Gasification of Iranian walnut shell as a bio-renewable resource for hydrogen-rich gas production using supercritical water technology
Gasification in supercritical water (SCW) media is known as an efficient and promising technology for obtaining hydrogen-rich gas from dry and wet bio-renewable materials. Gasification of walnut shell as the main hard nutshell produced in Kurdistan Province of Iran was investigated using a stainless steel batch micro-reactor. Effects of reaction time in the range of 10–30 min, feed loading in the range of 0.06–0.18 g, and temperature in the range of 400–440 °C were investigated to determine the condition for maximum hydrogen yield. Furthermore, carbon gasification efficiency (CGE) and hydrogen gasification efficiency (HGE) were calculated according to the elemental analysis and the yields of gaseous products. Total gas yield and hydrogen yield were directly correlated with temperature. Steam reforming of walnut shell was favored at higher temperatures. Also, walnut shell loading was inversely correlated with total gas and hydrogen yields while production of methane was favored by higher loading of walnut shell. Furthermore, hydrogen yield increased first, when reaction time increased from 10 to 20 min, and then decreased. Maximum hydrogen yield of 4.63 mmol/g of walnut shell was obtained at 440 °C, walnut shell loading of 0.06 g and reaction time of 20 min.
KeywordsGasification Supercritical water Hydrogen Walnut shell
These fermentable sugars are also dehydrated into 5-HMF (5-hydroxymethylfurfural) . Under a suitable condition, 5-HMFs are degraded into acids, alcohols, aldehydes and ketones [22, 23]. Eventually, these small molecules can be reformed to gaseous products.
Overall, the interaction between lignin, cellulose and hemicellulose in SCWG is not clearly specified. However, the amount of cellulose and hemicellulose and their availability in the structure can affect the conversion of lignocellulosic biomass into gaseous products.
In the methanation reaction, 3 mol of produced hydrogen are consumed by Eq. (6) for producing 1 mol of methane. Also, water-gas shift reaction consumes CO and water and produces H2 and CO2. Therefore, for hydrogen-selective SCWG process, methanation reaction should be decelerated.
Many experiments have been conducted for SCWG of biomass model compounds [25, 26, 27]. However, some research has been done for gasification of agricultural wastes and real biomasses. Useful reviews for SCWG of biomass have been proposed by Kruse, Guo et al. and Tekin et al. [28, 29, 30]. Rashidi et al. performed SCWG of bagasse using the same reactor with and without Ni/CNTs catalysts. The effect of bagasse loading and reaction times on hydrogen yield was studied. Hydrogen yield of 3.84 mmol/g was observed for non-catalytic tests at the temperature of 400 °C, the reaction time of 20 min, and bagasse loading of 0.15 g . Safari et al. reported the gasification performances of three different agricultural wastes including walnut shell and almond shell in a base case condition . Madenoglu et al. investigated the subcritical and SCWG of some hard nutshells in the absence and presence of the catalyst. Effect of temperature and catalyst was investigated on gaseous, liquid and solid products. Hydrogen yield was enhanced by increasing the temperature from 400 to 600 °C . Liu et al. reported the product identification and distribution from hydrothermal conversion of walnut shells into liquefied products using KOH and Na2CO3 catalysts . However, none of the previous studies performed a holistic analysis of gaseous products of SCWG of walnut shell.
The objective of this study is to investigate the hydrothermal gasification of Iranian walnut shell in SCW media for hydrogen-rich gas production. All of the main important parameters including temperature, feed content and reaction time were studied to observe the variation of gaseous products and to determine the optimum condition for hydrogen yield. Also, the simultaneous effect of reaction time and feed concentration was studied. Gasification efficiencies are also calculated using elemental analysis of walnut shell and the yield of the gaseous products. There is no work in the literature with a holistic analysis of hydrothermal gasification of walnut shell and its gaseous products. Therefore, the present study aims to study the gaseous products of walnut shell holistically and determine the optimum condition for hydrogen production.
Materials and methods
Elemental analysis of Iranian walnut shell
Mass fraction (%)
Reaction setup and experimental outline
Product analysis method
Gas samples were taken using tight syringes and injected into the gas chromatograph’s column. Gas chromatograph (Varian 3400 and Teyfgostar-Compact) was equipped with PORAPAK Q-S 80/100 (30 m long, 0.53 mm I.D) column, a methanizer and flame ionization detector (FID). Argon was used as carrier gas and oven temperature program was in the following: 40 °C isothermal for 5 min, increase in temperature from 40 to 75 °C in 17.5 min and isothermal in 75 °C for 5 min. The methanizer option enables the FID to detect levels of CO and CO2. During analysis, methanizer is heated to 380 °C with the FID detector body. When the column effluent mixes with the FID hydrogen supply and passes through the methanizer, CO and CO2 are converted to methane. GC was calibrated with standard gas mixture supplied by ROHAM Company in Tehran, Iran. The standard deviation for the results of gas composition was calculated to be ±2 %.
Results and discussion
Summary of experimental conditions and SCWG results
Reaction time (min)
Feed/water (mass ratio)
Main gas yields (mmole gas/g of walnut shell)
Effect of temperature
Effect of reaction time
Figure 5 depicts that hydrogen yield increases by increasing the reaction time, reaches a maximum at the reaction time of 20 min and then starts to decrease. The aim of this study is to optimize hydrogen production in SCWG of walnut shell. Hence, 20 min is the optimum reaction time for maximum yield of hydrogen when reaction time deviates from 10 to 30 min.
Methane yield also increased by a factor of 3.64. Extending the reaction time increases the methane yield from 0.39 to 1.22 (mmol gas/g of walnut shell). As reaction time increased from 10 to 20 min, the methane yield increased, while the total gasification yield increased by a factor of 1.28. Beyond 20 min of reaction time, the total yield of the product gas was not changed significantly while the composition continued to change. The decrease in hydrogen yield and increase in the methane yield can be associated to the methanation process (Eq. 7). According to Eq. (7), consuming 3 mol H2 generates 1 mol CH4 and consumes 1 mol CO. Meanwhile, downward trend of H2 yield should be sharper than the rising trend of CH4 and CO yields. When the objective of biomass gasification in SCW is hydrogen production, reaction (7) must be restrained and CO reacting with water to form CO2 and H2 (Eq. 6) must be enhanced. Figure 5 also shows that the yield of CO2 increases as reaction time increases from 10 to 30 min. This figure also shows that the CO yield decreases with time slightly which is due to the reaction of CO with water to form CO2 and H2 by increasing time. As shown in Fig. 6, the amount of total generated gas increased from 20.02 to 23.22 (mmol/g of walnut shell) when reaction time increased from 10 to 30 min.
Effect of walnut shell loading
Compared to the previous study of SCWG of walnut shell, this study resulted in the H2 yield of 2.44 mmol/g in 400 °C, which was slightly higher than that of Madenoglu et al. Also, the methane yield was nearly the same. There is no previous study on SCWG of Iranian walnut. Hence, this research presents a sustainable way for utilization of this bio-renewable agricultural waste.
The results of this study promise a sustainable process for making an alternative fuel from walnut shell which is an agricultural waste. Gasification of walnut shell in SCW media was performed under 13 different conditions to observe the effect of parameters including temperature, feed content and reaction time on the yields of main gaseous products considering the main reactions of the process. Promising results were obtained using holistic and comprehensive analysis of gaseous products and affecting parameters, for waste management sector and energy industry of Iran which has not been studied before. Ranges of variation of parameters were determined, considering the capacity of the reactor system and the obtained results from the literature. The temperature had a significant effect on the total gas and hydrogen yield so that as temperature increased, hydrogen yield increased while increasing the temperature may cause intensive energy consumption. So, the appropriate range of temperature was considered between 400 and 440 °C and the maximum hydrogen yield occurred in 440 °C. The effects of reaction time and walnut shell loading were investigated at this temperature. The maximum hydrogen yield of 4.63 (mmol gas/g of walnut shell) was observed at the reaction time of 20 min, walnut shell loading of 0.06 g and temperature of 440 °C.
The authors would like to thank the Iran Renewable Energy Organization (SUNA) for their kind support of this research.
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