Research articleNO reduction by different tar agents and model compounds in a drop-tube reactor
Graphical abstract
Introduction
Nitrogen oxides (NOx) from fuel combustion not only injures human health but also forms acid rain and photochemical smog [1], [2]. Several technologies have been developed to reduce NOx emission during fuel combustion, the priority way of combustion NOx control. Our previous work shown that the so-called circulating fluidized bed decoupling combustion (CFBDC) is a low-NOx combustion technology that has been well proven to be effective in lowering at least 50% NOx formation comparing to other traditional combustion technologies [3], [4], [5]. The decoupling combustion technology separates the combustion process into drying/pyrolysis of fuel and combustion of char and volatile products consisting of non-condensable pyrolysis gas and condensable tar. In CFBDC, the pyrolysis-generated volatile is sent to an intermediate position of the char combustor to allow for its co-burning with char. The co-burning of fuel pyrolysis products in such an intermediate position could be considered as a reburning way that can effectively reduce the NOx formed by burning char in the bottom of the combustor.
Fuel reburning is a well-known low-NOx combustion technique. Injecting the reburning fuel into the reburning zone establishes actually a fuel-rich atmosphere where fuel is decomposed to release some reactive species. The NOx formed in a primary combustion zone with excessive air then can be reduced by such reactive species to form N2 [6], [7], [8], [9], [10]. In CFBDC, the reduction of NOx involves the effects of all pyrolysis products including char, tar and pyrolysis gas (non-condensable). In recent years, lots of studies have demonstrated the significant effect of char [11], [12], [13] and reducing gas [14], [15], [16] on NOx reduction in reburning process, but there were very few studies on the NOx reduction by tar. Luo et al. [17], [18], [19], [20] revealed that the tar components are highly helpful in increasing the NO reduction efficiency. The tar derived from fuel pyrolysis is composed of many aromatic species including phenols, BTX and other compounds such as aliphatic, carboxylic acid and ester groups [21]. At high temperatures, the decomposition of these species releases some hydrocarbon and non-hydrocarbon species like CHi and HCCO radicals, CO and H2 gaseous [22], [23], which are well-known to be the important intermediates for NOx reduction reactions [14], [16], [24].
In our previous studies [25], [26], we found that the tar derived from pyrolysis of distilled spirit lees (DSL) is an efficient agent for lowering NOx emission in the CFBDC system. With the aim of further understanding the mechanism of low-NOx emission in CFBDC treating different fuels, this study is devoted to revealing the characteristics of NO reduction by different tar agents. Sawdust and coal were selected to obtain the tars used in evaluating NO reduction, and the results were also compared with that realized using the DSL-derived tar through our previous work [25]. In addition, the NO reduction efficiencies realized by five representative model tar compounds were also further evaluated to understand the different NO reductions for different tars and to figure out the contributions of different agents to the achieved NO reduction in reburning.
Section snippets
Tar preparation and properties
The NO reduction agent tested in this study included tars derived from pyrolysis of sawdust (SD) and Xianfeng (XF) coal in a nitrogen fixed-bed reactor at 500 °C for 30 min. The tar was collected via a condenser at 5 °C and further dehydrated over anhydrous MgSO4. The temperature 500 °C is quite close to the those adopted for fuel pyrolysis in CFBDC and it also guarantees the higher yield and diversified composition of the prepared tar. At other higher temperatures, the tar would be mainly heavy
NO reduction by tar agents
Following the previous studies [6], [12], [17], [25], the reburning SR is further considered as a key parameter in evaluation of the NO reduction by tars. Fig. 2 presents the variations of NO reduction efficiencies (η) with SR at reaction temperatures of 800, 900, and 1000 °C for both SD tar and XF coal tar agents. The results obtained in our previous work [25] for DSL tar agent were also shown to make an effective comparison. Generally, the achieved trends of NO reductions by the SD tar and XF
Conclusions
Experimental studies on NO reduction characteristics of tar agents including biomass tar and coal tar were conducted in a drop-tube reactor to simulate the reburning conditions of pyrolysis products involved in the so-called circulating fluidized bed decoupling combustion (CFBDC) process for biomass. The adopted biomass tar and coal tar were prepared by pyrolysis of sawdust (SD) and Xianfeng (XF) coal, respectively. The results were compared to that obtained by using tar from distilled spirit
Acknowledgment
The authors are grateful to the financial supports from the National Basic Research Program of China (2014BAC26B04, 2014CB744303) and the National Natural Science Foundation of China (U1302273).
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The first two authors contribute to this work equally.