Elsevier

Bioresource Technology

Volume 249, February 2018, Pages 744-750
Bioresource Technology

Effects of potassium salts loading on calcium oxide on the hydrogen production from pyrolysis-gasification of biomass

https://doi.org/10.1016/j.biortech.2017.10.083Get rights and content

Highlights

  • Effects of K salts loading on CaO on straw pyrolysis-gasification were studied.

  • 0.25 wt% KCl enhanced CO2 absorption of CaO and 0.25 wt% K2CO3/K2SO4 inhibited.

  • More KCl loading reduced CO2 absorption of CaO and raised catalytic effect of KCl.

  • 0.25 wt% KCl loading greatly improved the cyclic H2 production performance of CaO.

Abstract

The effects of potassium (K) salts loading on CaO on the H2 production from pyrolysis-gasification of wheat straw were investigated. The loading of 0.25 wt% KCl could significantly enhance the CO2 absorption capability of CaO. The CO2 concentration in the product gas decreased sharply from 20.83 to 11.70 vol%, and the H2 concentration increased from 48.2 to 55.5 vol%. While the loading of 0.25 wt% K2CO3/K2SO4 inhibited the enhancing effect of CaO. Further increasing the loading of KCl on CaO, the CO2 absorption of CaO declined, but the catalytic effect of KCl on the gasification process was promoted. The loading of 0.25 wt% KCl on CaO significantly improved the cyclic performance of CaO during the pyrolysis-gasification process. Higher H2 concentration and more CO2 absorbed by CaO were obtained with the loading of 0.25 wt% KCl even after 5 cycles compared with those of pure CaO in the first cycle.

Introduction

Biomass is the only renewable carbon source in the nature. It is also the only renewable which can be converted into gas, solid and liquid hydrocarbon fuels. Biomass/bioenergy ranks as the top four energy sources in 2011 world final energy consumption with the advantage of CO2 neutral emission and abundant amount (Chen et al., 2015, Heidenreich and Foscolo, 2015, Saidur et al., 2011, Sikarwar et al., 2016, Tock and Marechal, 2012). Its high efficiency conversion and high value polygeneration become the main focus, and have obtained increasing concerns nowadays (Chen et al., 2016, Chen et al., 2012, Jana and De, 2017, Yang et al., 2016).

Gasification is one of the promising ways to produce clean H2 from renewable biomass for high value utilization (Chang et al., 2011, Gnanapragasam and Rosen, 2017, Parthasarathy and Narayanan, 2014, Yao et al., 2016). However, the H2 concentration in the product gas still needs to be further improved due to the dilution of other carbonaceous gases (CO, CO2, and CH4) (Florin and Harris, 2008). Thus, CaO, as a CO2 absorbent, is introduced into the gasification process, and a novel technology called absorption enhanced steam gasification of biomass for H2 production has been developed in recent years (Acharya et al., 2010, Harrison, 2009, Li et al., 2014, Pröll and Hofbauer, 2008, Udomsirichakorn and Salam, 2014, Wei et al., 2014). The in-situ absorption of CO2 formed in the gasification process can shift the chemical reaction equilibrium to produce more H2 with a high concentration (Li et al., 2017a, Li et al., 2017b). Hanaoka et al. (2005) reported that with the addition of CaO, the H2 concentration and yield from steam gasification of woody biomass in an autoclave reactor increased significantly. Mahishi and Goswami (2007) investigated pine bark gasification in a two-stage fixed bed reactor in the presence of CaO in the reforming stage, the addition of CaO was found to greatly enhance the H2 production. The H2 yield, syngas yield and carbon conversion rate increased 48.6%, 62.2%, and 83.5%, respectively. Li et al. (2017b) used a fluidized bed reactor to study the corn stalk gasification with CaO enhancing, a H2 concentration of 61.23 vol% was achieved.

In the previous studies (Li et al., 2017a, Zhang et al., 2014), catalyst was also introduced into the absorption enhanced steam gasification process. For instance, MgO component in calcined dolomite was found to improve the CO2 absorption performance and catalyze the gasification process as well; while the addition of NiO/γ-Al2O3 catalyst could further increase the concentration and yield of H2 in the product gas significantly during the absorption enhanced steam gasification of biomass (Li et al., 2017a, Zhang et al., 2015). Furthermore, the circulation of CaO absorbent should be implemented in the practical gasification process in terms of economical consideration. Unfortunately, less attention has been paid to the cyclic H2 production characteristics of the absorption enhanced steam gasification of biomass. Considering that potassium (K) is relatively enriched especially in the agricultural biomass wastes, this inherent K would significantly affect the gasification process as well as the performance of the CaO absorbent. Meanwhile, K salts have also been directly used as catalysts to enhance the pyrolysis/gasification of biomass (Oike et al., 2014, Sueyasu et al., 2012, Zhang et al., 2014). Therefore, in this study, different types of K salts were introduced into the gasification process. They were directly loaded on the CaO absorbent. A two-stage pyrolysis-gasification system was employed to investigate the effects of K salts loading on CaO on the H2 production characteristics. The cyclic H2 production performance of CaO and 0.25 wt% KCl/CaO was also discussed.

Section snippets

Materials

Wheat straw was used as biomass material during the experiment. It was crushed and screened to 250–420 μm. Table 1 shows the ultimate and proximate analysis and heating value of the wheat straw. To minimize the effects of inherent K salts in biomass on the pyrolysis-gasification process, the wheat straw was pretreated by water-washing. 10 g of wheat straw was weighed and put inside a beaker. 200 ml of deionized water was added inside. The mixture was stirred/shaken for 2 h at room temperature.

Effects of different types of K salts loading on the H2 production of CaO enhanced pyrolysis-gasification of biomass

Fig. 2 shows the effects of the additions of KCl, K2CO3 and K2SO4 on the H2 production of CaO enhanced pyrolysis-gasification of wheat straw. It can be seen that, without any absorbent addition, the concentration and yield of H2 in the product gas was only 29.3 vol% and 5.88 mg/(g biomass), respectively. There were still a lot of CO (29.4 vol%) and CO2 (35.4 vol%) in the product gas. And the total gas yield was relatively high with a value of 243.2 mg/(g biomass). With CaO addition, the

Conclusions

The effects of K salts loading on CaO on the pyrolysis-gasification of wheat straw were investigated. 0.25 wt% KCl loading significantly enhanced the CO2 absorption of CaO, while the loading of 0.25 wt% K2CO3/K2SO4 inhibited. Further increasing the KCl loading, the CO2 absorption of CaO declined, but the catalytic effect of KCl on the gasification process was promoted. 0.25 wt% KCl loading significantly improved the cyclic performance of CaO. Higher H2 concentration and more CO2 absorbed by CaO

Acknowledgements

The authors wish to express their sincere thanks for the financial support from the National Natural Science Foundation of China (51376076 and 51622604) and the Foundation of State Key Laboratory of Coal Combustion (FSKLCCB1610). The experiment was also assisted by the Analysis Laboratory in the School of Chemical and Process Engineering at University of Leeds and the Analytical and Testing Center in Huazhong University of Science & Technology (http://atc.hust.edu.cn, Wuhan 430074 China).

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