrough dynamics and thermodynamic interactions of bio-syngas adsorption on commercial zeolites at varied operating conditions. During operation, zeolite 13X showed excellent hydrogen purification characteristics and completely restricted the propagation of impurities (CH4, CO, and CO2) in the region of interest. Similarly, zeolite 4A selectively captures CO2 and results in adjusting M to around 2.05. The results explain the interaction of multiple adsorbates in a competitive adsorption environment and clearly indicate the presence of roll-ups. This largely suppresses the competitive equilibrium loading of CH4 (by 98.6-99.2%) and CO (by 97.2-98.1%) and necessitates the use of dynamic desorption experiments for accurate assessments. Regarding this, two sweep gases – He and CO2, are thoroughly tested, and the advantages and limitations of using each have been underlined. The experimentally obtained competitive loadings are also compared with the predicted values derived from the extended Langmuir model, and a detailed discussion has been made. Additionally, as part of the parametric analysis, the breakthrough time, breakthrough and effective loading, adsorbate propagation rate, and competitive selectivities are analyzed and discussed. Finally, a general methodology, independent of the upstream process, is proposed for designing hydrogen purification systems. The novel approach adopts total effective working capacity as the design basis and considers the competitive and non-isothermal nature of adsorption.
In the next stage, to understand process dynamics and challenges working under real-operating conditions, a fixed adsorber is integrated to a laboratory scale downdraft biomass gasifier producing hydrogen rich (40-50 mol%) syngas. The respective adsorbents (zeolite 13X and zeolite 4A) are noted to successfully generate high purity hydrogen and M adjusted gas mixture and also restrict the propagation of nearly all contaminants, reducing the content of higher molecular weight, sulfur, and halogenated compounds to < 9.25 ppm, < 0.6 ppm, and < 1.5 ppm, respectively. Further, to improve separation process efficiency and zeolite lifetime, biomass-based granular activated carbon (AC) is tested as a potential cleaning element at the upstream of the separation unit. The AC showed high contaminant removal efficiency (≥ 90%), and demonstrated a notable regain of 78% and 73%, respectively, in its surface area and pore volume upon its thermochemical regeneration.
Lastly, in stage III, based on the outcomes of stage I and II, a low-pressure cyclic separation system is developed for continuous hydrogen production. The development is first carried out at a laboratory scale [feed flow rate (Qin): 4.8-7.2 Nm3/h], where a SCADA-PLC controlled vacuum pressure swing adsorption (VPSA) system working on a modified eight step Skarstrom cycle is developed and characterized using two practical biogenic feed streams. The characterization includes studying the effect of both extrinsic and intrinsic decision variables on key performance indicators. The assessment concludes adsorption pressure and feed flow rate as the main influencing factors and establishes N2 as the limiting gas impurity and CO2 desorption as the rate-determining step. Additionally, Pareto curves signifying the process capability to produce hydrogen of different grades in accordance with ISO 14687:2019 are generated. Subsequently, the learnings are extended to develop a pilot scale (Qin: 100 Nm3/h) separation system considering the coupling of adsorbent-process-operating conditions, using conventional scaling-up laws and safety elements, and incorporating a higher cyclic pressure ratio. The preliminary trials indicate encouraging operation with the continuous generation of high-purity hydrogen (99.99 ± 0.50 mol%) at an average recovery of 68.4 ± 3.5%. Further, the pilot system is tested for long-operating hours (~1500 cycles), where it displayed complete operational repeatability with textural properties and crystalline structure of the used adsorbent staying intact as compared to its virgin form (< 6% deviation). Moreover, the contaminant analysis performed on the produced hydrogen using a range of precision instruments confirms its Type I/II grade D quality and thus qualifies it entirely for PEMFC road vehicle application.
As an extension to the current work, a medical-grade oxygen (MO2) generator is developed under the aegis of IISc’s response to the COVID-19 pandemic during 2020-21. The workability of different adsorbents like Li-LSX and NaX is established for air separation at the fundamental scale, and a flexible oxygen purification unit capable of working in PSA and VPSA modes of operation is developed. The system performance is tested under steady and transient operating conditions, and several challenges faced in an industrial environment, mainly pertaining to drop in product purity and flow rate and adsorbent pulverization, are resolved through appropriate interventions. As an outcome, the detailed design, drawings and component information for generating 50 SLPM MO2 are released as an open source. Additionally, the technology at higher throughputs has been transferred to several licensees, and many plants with capacities ranging from 3-30 Nm3-MO2/h have been installed across India, which are running successfully.
Summarizing, the present work, for the first time, establishes and comprehensively examines the low-pressure adsorptive separation route to generate green fuels from feed streams generated using renewable and relatively abundant source of biomass. Although the focus has been on a multi-specie gas mixture from a near-ambient downdraft biomass gasification unit, the outcomes can be applied to any feed mixture obtained from different technologies, including anaerobic digestion, pyrolysis, reforming, etc. | en_US |
