Hydrogen production from steam gasification of corn straw catalyzed by blast furnace gas ash

https://doi.org/10.1016/j.ijhydene.2019.02.235Get rights and content

Highlights

  • The blast furnace gas ash improves the capacity of gasifier.

  • The catalytic reforming of the char layer is intensified, and H2 content is improved.

  • Temperature and S/B have a stronger influence on product gas characteristics.

Abstract

A lab scale gasifier was built to perform the gasification experiment. The effects of temperature and steam flow on the process were investigated, and the effects of the addition of blast furnace gas ash (BFGA) on product composition, the value of H2/(CO + CO2), the lower heating value (LHV) of product gas, and productivity are summarized. The experimental results clearly indicate that the addition of BFGA in the steam gasification of corn straw pellets effectively enhances the ability of the downdraft gasifier to produce hydrogen-rich gas. Compared with the non-catalytic gasification process, the addition of BFGA promotes the formation of H2, inhibits the generation of methane, CH4, and other hydrocarbon gases, CnHm, and increases the H2/(CO + CO2) ratio. Unlike the process without the BFGA, the LHV of the product gas with BFGA increases with increasing temperature. When the water vapor volume was 0.75 kg/h, the gas production rate was the same at 850 °C with BFGA and at 950 °C without ash gas. The addition of BFGA clearly leads to a significant improvement in the ability of gasifiers to produce hydrogen-rich gas.

Introduction

Not only will fossil fuels be depleted in a relatively short time, their overuse has adversely affected the environment. Biomass is the only renewable carbon source that can achieve zero emissions of CO2 because its carbon source comes from photosynthesis that absorbs CO2 from the air. The direct combustion method has a thermal efficiency of only 10–15%, which is a highly inefficient use of the carbon source. The high quality use of biomass has received increasing attention.

Hydrogen is an important chemical raw material in the chemical industry, which is considered to be the best energy carrier in the future [1]. In recent years, the demand for hydrogen in the energy and chemical industries has been increasing, which has led to the continuous development of hydrogen production technology. However, the current method of hydrogen production is still based on the use of fossil resources [2]. From the long-term strategy, the reserve and development of renewable resources (or energy) hydrogen production technology is an important issue in the current hydrogen production industry. Among them, the development and exploration of biomass hydrogen production technology have been paid much attention [3]. Biomass is the only renewable energy source that can directly produce hydrogen [4]. High-temperature steam gasification (HTSG) of biomass as an important research direction for the production of hydrogen-rich gas and syngas has received extensive attention from scholars all over the world [5], [6], [7]. Compared with air-steam and oxygen-steam gasification technology, the product gas produced by HTSG can reach more than 60% of H2 content [8], [9]. However, the HTSG process is an endothermic reaction process, so degradation of tar and increase of H2 yield are the key problems of this technology. The use of a catalyst can effectively crack and reform the tar and increase the H2 yield. Among them, alkali metals are very beneficial for reducing tar and promoting product gas quality [10], [11]. In the steam gasification hydrogen production reaction, the transition metal can accelerate steam reforming. It has the ability to promote the breakage of C–C bonds and C–O bonds [12], and can effectively reduce the formation of tar [13]. In the study of catalytic hydrogen production by NiO and CaO, Dou et al. found that they can effectively increase H2 production, while Ni can effectively promote ethanol reforming, and CaO can absorb CO2 [14], [15]. It has also been pointed out in research that the char has catalytic ability to crack and reform the tar [16].

Enhancing the ability of the char layer to the cracking and reforming of the pyrolysis volatiles during the gasification process can effectively improve the gasification and hydrogen production capacity of the reactor. Char has developed pore structure, high specific surface area, stable aromatic structure, rich surface functional groups and strong adsorption capacity, which can be used as a carrier for the catalyst. Therefore, some scholars have tried to use Ni [17], [18], Ni–Fe [19], Ca–Na [20], Pd–Cu [21], Co [22] and other catalysts were loaded onto biomass char. The experimental results showed that the char loaded catalyst had obviously effect on tar cracking and reforming. Klinghoffer et al. [23] discussed the production of char as a catalyst for tar cracking in biomass fluidized bed gasifiers. It was found that higher char surface area can increase the catalytic activity, but the pore size distribution also affects the activity of the catalyst. Zhang et al. [24] used Fe loaded on biomass char and lignite char as a gas component reforming catalyst. The results showed that all catalysts can reduce the concentration of CO2 and CH4 and increase the concentration of H2 and CO in the product gas. The iron containing species in the lignite char are favorable for the formation of H2, and the biomass supported iron catalyst has higher activity than the lignite char supported catalyst. Shen et al. [25], [26] proposed an effective method for in-situ catalytic dry conversion of biomass tar by loading Ni and Fe onto char by the method of solution immersion. The experimental results show that the tar conversion rates of Ni-carbon (calcined) and Nisingle bondFe-carbon (uncalcined) can reach 92.3% and 93%, respectively, and the condensed tar can be catalytically converted into non-condensed tar or small molecule gas. The calorific value of the gaseous product is increased. A large number of studies have shown that the catalytic effect of char-loaded Ni-based and Fe-based catalysts [27], [28], [29] is more obvious, and the Ni-based catalyst tar has a stronger degradation ability [30]. Even so, the cost of Ni is relatively high for in-situ catalytic processes.

The addition of the high purity additives described above greatly increases production costs, so there is a benefit to adding inexpensive industrial wastes rich in elements such as Fe, Ca, and Na to biomass to enhance the gasification process [31], [32], [33]. The BFGA, industrial waste, formed in blast furnace iron-making is a fine dust removed from the system by a variety of techniques, such as a gravity dust collector, a cyclone dust collector, or a bag filter [34]. Because the raw materials, fuels, and auxiliary materials used in the blast furnace iron-making process pass through different temperature zones inside the blast furnace, very complicated physical and chemical changes, including oxidation-reduction reactions, take place. This results in free states and complexes containing various substances, most of which are Fe2O3, ZnO, Al2O3, SiO, CaO, MgO, and PbO [30]. The BFGA has a small particle size and good fluidity [35]. If not handled properly, it may cause air and soil pollution, damaging ecological cycles and causing environmental hazards [36]. For example, smog and soil heavy metal pollution. BFGA has a high mineral content, with an average iron content of about 30% and a fixed carbon content of about 10%–30%. With the rapid development of the steel industry, blast furnaces have become much larger, and the amount of dust has increased year by year. Due to the lack of effective recycling for a long time, the space available for dust storage has diminished. China's steel output has exceeded 1.1 billion tons in 2014, and 100–120 kg of blast furnace BFGA is produced per ton of pig iron [37]. Therefore, in order to achieve sustainable development and cleaner production, blast furnace BFGA must be effectively recycled.

BFGA is rich in elements such as Fe, Ca, K, Na, Zn, and C. The carbon has a certain calorific value and can be used as a carbon source for gasification, and several other elements can be used as good gasification catalysts. In addition, the BFGA has a small particle size and can be mixed with corn straw without extra crushing. It can be used as an additive for biomass pyrolysis gasification to achieve catalytic gasification of corn straw. In this way, the increase in hydrogen production can be achieved, and at the same time, the effective recycling and disposal of waste can be satisfied. The waste can be resourced and rendered harmless. Based on the above, the downdraft gasifier is used as a steam gasification reactor, and BFGA is proposed as a new additive, which is evenly loaded into the biomass forming particles. Due to the catalytic action of the active substances in the BFGA, the biomass gasification and reforming process can be enhanced in the gasification reaction zone to realize efficient hydrogen production of biomass.

Section snippets

Materials

The corn straw was obtained from a farm in Baotou, a city in the Inner Mongolia autonomous region of China. The corn straw pellet was compression-molded using a pelletizing machine. Its bulk density was about 620 kg/m3, and the cylindrical particles were 8 mm in diameter and 15 mm–20 mm in height. Proximate and ultimate analyses were performed for a sample of pellets, and the results are given in Table 1.

The BFGA was obtained from the Baogang Group in Baotou. Table 2 shows its composition.

Gas components

Fig. 3 shows the variation of gas components in high temperature steam gasification of corn straw with BFGA. It can be seen from the figure that CnHm, CH4 and CO2 decrease with the increase of temperature, CO increases with the increase of temperature, and H2 content is less affected by temperature with temperature. CO is a product under high temperature and anoxic conditions. CO in the product gas mainly comes from CO volatilized from biomass pyrolysis in the pyrolysis zone; the reduction

Conclusions

BFGA is rich in elemental materials, such as Fe, Ca, and Na. When corn straw pellets are combined with BFGA, these active substances act as catalysts in the pyrolysis stage, the gasification stage, and the reforming stage of the pyrolysis volatiles in the gasification process. The BFGA promotes the in-situ catalytic gasification of the char layer and the reforming ability of volatile matter, effectively improving the H2 content in the product gas, and improving the rate of gas production in the

Acknowledgments

The authors are grateful for the financial support by Inner Mongolia Autonomous Region Natural Science Fund (2018MS05046).

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