Multiscale simulation of methane assisted fluidized bed biomass gasification

• Main Author: Bates, Richard Burton
• Other Authors: Ahmed F. Ghoniem.
• Format: Others
• Language: English
• Published: Massachusetts Institute of Technology 2017
• Subjects: Mechanical Engineering.
• Online Access: http://hdl.handle.net/1721.1/106789

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016. === Cataloged from PDF version of thesis. === Includes bibliographical references (pages 195-218). === Owing to increasing concerns that climate change poses an urgent threat to the existence of human society, there is a need to develop cost-effective and scalable technologies to produce renewable, drop-in transportation fuels. Fluidized bed biomass gasification (FBBG) is one of the most promising options for the thermochemical conversion of lignocellulosic biomass to synthetic liquid fuels. When biomass is introduced into the high temperature bed (700-900 °C), it rapidly devolatilizes and subsequently reacts with steam, carbon dioxide, and oxygen to form syngas (hydrogen, carbon monoxide) as well as a complex assortment of light gases and condensable compounds known as tar. The main technical challenges facing FBBG technologies are incomplete char conversion and generation of polycyclic aromatic hydrocarbons (PAH's), which require expensive cleanup steps to avoid downstream operational issues. Existing approaches to optimize the performance of FBBG have examined the manipulation of operational parameters such as temperature, pressure, in-bed additives, steam to carbon ratio (SCR), and air fuel equivalence ratio (ER). However, the optimization of FBBG through experimental studies has proven difficult because the extremely complex, coupled, physical and chemical phenomena obscure the actual causal mechanisms. Prior modeling efforts are deficient in several key areas including gas-phase chemistry and char conversion processes, rendering them unable to conclusively determine operating conditions which achieve high cold gas efficiency and complete char/tar conversion. The first part of this work describes the development of a flexible, modular, robust, coupled reactor network model (CRNM) enabling the steady-state simulation of a variety of feedstocks over a wide-range of conditions. The CRNM consists of three independently validated and parameterized sub-models that consider i) particle devolatilization, ii) char conversion, and iii) hydrodynamics and homogeneous reaction kinetics. For each sub-module, the dominant physico-chemical processes and modeling assumptions are identified using characteristic time-scale analyses. The proposed char conversion model describes simultaneous and competing particle-scale processes including gasification, combustion, inhibition, intra/extra particle mass transfer, attrition, and elutriation both under transient and steady-state conditions. Bed hydrodynamics is described using the two-phase theory of fluidization resulting in a network of idealized reactors. This enables the efficient solution of comprehensive gas-phase kinetics mechanisms (327 species and 10933 reactions). The second part of this study validates the CRNM by comparing its results with data from lab-scale steam/air blown gasification experiments performed in collaboration with the National Renewable Energy Laboratory (NREL) and the MIT Chemical Engineering Practice School. The experimental results show that the composition of tar is highly sensitive to the addition of air/oxygen, which appears to accelerate the conversion of lighter PAH's into soot precursors at a fixed operating temperature. Experimental data and modeling results agree that the char reacts with very significant fraction of air/oxygen, improving its overall conversion drastically and reducing the steady state bed inventory of char. The validated model is used to carry out a constrained parametric analysis and optimization of the key operating variables, feed location, and fluidizing agent options. Standalone biomass gasification with steam and air tends to result in a syngas with low H2:CO ratio (</=1). The addition of steam improves the hydrogen content and reduces tars slightly; however, complete conversion of the methane and tar compounds (>99%) is ultimately only possible if sufficient secondary air is injected into the freeboard to raise its temperature above 1300 °C. The modeling results demonstrate that methane and biomass act synergistically in the gasifier: the addition of methane acts to significantly improve the carbon yield and energy content of the syngas while the catalytic impact of minerals contained in the biomass act to promote the water-gas shift reaction in the bed region. === by Richard Burton Bates. === Ph. D.