Effect of fuel origin on synergy during co-gasification of biomass and coal in CO2
Graphical abstract
Introduction
Nowadays coal is the main feedstock used for energy production because of its large reserves, and it is expected to be applied as the energy resource for over 110 years (BP Statistical review, 2014). However, the huge consumption of coal over the past few decades has caused serious environmental impact locally and globally. Biomass is a renewable and clean energy source because of its replenishment, carbon neutrality, and low sulfur content, which can supply about 14% of the world’s energy consumption (Saxena et al., 2009). However, biomass fuels in their original forms have also widely dispersed and low energy-density character, and thus it is cost-prohibitive to run a stand-alone biomass conversion plants (Craig and Mann, 1996).
Combining biomass and coal as feedstock for energy production can offer several advantages in terms of environmental friendliness, production economy, and improved thermal efficiency (Craig and Mann, 1996). The environmental benefit of this approach is that the utilization of biomass can contribute to a CO2 neutral cycle (Hernandez et al., 2010). The economic superiority of co-processing biomass and coal lies in the economies of plant scale that can reduce specific operating costs to allow better use of biomass than in the case of constructing new decentralized plants fed exclusively with biomass (Saw and Pang, 2013).
Aside from the direct economic and environmental benefits listed above, a number of researchers have found synergy in co-processing of coal and biomass, in particular co-gasification. This synergy implies an attractive possibility of improving overall efficiency of co-processing systems. Previous studies on this issue have been conducted through different types of reactors such as thermogravimetric analyses (TGA) (Brown et al., 2000, Krerkkaiwan et al., 2013, Habibi et al., 2013, Ding et al., 2014), fixed-bed reactor (Howaniec and Smoliński, 2013, Fermoso et al., 2010, Rizkiana et al., 2014, Jeong et al., 2014), fluidized-bed reactor (Saw and Pang, 2013, Sjöström et al., 1999, Seo et al., 2010), and even an entrained-bed reactor (Hernandez et al., 2010). However, lack of synergy effects in coal/biomass blends during co-gasification was also reported. Collot et al. (1999) found some hints of synergy in the volatile yield in a fixed-bed reactor, but they were too small to constitute clear evidence of synergy. Kumabe et al. (2007) did not find any sign of synergies either in product distribution or in gas composition and process efficiency. Similar findings were reported by Aigner et al. (2011), who conducted co-gasification of coal and wood in a dual fluidized bed gasifier. Moreover, in addition to synergy and additivity effects, Habibi et al., 2013, Ding et al., 2014 reported that an inhibiting effect was observed during co-gasification of certain coal/biomass blends. The authors explained that this inhibiting effect was attributed to the formation of KAlSiO4 and comparable gasification rates of biomass char and coal char. In summary, synergy, additivity and inhibition behaviors were reported to occur for similar types of processes. Therefore, the occurrence of synergy during co-processing coal and biomass is generally inconclusive.
Contradictions on the existence of synergy between coal and biomass suggest that this issue requires further systematic research. In our previous paper (Zhang et al., 2016), we reported that the sample mass dependence of the char reactivity could be misdiagnosed as synergy or inhibition during co-gasification of coal and biomass, typically in the case of using conventional TGA as an experimental measure. A congruent-mass TGA method has been developed to overcome the limitations of the conventional TGA mode, which provides reasonable and reliable information on the synergy effect during co-gasification of coal and biomass (Zhang et al., 2016). This study reports further progress on this issue. Among all possible factors of governing synergy during co-processing biomass and coal, we believe that fuel origin may be an important determinant that should be considered. The purpose of this study is to investigate any synergies that result from different characters of fuels: biomass samples with different contents of potassium and silica and coal samples with different ranks and ash contents.
Section snippets
Materials and analyses
Three coals were used in this study. They were selected for variations in both coal rank and ash content: a bituminous coal (BC) from Shaanxi Province in China, a high ash lignite from the Inner Mongolia Autonomous Region in north of China (LC), and a low ash lignite from Indonesia (LI). Four kinds of biomass feedstocks were selected from either forestry wastes or agricultural residuals, including Chinese redwood (CR), soybean stalk (SS), orange peel (OP), and peanut shells (PS). All samples
Characterization of original biomass and coal
Table 1 presents the ultimate and proximate analyses of the investigated coal and biomass samples. LC and LI are apparently similar in coal rank and elemental composition, but significantly different in ash content (17.2 wt% in LC and 1.9 wt% in LI). BC is a higher-ranked coal than LC and LI, with the highest fixed carbon content among the three coal samples studied. Biomass samples commonly contain 71–78 wt% volatile matter, more than 2–3 times as much as the three coals contain. The ash content
Conclusions
Congruent-mass TGA provided conclusive evidence that synergy occurs only when free mineral potassium species are formed after complete gasification of biomass in coal/biomass blends. The SEM–EDS spectra revealed that the transfer of potassium from biomass to the surface of coal chars occurs during co-pyrolysis/gasification. High silica content in biomass could eliminate the catalytic activity of potassium, but not to the extent of inhibiting the reaction of coal/biomass blended char and causing
Acknowledgement
The authors are grateful to the financial supports from the National Natural Science Foundation of China (Grant No. 51376031).
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