A comprehensive study on NOx emission and fuel nitrogen conversion of solid biomass in bubbling fluidized beds under staged combustion
Introduction
In recent years, the utilization of biomass as alternative energy sources has become much more popular. According to BP Statistical Review of World Energy, the global consumption of renewable energy increases from 107 million tonnes of oil equivalent (Mtoe) in 2007 to 486.8 Mtoe in 2017, which accounts for 3.6% of the total consumption of primary energy [1]. Slade et al. [2] also did a systematic review on the estimates of global bioenergy potential, in which most of them are for 2050, and summarized that the most potential sources are energy crops, agricultural residues, forestry residues, wastes, and forestry, ranging from 3 to 1272 EJ.
For solid biomass, fluidized-bed combustion (FBC) is one of the most suitable and effective technologies for energy recovery because of its fuel flexibility and low emissions. As a matter of fact, the combustion tests of several kinds of biomass such as rice husk [3], [4], [5], [6], sawdust [6], [7], [8], and other agricultural residues [9], [10], [11] were successfully carried out in FBC and showed high combustion efficiency. Although the gaseous emissions, especially NOx (NO and NO2), are much lower in FBC, it is still a major concern for biomass combustion as a result of increasingly stricter regulations [12].
Mahmoudi et al. [13] have categorized the de-NOx techniques into combustion modification and flue gas treatment. By comparison, NOx reduction by means of flue gas treatment such as selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) is much higher than that by combustion modification, and the efficiency can reach up to 80–90%. However, the relatively high cost of catalysts especially noble metals and the risk of NH3 slip make these techniques less cost-effective, and the advantage of FBC would be greatly weakened if adopting the SCR system [12]. Therefore, this study mainly focuses on the effect of operating parameters on NOx emissions during biomass combustion in FBC.
Among controllable parameters in FBC, bed temperature and excess air are usually applied to study their effects on NOx emissions. Generally speaking, NOx emissions increase with bed temperature and excess air due to the enhanced reaction rate and increased oxidant, respectively. In some cases, however, inconsistencies would emerge. Some researchers have noted that there is no clear dependence between NOx and bed temperature [14], [15], [16], while others found that the NOx emission would begin to drop at a certain temperature [17], [18]. Similarly, Atimtay [19] indicated that NOx emission would remain constant with increasing excess air or show a slight increase even at the excess air ratio of 1.7 for co-combustion of peach stone and coal. From these references, it can be found that each fuel may exhibit different emission behaviors even at the same conditions. A more comprehensive study is therefore needed for better realization.
Air-staged combustion [8], [15], [20], [21], [22], [23], [24], [25] and flue gas recirculation (FGR) [26], [27], [28], [29] are widely applied as strategies to reduce NOx emissions in all kinds of furnaces owing to the cost-effectiveness, and the reduction efficiency can reach up to 30–60%. As is well known, the deeper the staged combustion, the lower the NOx emissions. In FBC, air is usually employed as an oxidant as well as fluidizing gas, so it cannot be ensured that the reduction under deeper staged conditions is induced by the deficiency of the oxidant or by the change of the gas flow, since NOx emission was also found to decrease with decreasing superficial velocity [30], [31]. As for FGR, it is reported that NOx reduction over char may be critical in FBC [32], [33]. Zhao et al. [34] indicated that the reactivity of biomass char toward NO-char reaction is 4–5 times higher than that of coal char at 850 °C. To achieve higher reduction efficiency, the application of FGR in biomass combustion appears to be an accessible method.
Fuel characteristic is generally considered to be the most important factor to affect NOx emissions [35], [36]. In particular, the content of fuel nitrogen (fuel-N) is commonly taken as a rough guideline for NOx emission of fuel. Chen et al. [37] burned 21 kinds of coals in a down-fired furnace and found a positive correlation between NOx emissions and nitrogen content. The inference is widely accepted, and the results of several studies also supported that fuel with higher nitrogen content would bring about more NOx emissions [38], [39], [40]. Nevertheless, Jenkins et al. [41] concluded a reversed trend from the collected data of biomass, and a rather scattered distribution of NOx production was observed. It is worth mentioning that these data were from different combustors, so the parameters varied case by case, which would strongly influence the fuel burnout as well as the nitrogen conversion. In any case, both results of Chen et al. [37] and Jenkins et al. [41] only showed broad tendencies.
The elemental ratio was also considered to be correlated well with the fuel-N conversion to NOx. Aho et al. [42] found that the N2O/NO ratio decreased with increasing O/N ratio of a fuel. Chyang et al. [43] proposed that the H/N ratio is a better indicator for fuel-N conversion to NO. Vermeulen et al. [44] concluded that the CH/N ratio is more representative than O/N and H/N ratios by comparing five sets of data. Konttinen et al. [45], however, applied the CH/N ratio to correlate with fuel-N conversion and found that there was no clear dependence between them, which was attributed to the possible NO reduction during biomass pyrolysis.
To comprehensively realize NOx emission behavior of biomass in FBC, the combustion tests of ten solid biomasses, as well as sub-bituminous coal for comparison, are summarized in this study. The effects of bed temperature, excess oxygen, staged combustion, and FGR on NOx emissions are quantitatively compared and discussed. Moreover, the correlation between fuel characteristics and fuel-N conversion is also analyzed, attempting to gain the connection between them.
Section snippets
Fuel characterization
Table 1 shows the fuel analyses of the solid biomasses together with the coal. As can be seen, the content of volatile matter in biomasses is above 60%, indicating that they are much easier to ignite than the coal. The nitrogen content of these fuels ranges from 0.2 to 16.8%, which is helpful to understand the NOx emission characteristics for a wide range of fuels. Moreover, this range of nitrogen content covers most of the fuels, so its correlation with NOx release behavior may be much
Bed temperature
The bed temperature in fluidized-bed combustion is regarded as one of the key parameters for fuel burnout and gaseous emissions. Higher temperature can facilitate the rates of several reactions, such as the oxidation of carbon and CO, but it may also induce elevated yield of NOx. Fig. 1 shows the effect of bed temperature on NOx emissions in the range of 700–900 °C. In order to give an overview of NOx emissions for all the biomasses, those under fixed bed temperatures, including camellia shell,
Conclusions
In this study, the effects of operating parameters (including bed temperature, excess oxygen, staged combustion, and FGR) on NOx emissions in FBC are comprehensively investigated. The results show that:
- 1.
NOx emissions increase monotonically with bed temperature and excess oxygen, in which the increment with excess oxygen is more noticeable than that of the other.
- 2.
Lowering bed temperature by water addition seems to be a perfect way to reduce NOx emission and agglomerate formation simultaneously,
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