An analysis of Brazilian sugarcane bagasse ash behavior under thermal gasification
© Fredericci et al.; licensee Springer. 2014
Received: 1 April 2014
Accepted: 25 August 2014
Published: 14 October 2014
Ashes from sugarcane were analyzed by X-ray fluorescence, ash content, X-ray diffraction, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). FactSage 6.4 database software was used to estimate viscosity at high temperatures (900 - 1550°C) of them.
The results showed that although ashes from sugarcane bagasse contain silica, most of its SiO2 is from soil contamination. Higher and lower silica samples treated at 1350°C for 20 minutes showed that the fine portion of fraction of the ashes melted at this temperature.
Sugarcane bagasse, as biomass, will be used in this work, due to its availability and promising potential. Brazil is the largest producer of sugarcane, with production concentrated in its South-Central region. Currently, the bagasse is used in boilers in the mills to produce energy and heat with typical 45% carbon content (dry mass) . Brazil’s sugarcane industry association (UNICA) reported that the amount of crushed surgarcane in the South-Central region in 2013/14 was 596.936 Mt, about 12% up from the 532.758 Mt produced in 2012/13  that generated approximately 118 Mt of wet bagasse. Even considering its utilization as fuel for energy generation (typically in steam boilers) in the production mills, Perrone et al.  report that there is still a 12% surplus of biomass that will increase proportionally with the increase of ethanol production, requiring solutions and innovative ideas in order to generate new economic value and opportunities for the sugar and alcohol industry. Elements including N, P, K, S, Ca, Mg, Fe, Zn, B, Cu, Mn, Cl and Si considered as macro-, micro- and functional nutrients are essentials for increasing and sustaining crop yields . At high temperatures these elements in bagasse or straw are involved in reactions leading to ash formation .
The temperature inside the gasifier should be sufficient to melt the ash that will be deposited on the wall of the gasifier forming a liquid slag which should flow out from the bottom of the gasifier . The typical temperature for highly efficient processes should be bellow than 1500°C. Entrained flow gasification works with liquid (bio-oil) or sub-millimeter solid particles (bagasse or straw particles). Entrained flow consists of a vertically placed cylindrical reactor at the top of which fuel (bio-oil or biomass) and a gasifying agent (air or O2) are inserted through a nozzle, usually in a swirling flow, forming a flame that carries either particles or droplets through the reactor as they undergo incomplete combustion, i.e. gasification. Residence times are small, temperature and pressure can as be high as 1500°C and 40 bar, respectively, ensuring low tar production .
The aim of this work is to analyze the ash from sugarcane bagasse generated from sugarcane mills in some regions of the São Paulo State – Brazil. The results obtained in this research will provide inputs for determining the preferred processing conditions for the gasifier.
2Methods and Experimental
Samples of bagasse and sugarcane and their sources
Bagasse samples collected in the mill
Location of the Mill
Araraquara – SP(*)
Iracemapólis – SP
Rio das Pedras – SP
Sorocaba – SP
Sugarcane sample collected in the field
Location of the Mill
Araraquara – SP
Twenty grams of bagasse were heat treated at 600°C for 30 min to generate ashes. Each experiment was repeated 10 times for each bagasse under the same conditions and the percentage of generated ashes was given as the average of them.
The samples collected from different mills were named with the following identification: BA, BI, BRP and BS. Ash samples from Araraquara region that were entirely processed in the IPT’s laboratory were named BAL and they were used for comparative purposes. In this case, the BAL sample stalks were milled in the laboratory and were dried at 80°C for 48 h for obtaining bagasse on a dry weight basis (about 2% humidity). Finally, they were heat treated at 600°C for 30 min. Table 1 shows the ash sample identifications and their mill.
Ashes were analyzed by X-ray diffraction (Shimadzu XRD 6000, using Co Kα radiation), X-ray fluorescence pressed powder pellet technique (Philips, model PW 2404), scanning electron microscopy (FEI-Quanta 3D FEG-SEM) coupled to an energy dispersive spectrometer (EDAX System), and sieving analysis for particle size distribution. The computer thermodynamic package FactSage 6.4, in the modulus Glass database, was used to calculate the viscosity of the ash as a function of temperature from 900°C to 1550°C.
About 0.2 g of ash was compacted in a stainless steel mold with a 0.5 cm diameter. The resulting cylindrical samples were put in an alumina boat crucible with 5 cm in length and were heat treated at 1350°C for 20 min in order to analyze the fusibility behavior of the ashes. The surfaces of the heat treated samples were analyzed by scanning electron microscopy using a JEOL-SEM model JSM 6300, coupled to an energy dispersive spectrometer Noran System.
3Results and discussion
3.1 Ash from bagasse
3.2 Ash composition and crystalline phases
Chemical analysis of the ashes obtained by X-ray fluorescence from BA, BI, BRP, BS and BAL samples (wt%)
3.3 Ash particle size distribution
Granulometric size distribution by sieving for the ashes obtained through the heat treatment of BA, BI, BRP, BS and BAL samples
−50 + 100
−100 + 170
−170 + 270
−270 + 325
3.4 Ash fusibility
Ash properties used to determine viscosities in the entrained flow gasifiers (Patterson and Hurst)
SiO2/ (SiO2+ Fe2O3 + CaO + MgO) (%)
If bagasse BA, BI, BRP and BS would be used for gasification, as received, coarse particles of SiO2 could became stuck and tightened on the inner and bottom walls of the gasifier, which could be dangerous for the process since the agglomeration of quartz particles on the bottom wall could cause a blockage of the gasifier. This could lead to severe unscheduled shutdowns and high operation maintenance costs . It is necessary to find technological and economically feasible alternatives for removing silica from bagasse before its gasification processing.
The ashes from sugarcane bagasse processed in the sugarcane mills from São Paulo State present quartz as the main crystalline phase from soil contamination. On the other hand, the ash from bagasse cleaned and processed in laboratory has potassium chloride and potassium sulfate as principal the crystalline phases, and SiO2 as a minor phase. Analysis from scanning electron microscopy of the surface of ash samples from mill bagasse heat treated at 1350°C for 20 minutes showed that the fine fraction of the ashes melt at this temperature and act as flux for coarse quartz sintering. This can cause aggregation of SiO2 on the bottom wall of the gasifier that can compromise the process when using sugarcane bagasse as biomass for gasification. It is very important to study possibilities for removing soil contamination before bagasse processing.
The authors kindly thank the Sugarcane Mills from São Paulo State - Brazil for donating the sugarcane bagasse, Ruben Spitz from Brown University, Claudia Maria G. de Souza and Miguel Papai Jr. of the The Center for Chemistry and Manufactured Goods – CQuim – for the X-ray fluorescence analyses.
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