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Optimization of parameters on the generation of hydrogen in combined slow pyrolysis and steam gasification of biomass Prakash Parthasarathy, Sheeba Narayanan, Selim Ceylan, and Nugroho Agung Pambudi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02429 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017
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Optimization of parameters on the generation of hydrogen in combined slow
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pyrolysis and steam gasification of biomass
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Prakash Parthasarathya, K. Sheeba Narayanana1, Selim Ceylanb, Nugroho Agung Pambudic a Fossil and Alternate Fuel Processing Laboratory, Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli-620 015, Tamil Nadu, India b Ondokuz Mayıs University; Faculty of Engineering, Chemical Engineering Department, Samsun, Turkey c International Institute for Carbon-Neutral Energy Research (I²CNER), Kyushu University, Japan ABSTRACT
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The generation of hydrogen in steam gasification can be improved by combining slow pyrolysis-
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steam gasification. The study investigates the effect of controlling parameters of slow pyrolysis-
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steam gasification on the generation of hydrogen. In the work, some native biomass wastes were
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first slow pyrolyzed and then the generated chars were steam gasified to generate hydrogen. In
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slow pyrolysis, factors such as temperature, solid residence time and particle size were
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optimized. Whilst in steam gasification, variables such as gasification temperature, residence
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time, steam to biomass ratio, catalysts, composition of catalysts, sorbents, composition of
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sorbents and effective catalyst-sorbent composition were optimized. Through slow pyrolysis, it
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was ensured that only high-quality char was available to steam gasification. It was found that the
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highest gasification temperature yielded the maximum hydrogen. It was noticed that an optimal
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residence time and steam to biomass ratio was essential to generate maximum hydrogen.
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Amongst the selected catalysts, KCl generated the maximum hydrogen. Of the selected sorbents,
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CaO offered the maximum hydrogen. The combination of catalyst-sorbent (KCl-CaO) yielded
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the maximum hydrogen. It was also observed that an optimal quantity of catalyst, sorbent, and
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catalyst sorbent was needed to generate maximum hydrogen.
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Keywords: Biomass, Slow pyrolysis, Steam gasification, Hydrogen 1
Corresponding author: Tel: 91 431 2503113, Fax: 91 431 2500133 Email:
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Energy & Fuels
1. Introduction
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As the fossil fuels are finite, polluting and pricey, the search for sustainable, harmless and
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inexpensive fuels has become intense worldwide. Since last two decades, hydrogen has been
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projected as the future of the fuel due to its varied advantages. Hydrogen is the fuel that has the
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highest calorific value (120 MJ/kg)1. It is the only fuel which, when burnt releases no harmful
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products. Also, hydrogen can be used as direct as well as intermediate storage fuel. As a direct
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fuel, hydrogen is employed in fuel cell technology and in internal combustion engines. Hydrogen
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is employed as an intermediate storage fuel in the production of gasoline, methanol, ethanol and
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other value-added chemicals. Depending on the application, hydrogen can be used either as a
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gaseous or liquid fuel, thus making it a versatile fuel.
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Hydrogen is generated from sources such as natural gas, liquid hydrocarbons, coal and
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biomass.
Processes
such
as
electrochemical,
photochemical,
photo-catalytic,
photo-
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electrochemical, thermochemical and biochemical are employed to generate hydrogen from these
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sources2. Among the various sources, only biomass generate hydrogen directly. The rest of the
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sources make use of electricity to electrolyze water into hydrogen. Further, biomass could
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generate hydrogen sustainably.
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Currently, the recovery of hydrogen from biomass is performed by two energy
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conversion methods such as biochemical and thermochemical method3. Biochemical methods are
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more appropriate for starch and sugar rich feedstock. However, they are not suited to convert
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lignocellulosic biomass4. Conversely, thermochemical methods can effectively treat lignin rich
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biomass. Also, the cost involved with the generation of hydrogen in thermochemical methods is
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more economical than biochemical methods5.
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Thermochemical methods of hydrogen generation are basically centered on two
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processes, namely pyrolysis, and gasification. Pyrolysis and gasification have been practiced by 3
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mankind since ages to generate energy, fuels and value-added chemicals6,7. The thermochemical
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route offers three modes of hydrogen generation from biomass8. The three modes are fast
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pyrolysis of biomass superseded by subsequent steam reforming of bio-oil, supercritical water
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gasification of biomass, and steam gasification of biomass. The process of fast pyrolysis
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generates less hydrogen yield. Supercritical water gasification is still to be proven and at present,
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its H2 production cost is not economical. Relative to other two processes, steam gasification
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generates more hydrogen. Furthermore, steam gasification is an established technology. Hence,
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in this study steam gasification was chosen.
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The performance of steam gasification could be enhanced by integrating slow pyrolysis-
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steam gasification techniques. The integration of these two processes : improve thermal
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efficiency, generate high quality syngas, augment hydrogen yield, and decrease tar yield9–11.
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There are many literatures on slow pyrolysis of biomass, steam gasification of biomass, and
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steam gasification of char12–16. However, investigation on slow pyrolysis-steam gasification of
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biomass aimed at improving hydrogen yield has not been done yet. Henceforth, in the current
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study, the influence of slow pyrolysis-steam gasification parameters on hydrogen generation was
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studied. In slow pyrolysis, the effect of temperature, solid residence time and particle size on
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char generation was investigated. In steam gasification, the influence of gasification temperature,
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residence time, steam to biomass ratio (S/B), catalysts (NaCl, K2Cr2O7 and KCl), composition of
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catalysts, sorbents (Al2O3, CaCl2, CaO) composition of sorbents and effective catalyst-sorbent
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composition on hydrogen generation was studied.
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2. Materials and Methods
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2.1.Biomass and Characterization of samples
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The photographs of the selected biomass source are presented in Figure 1. Though wheat
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is not cultivated in Tamilnadu, usage of wheat husk as fodder is a common practice here. The 4
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collected biomass wastes were sundried until the moisture content was below 15 %. The true
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density of samples was measured using pycnometer. ASTM E873-06 was followed to measure
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the bulk density of samples. The mean particle diameter of the samples was determined as
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followed elsewhere17. The proximate analysis was done in line to ASTM D3173, ASTM D3174
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and ASTM D3175 standards. An Elementar Vario EL III analyzer was used to perform elemental
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analysis. The higher heating value (HHV) of samples was determined using a correlation
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followed elsewhere18. Table 1. presents the results of density, particle size, proximate, ultimate,
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and heating value analysis of the samples. The ash analysis was done using FEI ESEM Quanta
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200 scanning electron microscope (SEM) with Energy Dispersive X-ray analysis (EDAX)
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detector. The major mineral composition present in the ash of samples is presented in Table 2.
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The analysis procedures employed elsewhere was followed to determine the wet chemical
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analysis of the samples19,20. The results of wet chemical analysis are presented in Table 3.
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2.2.Bench scale slow pyrolysis
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Before performing the combined slow pyrolysis-steam gasification, it was necessary to
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determine the effective operating conditions suitable for generating optimized char yield in slow
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pyrolysis. Hence, bench scale slow pyrolysis experiments were conducted first. Experiments
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were performed in a tubular reactor and the heat for the system was supplied using a tubular
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furnace. For every experimental run, exactly 5 g of samples were loaded into the reactor.
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Nitrogen gas of purity 99.95 % was purged for 15 min into the system before and after pyrolysis.
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The char collected after was quantified and taken for further analysis. Pyrolysis is the process in
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which the volatiles are liberated, and carbon content of the feed is enriched. For char to be used
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as fuel, the carbon content should be greater than 75 %21. Hence, in this study, pyrolysis
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parameters were optimized till high quality char (≥75 % carbon content) was obtained. This 75
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% carbon content includes primarily fixed carbon and some amount of volatile carbon22. 5
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First, slow pyrolysis experiments were performed at different temperatures of 300, 350, 400,
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450, 500 and 550 °C at a residence time of 30 min using samples of average particle size (refer
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Table 1). Then, with an optimized temperature of char yield (75 % carbon content), experiments
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were conducted at a varying solid residence time of 10, 20, 30, 40, 50 and 60 min with samples
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of average particle size. Finally, with optimized temperature and solid residence time,
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experiments were conducted changing feedstock particle size of 90
