Property analysis of pyrolysis product from cotton stalk heating by microwave

Abstract: The use of microwaves for heating is well established in society, and is being used in domestic and some industrial processes. However, it has the potential to be introduced and applied to many other industrial heating processes, which offers unique advantages not attained from conventional heating. In this sense, microwave technology was being explored as one method to assist in pyrolysis process of cotton stalk. A study was carried out on the effect of microwave on the liquid and solid products, then the physicochemical properties of pyrolysis products were analyzed by gas chromatography and mass spectrometry (GC-MS), isothermal adsorption analyzer (BET) and fourier transform infrared analyzer (FT-IR). The results showed that with the rising of the pyrolysis temperature, the yields of gas and liquid products had shown opposite trend, and the highest yield of bio-oil was around 500℃ (about 40%), however, the yield of char gradually decreased. Furthermore, within 550-650℃ the changing rate became weaker. The bio-oil had a complex composition, and major compounds in the liquid product were acetic acid, levoglucosan and phenolic substances. Besides the content of decreased acetic acid and levoglucosan with the increasing temperature, while acetic acid obtained by microwave heating was significantly lower comparing with conventional electric heating, which made the acidity of liquid products decrease, the high content of levoglucosan in the cotton stalk oil was discovered under microwave heating, which could go up to 19.86%. In addition, phenolic substances went up with the increasing temperature when below 450℃ then multi-substituted phenolic substances had been declined but the content of phenol and hydroquinone increased when above 450℃, so it changed little in total phenolics. Conventionally pyrolysed char could not be used for further application since there persisted large and deep cracks due to the overheating of surfaces, which made them fragile and lost porous nature. Analysis of the product showed the adsorption loop type was L3. Moreover, microwave heating technique was conducive to the formation of the developed pore structure and the specific surface area and micro-pore surface area were first increased and then decreased as the temperature rose, the maximum of SBET and Smic reached 400.29 m2/g and 276.93 m2/g at 550℃, which was known as microwave heating and was highly effective in pyrolysing biomass, specifically the large size which otherwise if not possible but would had been difficult. The surface functional groups of char at 350℃ mainly included O-H (3 500-3 000 cm-1), C-Hn(2 970-2 860 cm-1), C=O(1 730-1 700 cm-1), C=C(1 632 cm-1), C-O-C(1 260 cm-1) and C-H(900-700 cm-1), then gradually decreased as temperature increased. Only small amount of aromatic structure peak (around 1 450-1 600 cm-1) could be detected at 650℃, which indicated the char had been close to completely pyrolysis. The comparative results showed that microwave pyrolysis had a very good prospect on the optimization of bio-oil, getting high value-added products such as levoglucosan, and also the preparation of activated char with high specific surface area.