the interplay between chemistry and transport phenomena during the fast pyrolysis of cellulose, lignin and biomass
Pushkar Marathe is a PhD student in the research group Sustainable Process Technology. His supervisor is prof.dr. S.R.A. Kersten from the faculty of Science and Technology.
Depleting fossil energy resources and the changes in climatic conditions have propelled humanity to find alternative sustainable resources to satisfy the increasing demand. Lignocellulosic biomass is one of the carbon-based renewable resources which can be used to produce fuels and chemicals. Fast pyrolysis is one of the promising technologies in which lignocellulosic biomass is decomposed at ~500 °C and in the absence of oxygen, to produce oil (~75%), char (~12%) and gas (~13%). Pyrolysis oil can be upgraded to fuel or platform chemicals via downstream processing.
Literature suggests that the yields of products and their composition obtained during the pyrolysis are affected by the competition between transport phenomena and chemistry. The research described in this thesis aims at advancing the current understanding of the three simultaneously occurring processes, viz. chemical reactions (decomposition, cracking, polymerisation), heat transfer and mass transfer (evaporation, sublimation, random ejection), and their interplay during the fast pyrolysis of cellulose, lignin and lignocellulosic biomass.
For that, a dedicated screen-heater reactor was used, which was designed: 1) to minimise non-isothermality (heating rate: ~5000 °C s-1), 2) to control the reaction time inside the reacting particle by the escape rate of compounds from the reaction zone by variation of the pressure, and 3) to minimise reactions outside the reacting particle by minimising hot vapour residence time (~20 ms) and fast quenching of the products (~-180 °C). Experiments were also carried out in a bench-scale 1 kg h-1 fluidised bed unit equipped with a fractional condensation system to investigate the effect of hot vapour residence time (~2 s).
Qualitative and quantitative characterisation of pyrolysis products was done using various analytical tools. In order to understand the reaction mechanisms, quantitative analysis of species present in pyrolysis oil is necessary. The suitability of gas chromatography (GC) and liquid chromatography (LC) for the quantification of levoglucosan (LG) and hydroxyacetaldehyde (HA) was evaluated. It was found that both GC and LC can principally determine LG quantitatively in pyrolysis oils. However, depolymerisation of oligo-anhydrosugars in GC owns a risk of overestimation of the LG yields. HA can only be determined quantitatively by LC because of its reactions during the high temperature (~250 °C) injection in GC.
It was found that in the absence of potassium salts, cellulose could almost entirely be converted to anhydrosugars while producing hardly any gas (<1%) and char (<1%) in a wide temperature range of 450 to 765 °C. Depolymerisation of cellulose to anhydrosugars was identified to be a true primary reaction; gas and char formation secondary. Mathematical models were developed, including the interaction between chemistry, heat transfer and mass transfer. The escape rate of products from the hot reacting particle was identified as a crucial process affecting the DP distribution of anhydrosugars.
The potassium concentration in cellulose was varied to mimic the mineral-rich and pre-treated feedstock and subsequently pyrolysed at 530 °C in screen-heater and fluidised bed. Potassium was found to be catalytically active even when the escape rate of the product away from the reaction front was extremely high (milliseconds). The yields of oil and anhydrosugars decreased significantly as a function of potassium concentration, while the production of other products (water, gases, light oxygenated compounds) was enhanced. The production of char was found to be independent of the escape rate of products at any given potassium concentration. It could be concluded that pyrolysis at reduced pressure, i.e. by fast removal of products from the hot reaction zone, can improve the oil and sugar yields, but only for low alkali and alkaline earth metals content feedstock.
The effect of molecular weight on the competing physio-chemical processes was investigated by pyrolysing 14 lignins (350 – 1900 Da) in the screen-heater at 0.5 kPa and 100 kPa. A population balance model was developed, which includes simultaneously occurring cracking reactions, polymerisation reactions and mass transport away from the reaction zone. The model was able to predict all experimentally observed trends after parametrisation. The molecular weight distribution was found to be one of the crucial characteristics of the lignin feedstock, which has a significant influence on the pyrolysis product distribution. Upwards 530 °C, the temperature turned out to have only a minor influence on the yields and composition of the oils produced, whereas the system pressure was identified as the main steering wheel to manipulate the product yields and the molecular weight of the oils. HSQC NMR analysis of lignins and its oils showed that the yields of oil and char and the number average molecular weight of oils were found to be independent of the number of ether linkages in the lignin feedstock.
Fast pyrolysis of acid-leached bagasse was carried out in the pressure range of 0.005 to 100 kPa in screen-heater. At the lowest pressure, the total yield of C6-anhydrosugars (sum of DP1 to DP6) was as high as 73% of the poly-C6-sugars in the feedstock. A mathematical model, including again reaction and mass transfer away from the reaction zone, was able to predict the measured decrease in a total yield of C6-anhydrosugars and the shift to lighter C6-anhydrosugars as a function of increasing pressure. At identical pressure and temperature, the total yield of C6-anhydrosugars obtained from acid-leached pinewood was the same for the screen-heater and fluidised bed. As a result of the longer hot vapour residence time in the fluidised bed, DP≥2 C6-anhydrosugars depolymerised towards DP1.
In nutshell, besides the heating rate of sample, hot vapour residence time of products and the temperature during the pyrolysis, the system pressure is the key parameter, which alters the residence time of products in/on the hot reacting particle, thereby, providing a means to steer the yields and composition of the products of pyrolysis.