A theoretical and experimental study on the pyrolysis of softwood sawmill residues to py-oil

Papari, Sadegh (2016) A theoretical and experimental study on the pyrolysis of softwood sawmill residues to py-oil. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Pyrolysis oil (py-oil) that is obtained from biomass, through thermochemical conversion in the absence of oxygen, is a possible sustainable source of renewable energy and for useful chemicals. Based on our existing infrastructure which is petroleum-based, py-oil can be an attractive alternative/blend for fossil fuel. The fuel application of py-oil can be limited by high water content, low heating value and high total acid number. Lab-scale and pilot-scale, pyrolysis experiments on forestry residues were performed to determine the impact of key parameters on py-oil yield and properties. The overall objectives of this study were to; determine the important operatorial factors and scale up of the pyrolysis of woody biomass, determine the range of conditions at lab scale for optimal py-oil yield and fuel properties, use these results to optimize the pilot scale auger unit, and develop a process model to simulate the process and be used as a design tool. In Chapter One, an overview on the first and the second generation of pyrolysis, the scopes, the objectives and the significance of this study along with a summary of the thesis chapters was outlined. In Chapter Two, the literature was reviewed to identify the impact of reactor operating conditions on py-oil yield and properties. The results indicated that the key parameters are a faster heating rate and a shorter vapour residence time which produce a higher py-oil yield (up to 75 wt.%) with a higher heating value (up to 22 kJ/kg). In addition, the published empirical and process models for pyrolysis of woody biomass were investigated in order to better understand the applied heat/mass transfer equations, assumptions, kinetic models, and the method of solution. The reported kinetic models in literature were compared with our experimental data obtained from the lab-scale reactor in order to find the ―best‖ model. In Chapter Three, the impact of three process parameters including temperature, N2 flow rate, and biomass particle size were investigated on py-oil yield and water content using response surface methodology coupled with central composite design (RSM-CCD) in a lab-scale tube furnace reactor. The results indicated that a 500-550 ᵒC temperature, a 500 mL/min N₂ flow rate, and a 0.1-0.5 mm particle size produced the optimum oil for the lab-scale reactor. The quadratic CCD model with factor interactions better predicted the experimental data compared to the quadratic model without parameter interactions. In addition, the results showed that the secondary tar cracking should be included in pyrolysis reactions at a temperature higher than 550 ᵒC, since some condensable organics convert to non-condensable gases by these reactions. After finding the optimum conditions of the lab-scale tube furnace pyrolysis reactor in Chapter Three, the impact of feedstock quality (particle size, moisture content, and age of feedstock) on py-oil yield, higher heating value (HHV), total acid number (TAN), and water content was investigated in the lab-scale reactor (Chapter Four). The results illustrated that the initial moisture content has a little effect on the water chemically produced during pyrolysis. Particle size reduction did not have a significant effect on HHV. The aged feedstock produced a slightly lower py-oil yield and higher water content compared to the fresh feedstock. In addition, a qualitative assessment of the pyrolysis heat of reaction was performed in the lab-scale reactor. The results illustrate the overall endothermic nature of the pyrolysis of this type of biomass (balsam fir wood). This result was helpful in next Chapter (i.e. process modeling). In Chapter Five, a process model was developed for the 2-4 kg/h auger reactor with assuming plug flow model for both solid and gas phases. Process modeling is typically used as a tool in process optimization, scale up, and reactor design to reduce the capital and operating cost of a pyrolysis system. The transport equations for each phase are combined with the kinetic model to predict py-oil, bio-char, and non-condensable gas yields. The process model was validated with the experimental data obtained from this reactor and showed good agreement (with approximately 10% average relative deviation). The model was used to predict py-oil yield as a function of temperature, feed flow rate and reactor pressure. In Chapter Six, the impact of process variables (temperature, feed flow rate, and vacuum fan speed) on py-oil yield, water content, and more importantly phase separation were investigated in the 2-4 kg/h auger reactor. In the optimum conditions (a 450-475 ᵒC temperature, a 2415 rpm vacuum fan speed, and a 4 kg/h feed flow rate) a single phase softwood oil was obtained with 53 wt.% yield and 26 wt.% water content. A comparison between different sawmill residues (softwood shavings, hardwood sawdust, and softwood bark) at similar conditions showed that hardwood and softwood produced a single phase oil with a higher oil yield (53-55 wt.%) and a lower water content (25-26 wt.%) compared to bark (39 wt.% oil yield and a 33 wt.% water content).

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