In the study of thermochemical reactions, the term pyrolysis refers to the multiplicity of thermal breakdown reactions that take place, when substrates are heated without adding oxygen. Pyrolysis is often used as a key processing stage in the conversion of solid fuels to gaseous and liquid fuels or chemical feedstocks. Pyrolytic reactions, therefore, remain the focus of much research activity.
The highly reactive nature of primary products released during thermal breakdown is a distinguishing factor of pyrolytic reactions. Resultant volatile products may undergo secondary reactions with each other, or crack “in flight” to produce lighter components, or react with the pyrolyzing solid matrix. Heavier volatiles (“tars/oils”) may condense on solid surfaces in the reaction zone to form an amorphous secondary char. Thus, product distributions may be significantly altered by changes in sample and reactor configuration, as well as through the selection of key reaction parameters, such as temperature, heating rate and gas flow profiles through the reactor. In pyrolytic work, it is vitally important to recognize that the designer must maintain the ability to promote or, alternatively, suppress these secondary effects, depending on the desired outcome of the process. The design of pyrolytic processes requires, therefore, unpicking a complicated matrix of physical and chemical factors, in order to evaluate and, indeed, to anticipate eventual product distributions.
Clearly, pyrolysis is a well worked field, with antecedents going back to the pre-historic charcoal burner. However, the multiplicity of factors contributing to final outcomes inevitably lead to a plethora of experimental results that do not often fit into neat, compact patterns. To address the problem, a team of researchers from Queen Mary University of London, London, UK: Meredith Rose Barr and Dr. Roberto Volpe together with Professor Rafael Kandiyoti at Imperial College London have attempted to bring a degree of order into this complex design problem. In this analysis, comparing results from different types of reactors was used as a principal tool in evaluating the role of particular design elements under similar experimental conditions, mostly relying on experiments performed in their own laboratories over a period of nearly three decades. The work was recently published in the research journal, ACS Sustainable Chemistry & Engineering.
Surprisingly large differences were recorded when results from different reactor configurations were compared under similar experimental conditions. In the case of pure cellulose, char yields from pyrolysis experiments have been reported to vary between 1 and 26%, as a function of changes in reactor design and associated operating parameters. Another set of data showed how secondary reactions of primary pyrolysis products directly affect sample weight losses determined in thermogravimetric balances, leading to errors originating from equipment design.
In attempting to assist further research, the authors also focused on identifying the ranges of experimental conditions where different reactor types provide more dependable data. In the temperature interval between 300 and 550 °C, where most biomass thermal breakdown takes place, the efficiency of heat transfer between sample and reactor components was found to be a key parameter. Compared to challenges encountered in using wire-mesh reactors and thermogravimetric balances in this temperature range, fluidized-bed reactors appear to owe their greater reliability in sample weight loss (total volatile) determinations, to intimate solid-solid contact within the fluidizing bed itself.
In summary, the study examined the relationships between experimental design and the outcomes of pyrolysis experiments, presenting detailed discussions of salient points and basic concepts that contribute to the design of pyrolysis reactors. The authors also showed how different types of reactors may prove more useful than others in teasing out the fundamental thermal responses of particular samples over specific temperature ranges.
Meredith Rose Barr, Roberto Volpe, Rafael Kandiyoti. Influence of Reactor Design on Product Distributions from Biomass Pyrolysis. ACS Sustainable Chemistry & Engineering 2019, volume 7, page 13734−13745.