Experimental results for slow and vacuum pyrolysis of sugar cane bagasse, in the same reactor allowing the comparison of these two processes, are reported. The experimental results showed that vacuum pyrolysis leads to a higher BET specific surface area whereas slow pyrolysis seemed to favour the HHV of charcoal. Detailed yields of products are presented and the influence of temperature and heating rate were studied using a design of experiments and an ANOVA analysis. From the results the optimum experimental conditions to maximise the yields of char and bio-oil products, as well as their heating value and specific surface area characteristics, were established. The optimal yields of bio-oil for vacuum pyrolysis were obtained at 400-500 °C and a heating rate of 15-24 °C min-1, and for char the corresponding values are 340-350 °C and 18-24 °C min-1. Slow pyrolysis produced the highest char yield. The optimal ranges of temperature and heating rate differ from that of vacuum pyrolysis mainly due to the short residence time of the vapours in the case of vacuum pyrolysis. Optimum conditions for bio-oil and char yields did not correspond with conditions to optimize the BET surface and HHV for chars, and to minimize the water content of the products.
72
Keywords:
Vacuum/ slow/pyrolysis/ sugarcane bagasse/comparison
4.2 Introduction
Thermo-chemical processes such as pyrolysis and gasification have recently become a topic of interest for conversion of biomass into clean energy and valuable products. The choice of the process depends on the desired product. Fast pyrolysis leads to a high yield of bio-oil while vacuum and slow pyrolysis offer a good compromise for the production of char and bio-oil, providing high yields and with superior quality of the char products [1]. Bio-oil potentially represents a valuable liquid fuel for boilers, while its chemical composition suggests that it is a challenging matrix for isolation of chemicals, as well as nutritional and pharmaceutical products. The char represents a good feedstock both for boiler fuel and the production of activated carbon. Comparative studies have been carried out between the different types of pyrolysis: slow, fast and vacuum [2-9]. They differ in terms of chemistry, overall yields and quality of products. A precise comparison is possible if the pyrolytic treatment is carried out in the same reactor. Nevertheless, some authors compared the trend of pyrolysis products from various reactors and they justified the deviations in absolute values by differences in the reactor configurations and conditions [5-9]. The design of the reactor can induce major differences in the results which do not come from the pyrolytic process and the comparison could be difficult to evaluate. This study proposes to convert through vacuum and slow pyrolysis biomass in the same reactor.
It is recognized that biomass surpasses many other renewable energy sources, because of its abundance, high energy values and versatility. Since bagasse is the most abundant crop waste in the world [10], it may be used as an energy product without directly compromising production of food and thus affecting food prices.
The influence of process parameters such as temperature, pressure, heating rate and residence time has been extensively studied [11-15]. When temperature increases under normal pyrolysis conditions the char yield decreases and the release of volatile matter increases. With regards to the char quality, an increase in temperature will increase the ash and fixed carbon content. Consequently, there is a decrease in volatile matter in the char with an increase in pyrolysis temperatures. Therefore, higher temperatures yield chars of greater quality, although the char yield diminishes [11]. The optimal yield for bio-oil is reached at an intermediate temperature. At higher temperatures more compounds are degraded, leading to the formation of non-condensable gases and a decrease in bio-oil yields [12]. Concerning the pressure, it is widely accepted in scientific literature that higher pyrolysis pressures will increase the number of re- condensation reactions, which leads to more of the vapours becoming trapped on the char, thereby
73
increasing the water content in bio-oils while reducing their quality. The presence of water in bio-oil lowers the energy density and the flame temperature of the oils; it may lead to ignition difficulties, and may cause pre-evaporation of the oil resulting in injection difficulties during preheating [12]. The heating rate in pyrolysis substantially influences product yields, with higher heating rates leading to higher liquid product yields [13]. The mass transfer restrictions that apply to the volatile contents of biomass can be improved by slowing down the pyrolysis reactions. A longer residence time favours secondary reactions such as thermal cracking, repolymerization, and recondensation, thereby minimizing liquid yield. However, employing a higher heating rate removes these limitations on the yield of bio-oil. As a result, the oil yield increases substantially under fast pyrolysis conditions [14]. Moreover, the heating rate also has an effect on the BET surface area of the char produced in the pyrolysis process. A low heating rate of 5 ºC min-1 was not effective to remove volatile matter from char during the pyrolysis of pistachio-nut shells. For a low heating rate of 5-10 ºC min-1 the subsequent activation of char resulted in an insufficient pore structure for the production of activated carbon. However, for higher pyrolysis heating rates of 20 ºC min-1 and above, the devolatilization reactions were sufficiently intense, resulting in a highly developed porous structure. This porous structure will lead to the development of mesomacropore structures during the subsequent activation, as the CO2 molecules can diffuse into the pores more easily [15].
Consequently, the BET surface area and micropore volume increase for pyrolysis heating rates greater than 10 ºC min-1, resulting in char that is more suitable for activated carbon production. Finally, increasing the pyrolysis residence time will increase the fixed carbon content of the char, because of an increase in the fraction of volatile matter released from the char during pyrolysis [15].
This study presents the different experimental conditions applied in statistically-designed experiments conducted in vacuum and slow pyrolysis of sugar cane bagasse. The objectives of these subsequent experiments were: firstly, to determine the reproducibility of the results concerning char and bio-oil yields, and secondly, to determine the optimal experimental conditions, in particular temperature and heating rate, to maximise the yields of char and bio-oil, together with the calorific value of these products. The BET surface area of chars produced under the selected experimental conditions was also determined to estimate their value as feedstock for the production of activated carbon.