Theoretical and experimental analysis of biomass gasification processes using the attainable region theory

Abstract

There are limits on performance of processes and reactions set by material balances and by thermodynamics. The interaction of these theoretical limits and how they influence the behaviour of reactions and equipment is of interest to researchers and designers. This thesis looks at the conversion of biomass to gaseous products under various conditions, including a range of temperatures from ambient to 1500 ⁰C and in the presence or absence of oxygen. The limits of performance of the material balance can be represented as an Attainable Region (AR) in composition or extent space; we call this the MB-AR. The MB-AR represents all possible material balances that can be achieved for a given a set of feeds and set of possible products. The dimension of this space depends on the number of independent material balances. The extreme points of the MB-AR are of particular interest as these define the limiting compositions and the edges of the boundary of the MB-AR represent the limiting material balances. The MB-AR does not depend on temperature. The thermodynamic limits of performance of can be represented as an AR in the space of Gibbs Free Energy (G) and Enthalpy (H); this is called the G-H AR. The G-H AR is always two dimensional, no matter what the dimension of the MB-AR. Extreme points in the G-H AR are also extreme points in the MB-AR are; however not all extreme points in the MB-AR are extreme points in the G-H AR. The extreme points in the MB-AR are transformed by calculating G and H of the points at the condition of interest (reaction temperature and pressure). It is then necessary to find the convex hull in G-H space of this set of transformed points which gives us the boundary of the G-H AR. The extreme points in the G-H AR can be associated with material balances and the extreme point with the minimum G represents the global equilibrium or equivalently the most favoured material balance for the system. The edges of G-H AR are defined by the lines between neighbouring extreme points in the boundary of the G-H AR. These edges represent the limiting material balances in terms of defining the extremes of the G and H of the system. The G-H AR depends on the feed and products through the MB-AR, but also depends on temperature (and pressure). The set of points which are extreme points of both the MB-AR and the G-H AR changes with temperature. Geometrically, the transformed set of extreme points for the MB-AR moves in the GH space as temperature is changed and they move at different rates. Hence when finding the convex hull in the G-H space of the transformed extreme points of the MB-AR, G-H points become either boundary (extreme) points or move into the convex hull at different temperatures. Thus, the material balance which corresponds to the global minimum in G may change with temperature, as do the material balances which are associated with the edges of the G-H AR. Experiments are performed on biomass anaerobically at ambient temperature using microbes as the catalyst, and the products of this process are called biogas. The experiments were performed in a nitrogen plasma system on biomass at higher temperatures (400 ⁰C to 1000 ⁰C) also in the absence of oxygen, and this process would typically be referred to as pyrolysis. Oxygen was added to the plasma system and operated at temperatures between 700 ⁰C and 900 ⁰C, and this would typically be referred to as gasification. Thus, it was able to change the MB-AR by presence or absence of oxygen. By changing operating temperatures, the G-H AR is effectively changed with either the same or different MB-AR’s. The experiments show that in all cases, the product tends towards minimum G. Although this might not be surprising at the higher temperatures, minimizing G is not thought to be the driving force in microbial systems. An important insight from this is that if one were to try and make hydrogen only in a biological system, the system would need to have organisms that make hydrogen only. This is because the material balance that produces hydrogen has a lower change in G than the material balance that make methane. Thus, if there was a consortium of organisms and some of them could make methane, the methane producing organisms would dominate as they have the higher Gibbs Free Energy driving force. If the boundary of the G-H AR around the minimum G is fairly flat, or if many of the extreme points of the MB-AR lie close to the minimum G in the boundary of the G-H AR, then there are many material balances that will give the same G and H. Thus, there are a range of compositions with similar G and H and how one approaches the minimum G will determine the chemical composition of the product. This has important implications for the design, scale up and operation of equipment if a particular product is desired rather process efficiency. The low temperature anaerobic route to gasifying waste, using microbes as catalysts, has a very simple G-H AR, and the preferred products are CH4 and CO2, known as biogas. These units should be relatively stable to operate as none of the other products have G’s that are as negative as that of the biogas. Although not part of this thesis, small-scale anaerobic digesters were installed in communities and these do run easily and stably with fairly little intervention from the operator which seems to support our conclusion. We however could ask, why then have simple technologies, such an anaerobic digestion, not been widely adopted in Africa? To this end we worked with communities and spoke to people about their knowledge about the technology, their concerns and their possible interest in using new approaches to supply energy for cooking and lighting. We found that people were not aware of the technology but would be very interested in adopting a technology that supplied energy cheaply. To our surprise however, their major concern was around hygiene and safety, in that if the gas was made from “poo” how could the gas be clean and would cooking with it not contaminate the food and make people sick? This in hindsight is a very reasonable concern, although it had never occurred to us that this would be a perception. Engineers will have to work with social scientists and psychologist, amongst others, to address the concerns and needs of communities in order for sustainable technologies to be successfully adopted by communities. In summary, this thesis presents a tool for analysing biomass conversion to gaseous products in general, whether microbial or thermal. This tool gives insight into what is achievable, what the major factors are that affect the favoured product and how this can be manipulated to improve efficiency from an overall material and energy point of view.