Elsevier

Fuel Processing Technology

Volume 135, July 2015, Pages 73-79
Fuel Processing Technology

Effect of char on co-pyrolysis of biomass and coal in a free fall reactor

https://doi.org/10.1016/j.fuproc.2014.10.022Get rights and content

Highlights

  • In-situ volatiles–char re-contact was achieved in a free fall reactor.

  • Volatiles–char re-contact effectively enhanced tar generation.

  • Co-pyrolysis and volatiles–char interactions favored acidic component generation.

  • Volatiles–char re-contact during co-pyrolysis decreased asphaltene content in tar.

Abstract

Fast pyrolysis and co-pyrolysis of pine sawdust (SD) and sub-bituminous coal (SB) were carried out in a free fall reactor. There is a gas–solid separator at the bottom of the reactor, where a controlled re-contact of the primary volatiles with the nascent char could be achieved. The experiments of the individual fuels with the volatiles–char re-contact resulted in increased tar yield and decreased the light hydrocarbon gas concentrations compared with that without the volatiles–char re-contact. During the co-pyrolysis of SD and SB, synergies were observed to have produced more tar at different biomass blending ratios (BR). Particularly, the re-contact of the volatiles–char effectively enhanced tar generation and suppressed water formation. The volatiles–char interactions promoted the tar generation mainly at the char bed temperature range from 600 °C to 700 °C. At the optimum condition with the BR of 0.50 and char bed temperature of 700 °C, the tar yield increased up to 28.7%. The co-pyrolysis and volatiles–char interactions favored the acidic components generation in the liquid products. Especially, the volatiles–char re-contact during co-pyrolysis improved the quality of tar by decreasing its asphaltene content.

Introduction

The growing shortage of oil resources and increasing oil prices have been promoting an urgent search for alternative liquid fuels. However, in the foreseeable future, coal is believed to be a major energy source in China, and a large number of coal tar are produced each year through coal carbonization and liquefaction [1], which can be further processed to produce liquid fuels. But the tar yield of coal pyrolysis is low, and in which the content of heavy components is high owing to its low H/C molar ratio. Accordingly, to increase the tar yield and quality, it is necessary to supply H to coal pyrolysis from other sources [2], [3], [4]. Compared with coal, biomass is becoming important as a renewable alternative energy resource and more environmentally friendly when utilized for energy production due to its CO2 neutrality. Besides, its high thermochemical reactivity and high content of volatiles facilitate the thermal conversion and upgrading of the fuel, and the high molar ratio of H/C in biomass suggests that biomass could act as H-donors in co-pyrolysis of biomass and coal blends and some synergetic effects to produce more volatile products might be expected. Recently, interest in co-pyrolysis of coal and biomass has been growing significantly [5], [6], [7], [8], and synergies to produce more volatile products could be observed. The main reasons for synergies are the hydrogen transfer from biomass to coal and the catalytic effect of inorganic species, especially alkali and alkaline earth metal during the co-pyrolysis [9], [10], [11], [12], [13].

The pyrolysis process of solid fuels includes the primary degradation of fuel particles and the secondary reactions of volatiles, involving a series of physical and chemical processes. Char, one of the main intermediate products in the pyrolysis reaction system, not only has many cavities but also contains some active sites and inherent metallic minerals [14], [15]. It will undoubtedly play an important role in the secondary reactions of primary volatiles. Thus the volatiles–char interactions have significant effects on the product distributions during the co-pyrolysis process. Recently, many studies involving the catalytic effect of char on the tar reforming during the pyrolysis of biomass and coal have been conducted and most of these studies [15], [16], [17], [18], [19], [20], [21], [22], [23] focused on the off-situ char catalysis. But it is different from the nascent char, because the structure characteristic and reactivity will change during the processes that the nascent char is cooled down and again heated to high temperature from low temperature [24], [25]. Xu et al. [26] reported that in-situ char has a higher capability of cracking tar than off-situ char at the temperature as high as 1100 °C. But little attention [27] was paid to the interactions between in-situ char and volatiles, especially under moderate conditions.

In the present study, examinations have been made on the effects of in-situ char–volatiles interactions on the pyrolysis and co-pyrolysis of biomass and coal, employing a particular type of free fall reactor that enables investigation of the secondary reactions of primary volatiles in the presence of char bed.

Section snippets

Feedstocks

The biomass feedstock used in this study is pine sawdust (SD) from Dalian, China and the coal is sub-bituminous coal (SB) from Wyoming's Powder River Basin, USA. The feedstocks were sieved, classified and dried for 3 h at 105 °C before experiments. The particle sizes of biomass and coal were in the range of 0.45–0.90 mm and 0.30–0.45 mm, respectively. The proximate and ultimate analyses of the feedstocks are shown in Table 1.

Experimental apparatus and methods

The fast pyrolysis and co-pyrolysis of biomass and coal were conducted in

Pyrolysis of individual fuel

Fig. 2, Fig. 3 show the product distributions and gas compositions from fast pyrolysis of the individual SD and SB, respectively, with and without nascent char bed at the reactor temperature of 700 °C and the gas–solid separator temperature of 600 °C. As seen in Figs. 2(a) and 3(a), in the case of with char bed, the tar yield increases significantly while the char yield decreases compared to the result without char bed. For example, the tar yield from biomass increases from 5.2 wt.% (daf) in

Conclusion

Fast pyrolysis and co-pyrolysis of SD and SB in a free fall reactor were investigated in order to determine the influence of nascent char on the secondary reactions of primary volatiles. The nascent char has shown encouraging results for the production of tar either for individual pyrolysis and co-pyrolysis. During co-pyrolysis, the presence of char bed can effectively promote tar generation and suppress water formation, the water yield reduces to a minimum value (3.7%) at the BR of 0.50. The

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51306029), the China Postdoctoral Science Foundation (No. 2013M530920), the Fundamental Research Funds for the Central Universities (No. DUT12RC(3)58) and the Program for Liaoning Excellent Talents in University (2009R10).

References (39)

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