Influence of pretreatment for deashing of sugarcane bagasse on pyrolysis products
Introduction
Sugarcane bagasse—the residue left after juice extraction is a waste available in abundance worldwide. About 1.34 Gt of sugarcane was produced globally in 1999, which equates to approximately 375 MT of bagasse, 50% of which is typically burned [1]. India is the second largest producer of sugarcane next to Brazil with a production of 300 MT of sugarcane in 1999–2000. In India, about 4 million hectares of land is under sugarcane farming with an average yield of 70 t ha−1. Besides Brazil and India, Australia, South Africa, Cuba, China, tropical and subtropical countries also are major contributors to world production of sugarcane. Thus, sugarcane bagasse has a strong potential in displacing fossil fuels and can be extensively used in boilers, turbines and furnaces for power generation.
Generating power by direct combustion of sugarcane bagasse in boilers has a maximum efficiency of about 26%. Combustion systems with low efficiency are traditionally used in sugarcane plants [2]. In populated areas, bagasse-fired boilers can be one of the major health hazards due to airborne fly ash. Recently, the overall efficiency of the process is being greatly improved by cogeneration, wherein; the improvement comes from the proper utilization of heat and effective waste heat recovery. On the other hand, pyrolysis offers an effective utilization of the “fuel energy” itself giving energy-dense liquids (easier to handle, store and transport), charcoal (improved solid fuel) and gaseous fuel. The ability to decouple the fuel production from the application is unique to pyrolysis liquids and is a major advantage over gasification and combustion, which must use the energy products immediately and cannot store or transport them. Thus, transformation of bagasse into high-density renewable fuels, like charcoal and bio-oil, can significantly increase the profitability of sugarcane plantations [3].
Fast pyrolysis at medium temperature and low vapour residence times are known to be most suitable conditions for maximizing liquid products from biomass. To achieve the fast heating rates, fluidized and entrained bed reactors have been extensively used. In case of fluidized bed pyrolysis, extensive particle entrainment with the vapours has been reported. Vacuum pyrolyser—the low turbulence inside moving and stirred-bed pyrolysis reactors developed by Pyrovac [4], [5] has been reported to have reduced carryover of particles in condensable products. The vacuum pyrolysis provides the required low vapour residence time with a slight compromise in the heating rate achievable and thereby the reduction in liquid products.
Sugarcane bagasse pyrolysis has been referred to by many authors. The role of parameters like peak temperature and tar yield has been investigated [1], [6]. Total condensates of the order of 40–60% have been reported on dry bagasse basis. Vacuum pyrolysis of sugarcane bagasse has been reported first by Pakdel et al. [7]. The recent studies reported by Perez et al. [2] gives a very elaborate and extensive understanding about vacuum pyrolysis of sugarcane bagasse particularly in the assessment of the yield and the product characteristics. The ageing tests reported therein are unique and gives an insight into the ageing process of the bagasse oil. This is of interest, particularly in its application as a fuel. The major deterring factor in the wide usage and acceptability of the bio oils are its change of physico-chemical characteristics during storage along with the corrosivity in the commonly used storage medium. The oil undergoes polymerization thus resulting in an increase in viscosity with time. During ageing etherification and esterification reactions occur between hydroxyl, carbonyl and carboxyl group components [8], [9], [10], [11]. The presence of condensation reactions during ageing is confirmed by increase in water content in the oil with time [2]. It is also mentioned in the literature [12], [13] that this instability may be attributed to the presence of alkali metals in the ash, which are being carried over/entrained by the char particles with the vapours. These alkali metals catalyse the polymerization reactions and thereby increase the viscosity. Moreover, these alkali metals in ash, form deposits in combustion applications, particularly in turbines, where the damage potential is considerably high. Therefore, the present study aims at understanding the influence of this ash on the stability as well as pyrolysis product yields upon vacuum pyrolysis of sugarcane bagasse.
Section snippets
Biomass properties
The properties of the sugarcane bagasse used for the present study are given in Table 1.
Preparation of sample
The oven-dried bagasse sample ground to pass through 60-mesh sieve (⩽250 μm particle size) has been taken for the investigation.
Experimental
Pyrolysis experiments were carried out in a packed-bed reactor of 3-in NB pipe made of stainless steel. The reactor was electrically heated at a maximum temperature of 500 °C under an initial reactor vacuum of 5 kPA. At the end of each pyrolysis run the reactor was cooled to room temperature under nitrogen flow and the char was collected from the biomass basket hung inside the reactor. The volatiles removed on pyrolysis are gradually condensed in a pre-weighed condensing train. The condensates
Effect of deashing treatments on pyrolysis product distribution of sugarcane bagasse
The effect of deashing treatments on sugarcane bagasse pyrolysis product distribution (char, gas, and total liquid including oil fraction), based on treated bagasse basis, has been shown in Table 6 columns A. Columns B in Table 6 represent the same on original bagasse basis, i.e. all values are inclusive of mass reduction during leaching.
As expected, the pyrolysis product distribution relates well to the chemical composition of bagasse as follows. In case of water leaching, the extractives are
Microorganism and culture media
In the present study, the leachates, which result from different pretreatment processes of finely powdered sugarcane waste, was fermented for ethanol by yeast Saccharomyces cerevisiae Yeast strain Saccharomyces cerevisiae (Distillers yeast) provided by Burns Philip India Ltd (Kegaon-Uran, India) was used for the study. The strain was maintained on culture medium containing glucose 20 g/l, peptone 5 g/l, yeast extract powder 5 g/l, and agar 25 g/l.
Ethanol fermentation with leachate
The ethanol fermentation was carried out in the
Results and discussion
All the leachates were analysed for glucose content and the results are presented in Table 12. It shows that, an average of 55% of total weight loss of biomass during leaching process is contributed by glucose and the rest might be due to pentoses like xylose and other sugars. The glucose concentration of leachate being high (78%) in case of pretreatment with higher biomass-to-water ratio.
All the leachates were first neutralized with 0.1 N NaOH solution and tested for the viability of cell
Conclusion
Pre-treatment of bagasse with water, dilute HCl solution and dilute HF solution shows a remarkable change in the pyrolysis product distribution by virtue of a combination of a change in the organic constituents and the selective removal of inorganic ash elements. Mild HF solution is effective in reducing the ash content of the biomass to a negligible amount. Moreover, this treatment effectively increases the oil yield by ∼69% (on the basis of original wt of bagasse before treatment) compared to
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The HF has been used only to achieve complete removal of ash for the present study and by no means a suggestion toward using for commercial application.