PYROLYSIS KINETICS OF BIOMASS FEEDSTOCKS AND BIO-OIL PRODUCTION

Abstract

Due to scarcity of fossil fuels and greenhouse gas emissions, during its combustion, renewable and cleaner energy sources are being investigated around the world in recent years to meet the energy security and environmental sustainability. Biomass is the only renewable energy source that can compete with fossil fuels in terms of production of fuels, materials and chemicals. It is non-toxic, carbon neutral, bio-degradable and abundantly available. Important types of biomass include crops as well as forest, agricultural and industrial wastes. For the utilization of crops, systematic procedures are normally followed, however, the other types are treated as waste and not used in suitable manner for useful applications. Pine needles (forest residue), sugarcane bagasse (agricultural waste) and wood sawdust (industrial waste) are three abundantly available biomass in India. Pyrolysis is the most promising thermochemical conversion process for conversion of biomass/organic materials into biofuels. It is advancing at a rapid rate and has promising potential for commercialization. It converts biomass/organic materials into bio-oil (liquid), bio-char (solid) and gases (pyro gas) by heating in inert atmosphere. The pyrolysis products can be utilized in different ways. Bio-oil can be used as a renewable energy source for energy production as it has higher energy density than biomass. In addition, it can be upgraded to the refined fuel and also be served as raw material for useful chemicals production. Bio-char has high carbon content with higher calorific value than biomass. It can be used as source of energy. In addition, it can also be utilized as a precursor for activated carbon production. In recent years, application of bio-char has gained enormous attention as it can be used as a fertilizer when mixed with soil. It also enhances the quality of soils by increasing the retention and availability of water and nutrients in soil. Pyro gas mainly consists of H2, CO, CO2, CH4 and other light hydrocarbons. It can be used to partially fulfill the energy requirements for the pyrolysis or can be used for natural gas or syngas production. The compositions and yields of pyrolysis products are highly dependent on nature of feedstock and their particle size, temperature, heating rate, reactor types and vapor residence time. Biomass pyrolysis is a complex process that comprises different chemical and physical processes in the form of parallel as well as competing reactions. Thus, physicochemical ii characteristics of biomass and its pyrolysis kinetics are important to model, design and develop a biomass conversion system through pyrolysis. Kinetic parameters of pyrolysis process are also important for predicting the behavior of reactions and optimization of the process to achieve desired product distribution. Thermogravimetric analysis (TGA) is the most common method used for kinetic analysis of pyrolysis process. There are many methods used for evaluating the pyrolysis kinetic parameters of biomass/ waste from TGA data. These methods are of two types such as model-fitting and model-free methods. In the model fitting methods, data are fitted with different models for acquiring the best statistical model from which the various kinetic parameters are calculated. These methods have ability to directly find out the kinetic parameters from a single TGA curve. Instead of model-free methods require TGA curves at different heating rates to find the kinetic parameters without taking any assumptions about reaction function. Recently, some articles have been published on the evaluation of the pyrolysis kinetic parameters (activation energy, pre-exponential factor and reaction order) of different biomass from TGA data using iso-conversional model-free Kissinger-Akahira-Sunose (KAS) and Ozawa-Flynn- Wall (OFW) methods as well as model- fitting Coats-Redfern methods. However, there is hardly any literature available on kinetic evaluation of pine needles (PN), sugarcane bagasse (SB) and wood sawdust (WSD) pyrolysis using KAS, OFW and Coasts-Redforn methods. Different types of batch, semi batch and continuous reactors have been used for pyrolysis of different biomass. Continuous reactor can give more bio-oil production rate per unit reactor volume than batch and semi batch reactor. However, complexity of operation is maximum in continuous reactor and design of continuous reactor requires more understanding of the process. Moreover, when a new biomass is used, batch and semi batch rectors are utilized to investigate the process characteristics and generate data for kinetics and reactor design. However, the pyrolysis of PN, SB and WSD in a fixed bed semi batch reactor especially to study the effect of pyrolysis parameters on products yield, complete characterization of products as well as their application potentials are also less reported. Activation of PN, SB and WSD bio chars is also rare in literature. In the present investigation the physicochemical characteristics and fuel properties of PN, SB and WSD such as proximate composition, elemental composition, heating value, lignocellulosic composition, particle size, bulk density, surface functionality, crystalline nature and surface morphology have been determined to understand their suitability for use as pyrolysis iii feedstock(s). Results reveals that PN, SB and WSD have low moisture (5.03-5.87 wt.%), ash (3.1-3.78 wt.%), nitrogen (0.1-0.45 wt.%) and sulphur (0.06-0.07 wt.%) content as well as considerable amount of carbon (44.86-46.38 wt.%) and high volatile matter (75.87-80.20 wt.%) content, which suggest that these biomass (PN, SB and WSD) have good potential for production of bio-oil through pyrolysis. The HHV 19.22 MJ/kg for PN, 17.94 MJ/kg for SB and 18.34 MJ/kg for WSD is comparable to the HHV of other biomass currently used to generate renewable energy and to obtain liquid fuel through pyrolysis. Thermal degradation behaviour of PN, SB and WSD shows that the main devolatilization of these biomass occurs in the temperature range of 200 oC-500 oC. The kinetic analysis results found that the average activation energy calculated from KAS and OFW methods for PN are 70.97 and 79.13 kJ/mol, respectively; for SB these values are 91.64 and 104.43 kJ/mol, respectively, and for WSD these are 164.24 and 173.41 kJ/mol, respectively. This shows that WSD is more thermally stable than PN and SB. The value of activation energy varies with degree of conversion for all three biomass. The degree of conversion of PN, SB and WSD on heat treatment, computed by Coats-Redfern method, is found to be in good agreement with experimental conversion for all the three biomass within error limit of 3.2 % to 11.6 %. The pyrolysis of PN, SB and WSD has been performed to produce bio-oil, bio-char and pyro gas. Studies have been conducted in a fixed bed reactor operated in semi batch mode to find out the effects of process parameters such as temperature, heating rate, biomass particle size and N2 flow rate on products yield as well as to find out the optimum process conditions. Maximum bio-oil yield of 44.16 wt.% is found for PN pyrolysis at optimum conditions temperature 550 oC, heating rate 50 oC/min, N2 flow rate 100 cm3/min and biomass particle size range 0.6 < dp >1 mm. For SB pyrolysis, maximum bio-oil yield of 45.23 wt.%, is achieved at optimum temperature 500 oC, heating rate 50 oC/min, N2 flow rate 100 cm3/min and biomass particle size range 0.5 < dp >0.6 mm. Whereas, for WSD pyrolysis, maximum bio-oil yield of 41.39 wt.% is found, at optimum temperature 500 oC, heating rate 50 oC/min, N2 flow rate 100 cm3/min and biomass particle size range 0.6 < dp >1 mm. The yield of bio-oil initially increases with increase in temperature, reaches maximum value at the intermediate temperature of 500-550 oC and decreases thereafter, gaseous products yield increases continuously with temperature, however the bio-char yield decreases with iv increase in temperature for all the three biomass. The effect of particle size signifies that the biochar yield increases and gaseous products yield decreases with increase in biomass particle size. Whereas the yield of bio-oil is not significantly affected by the biomass particle size although maximum bio-oil yield is found at intermediate biomass particles size. As N2 flow rate increases the yield of gaseous products increases whereas bio-char yield decreases; bio-oil yield initially increases with N2 flow rate, achieves optimum value and decreases thereafter at higher N2 flow rate. The pyrolysis products (bio-oil, pyro gas and bio-char) have been characterized and assessed for their utilization potential. The properties of bio-oils are evaluated by the ultimate, FTIR, GC-MS and 1H NMR analysis. Further, calorific value, ash and water content as well as pH of bio-oils are determined. Other properties of bio-oils such as density, kinematic viscosity flash point, pour point and Conradson carbon residue are also determined. Bio-chars have been characterized through the proximate and ultimate analysis, calorific value determination, thermal analysis, pH, FTIR, XRD, EF-SEM analysis, BET surface area and pore volume determination. The composition of pyro gas is analyzed by the GC. Results demonstrates that the bio-oils have higher carbon and lower oxygen content as compare to their respective biomass. The empirical formula of bio-oils are found as CH1.52O0.40N0.012, CH1.31O0.33N0.009 and CH1.52O0.47N0.011 for PN, SB and WSD bio-oil, respectively. The bio-oils have high energy contents of 23-26 MJ/kg hence, these can be combusted in existing heating application systems. The bio-oils are found to have high oxygen of 28-35 % and water content of 17-22 %, thus further up-gradation and refining is required to meet the desired quality similar to diesel and heavy fuel oil. The present bio-oils are found acidic in nature due to its low pH (2.8-3.6). The carbon distribution range of volatile compounds present in the bio-oils are found as C4-C15 for PN bio-oil, C4-C15 for SB bio-oil, C5- C12 for WSD bio-oil, which covers the carbon range of gasoline and diesel. Further, these can be used as feedstocks for chemicals production as phenols and furfural are present in these. The spectroscopic and chromatographic analysis of bio-oils suggest the presence of aliphatic, olefinic and aromatic as well as oxygenated, nitrogenous compounds in the bio-oils. Elemental analysis of the bio-chars reveals that these have high carbon (63-69 wt.%) and low nitrogen (0.72-1.02 wt.%) and sulfur (0.01-0.03 wt.%) content with HHV of 22-25 MJ/kg, thus these can be utilized as a solid fuel and precursor for activated carbon production. The present bio-chars are found alkaline in nature due to high pH (8.1-8.8). Thus, these can be used for soil amendment to v neutralize soil acidity as well as to improve the soil quality and raise the crop productivity. Thermal analysis of bio-chars in oxidizing atmosphere suggests that these can be utilized as solid fuel for energy generation through combustion. FTIR analysis of bio-chars shows the presence of more aromatic compounds, hence these are suitable for carbon sequestration. Surface morphology of bio-chars reveals the presence of various heterogeneous pores and rough texture of bio-chars. The EDX analysis confirms that the bio-chars have various inorganic matters such as N, K, Na, Mg, Al, Si, Fe and Ca, which are the important nutrients for soil fertility. Chemical activation of all the three bio-chars enhances their carbon content by 12 - 20 %. However, the BET surface area and pore volume of activated bio-chars increase 25-75 times and 3-9 times, respectively, due to chemical activation. Thus, the activated bio-chars can be utilized as adsorbent or catalyst.