Oral Defense Examination : Evaluation and Optimization of Biomass and Solid Waste Gasification System for Clean Energy Production: Reactor Modelling, System Integration, and Emission Measurement

Speaker Yao Zhiyi (Supervisor: Prof Wang Chi-Hwa)

Host Department of Chemical and Biomolecular Engineering

Date/Time 09 Feb - 09 Feb, 2.00PM

Venue E5-03-23 , Faculty of Engineering, National University of Singapore

Synopsis

Biomass is widespread, CO2-neutral energy source and it is considered as a potential substitute for fossil fuels due to increasing demand for clean energy. Among the plethora of biomass-to-energy technologies, gasification is an efficient and environmentally friendly technology to realize energy conversion from biomass.

In the first work, both experimental and numerical study were conducted to investigate the various aspects of interaction within a fixed bed downdraft gasifier. A coupled representative particle and fuel-bed model was developed based on a multi-scale approach to predict the production rate and quality of both syngas and biochar. Energy application and economic evaluation have been conducted based on the model outputs. Experimental work was also conducted on the fixed-bed downdraft gasifier results under different operating conditions (equivalence ratio, moisture content of feedstock, and air inlet location).

The second study proposed a hybrid two-stage municipal solid wastes (MSWs) disposal process and its feasibility study was carried out. At the first stage, anaerobic digestion of municipal solid waste was conducted by mixing with anaerobic sludge. At the second stage, gasification was added as post-treatment for AD residue to produce syngas. In this study drying process was found to play an important role in determining overall energy efficiency of the hybrid system. From an energy point of view, optimal moisture content of AD residue was found to be 30% in co-gasification of 80wt% wood chips and 20wt% AD residue. The highest energy efficiency could reach 70.72% under this operating condition.

Biomass gasification and pyrolysis are not emission-free technologies. Thermochemical treatment of biomass is found to be one of the most important sources of PM2.5 (particles of an aerodynamic diameter smaller than 2.5 μm) in both developed and developing countries. In the third work, we characterized the PM emission potential of the gasification and pyrolysis process. We proposed a particle respiratory deposition-based cyclone design scheme by combining particle-removing modelling (by cyclone) and particle respiratory deposition modelling. Sensitivity analysis showed that the cyclone collection efficiency was found to be most sensitive to the cyclone vortex finder diameter (D0). This work could potentially serve as the basis for proposing health protective measures against the particulate pollution from gasification and pyrolysis technologies.

The last work involved the measurement of particle number concentration, particle size distribution, and size-dependent chemical compositions at a bus stop, alongside a high way, and at an industrial site in a tropical city. Existing respiratory deposition models were employed to analyse the deposition distributions of particles and selected non-carcinogenic and carcinogenic elements in the human respiratory system with respect to some potential exposure scenarios related to bus stop, highway, and industry, respectively. Health risk assessment was conducted following the US EPA supplemented guidance to estimate the risk of inhalation exposure to the selected elements (i.e. Cr, Mn, Ni, Pb, Se, and Zn) in the scenarios.

Biography