Xiang Xiaoab,
Ping Fang*ab,
Jian-Hang Huangab,
Zi-Jun Tangab,
Xiong-Bo Chenab,
Hai-Wen Wuab,
Chao-Ping Cenab and
Zhi-Xiong Tangab
aSouth China Institute of Environmental Science, Ministry of Ecological Environment of P. R. China, Guangzhou 510655, Guangdong, China. E-mail: fangping@scies.org
bThe Key Laboratory of Water and Air Pollution Control of Guangdong Province, Guangzhou 510655, Guangdong, China
First published on 24th July 2019
An experimental study on the effects of CO2 concentration on the release of reducing gases and the NO reduction efficiency by sludge reburning was carried out in a pilot scale cement precalciner. The results indicate that sludge reburning shows an ideal NO reduction activity. The best NO reduction efficiency of 54% is reached when the CO2 concentration is 25 vol%. Characteristic analysis of the sludge shows that the main types of reducing gases generated by sludge reburning are HCN, NH3, CO and CH4. Among them, CO2 concentration plays a crucial role in the release of HCN, CO and CH4. The mechanistic study indicates that NO reduction is dominated by homogeneous reduction during the sludge reburning process, in particular the reducing gases of CO and NH3 have significant influences on the NO reduction. Meanwhile, the effect of CO2 concentration on NO reduction is mainly due to the difference in CO release. The results of the present study not only provide insight into the mechanism of NO reduction by sludge reburning, but could also contribute to the development of NOX removal technology in the cement industry.
The production of cement consumes large quantity of original materials (clay, limestone, and fuel), which produces lots of air pollutants such as particles, SO2 and NOX. The cement industry has become the third largest industrial NOX emission source in China.13 Several NOX emission control technologies, such as low-NOX burners, staged combustion (air and fuel staged) and selective non-catalytic reduction (SNCR), have been developed. Among them, low-NOX burners and staged combustion technologies have been widely applied to control NOX emission due to the advantages of relatively low investment and operation costs. However, their removal efficiency is only roughly 30%. SNCR is also regarded as a proper technology in considering the temperature range of cement precalciner (850–1200 °C).14,15 Its NOX removal efficiency is approximately 40%. Nevertheless, the SNCR technology will consume a large amount of ammonia and cause the ammonia escape. Therefore, more efforts should be put into the development of more efficient and cost-effective NOX removal technology for cement industry in view of energy conservation and environmental protection.
With the gradual increase of sludge treatment in cement kiln, some researchers have found that co-processing sludge in cement kiln contributes to the decrease of NOX emission.16 For instance, Fang et al.14 discovered that combustion temperature, O2 concentration, sludge dosage and feed point influenced the NO reduction substantially. Lv et al.17 found that co-processing of sludge in cement kiln was beneficial to NO reduction, and the yield of NH3 released from sludge affected NO reduction remarkably. During sludge reburning under hypoxic conditions, various reducing species, such as short-chain hydrocarbon (mainly CH4), CO, HCN and NH3, are generated. These reducing species can effectively reduce NO to N2 by producing a large number of C-containing (CHi) and nitrogenous (NHi) radicals.15,18 It is worth noting that CH4 is firstly decomposed into CHi radicals, which react with NO to form HCN, and then HCN can reduce NO again by forming NHi radicals. Of course, due to the limitation of residence time, HCN may also be a precursor to generate NO.14,19 Although it has been reported that co-processing of sludge in cement kiln has synergistic denitration effect, the reaction mechanism between reducing gases released and NO reduction has not been well studied. In particular, the working conditions of the cement precalciner are relatively complicated, especially the CO2 concentration can be higher than 30 vol%.20 Besides, there existing coupling effects between the decomposition of cement raw meal and the combustion of sludge.21 Therefore, in order to elucidate the influence of CO2 concentration on NO reduction and its mechanism during sludge reburning in cement kiln in the presence of cement raw meal, the characteristics of reducing gases (HCN, NH3, CO, CH4) released and the properties of NO reduction should be researched in detail.
In this study, the types and sources of reducing gases released during sludge combustion were analyzed, and the influences of CO2 concentration on the release of reducing gases were systematically investigated. Meanwhile, the influences of CO2 concentration on the dynamic variation process of NO reduction by sludge reburning were researched. Furthermore, the relationship between homogeneous and heterogeneous NO reduction was also explored. On the basis of the above discussions, the results are expected to disclose the mechanism of NO reduction by sludge reburning in a pilot scale cement precalciner, which can provide theoretical references for improving the technology of industrial application and realizing the efficient disposal of sludge.
Sample | Proximate analysis (wt%) | Ultimate analysis (wt%) | ||||||
---|---|---|---|---|---|---|---|---|
Mad | Vad | Aad | FCad | C | H | N | S | |
Sludge | 6.71 | 30.42 | 58.82 | 4.05 | 27.11 | 3.13 | 3.62 | 0.76 |
Sludge char | 1.24 | 1.85 | 92.57 | 4.34 | 2.58 | 0.18 | 0.21 | 0.43 |
In order to maintain the constant temperature in the reaction tube and reduce the metrical deviation of gaseous products concentration, the added amounts of sludge/char and cement raw meal were 0.1 g and 1 g, respectively. Each experiment was repeated to ensure the reliability of the experimental data. The data deviation of repeated experiments was not more than 5% and the variation trend was consistent. In addition, the cement raw meal used in this experiment was prepared according to the saturation ratio, the silicic acid ratio and the aluminum oxygen ratio. The main components of cement raw meal were shown in Table 2.
Composition (w/g) | ||||||
---|---|---|---|---|---|---|
CaCO3 | SiO2 | Al2O3 | Fe2O3 | MgO | K2CO3 | Na2SO4 |
77.46 | 13.47 | 3.01 | 1.98 | 2.48 | 0.264 | 0.186 |
The O2 concentration was 3 vol%, the CO2 concentration was set as 0, 10, 25, 30 and 35 vol%, respectively, and N2 provided the balance. When the experiment of NO reduction was conducted, the initial concentrations of NO and SO2 at reactor inlet were designed as 600 and 200 mg m−3 respectively based on the atmospheric composition in cement precalciner. Other atmospheric conditions were consistent with the experiment condition of reducing gases released. Meanwhile, the reaction temperature of the conical zone was controlled as 900 °C.
FTIR spectrometer coupled with TGA can provide useful online observation of gaseous products released from the combustion process of sewage sludge. The three-dimensional FTIR absorbance spectra of gaseous products released from the combustion of the sewage sludge is shown in Fig. 3(a). Regions of strong IR absorbance can be identified, which is the absorption peak generated by CO2 anti-symmetric stretching vibration (2400–2240 cm−1). Fig. 3(b) shows FTIR absorbance stack spectra of gaseous products at 80, 290 and 520 °C, which are respectively screened from the maximum loss rates of three stages from DTG. At 80 °C, the broad band at 4000–3400 cm−1 corresponding to the vibration (O–H) of H2O. The decomposition of CO bonds at around 1780–1600 cm−1 led to the yield of CO. With the temperature rising to 290 °C, it is evident that there exist more gaseous products in addition to H2O and CO. The C–H bonds at around 3200–2700 cm−1 revealed that the compounds of alkyl and aliphatic hydrocarbons exist. The decomposition of C–H bonds led to the yields of CH4, C2H4, C2H6 and other light hydrocarbons. The C–N bonds at around 1340–1020 cm−1 attribute to the presence of amides, which can be decomposed to generate NH3. Furthermore, a strong HCN absorption peak appears at 748 cm−1.22,24 When the temperature is 520 °C, the gaseous products contain H2O, CO2, CO, HCN and hydrocarbons. The above results further indicate that the reducing gas of NH3 is mainly derived from low-volatility components and can be completely released at the initial stage of combustion, while CO, HCN and hydrocarbons are slowly converted from thermally unstable and stable functional groups.25
XPS measurement is carried out to investigate the chemical state of sewage sludge. As shown in Fig. 4(a), the C 1s spectra can be fitted with four characteristic peaks corresponding to COOH (288.3 eV), CO (287.2 eV), C–O (285.9 eV) and C–C/C–H (284.8 eV). The C–C/C–H is the major carbon functional group of sewage sludge, which accounts for about 60% of total carbon in sewage sludge. The contents of other functional groups (C–O, COOH, and CO) are 20%, 11%, and 9%. Fig. 4(b) shows that the nitrogen functionalities in sewage sludge are mainly presented as protein-N (53.73%), amine-N (33.65%), inorganic-N (11.54%), and pyrrole-N (1.08%), the corresponding electron binding energies of which are 400, 399.1, 401.7, and 400.6 eV, respectively.26 It is worth noting that the relative surface content of protein-N is much higher than that of other nitrogen functionalities.
In combination with the FTIR observation (Fig. 3), it is inferred that the CH4 release is due to the secondary cleavage of long fatty chain (C–C/C–H) and fracture of aliphatic chain attached to oxygen atom (COOH, CO, C–O). The thermal cracking of ether bond, hydroxyl group and oxygen-containing heterocycles is the main route for CO formation. NH3 is derived from the decomposition of inorganic-N and the deammoniation of amine-N which is produced by the protein pyrolysis. HCN is generated from the cracking of pyrrole-N, nitrile-N and heterocyclic-N, while the nitrile-N and heterocyclic-N are produced by the pyrolysis of protein.27,28
Fig. 5 Releasing rates (a) and yields (b) of HCN during sludge combustion under different CO2 concentrations. |
It is well-known that CO can be generated by the gasification reaction between CO2 and char.32 It can be speculated that the gasification rate is the main factor affecting the CO release under different CO2 concentrations. The releasing characteristic curves of CO are depicted in Fig. 6. The gasification rate gradually increased because the increase of CO2 concentration (0–25 vol%). The promoting effect is stronger when the CO2 concentration is higher due to a larger peak value and more CO released. However, when the concentration of CO2 further increased (25–35 vol%), the CO yield decreased. In combination with the influence of relatively higher CO2 concentration (25–35 vol%) on HCN release in Fig. 5, the excessive CO2 can reduce the local reaction temperature, which decreases the gasification rate between CO2 and char. Therefore, the inhibition effect of local reaction temperature decreased is obviously greater than the promotion effect of reactant concentration increased, which leads to the gradual decline of CO released at a relatively higher CO2 concentration (25–35 vol%). The above results indicate that an appropriate concentration of CO2 plays a key role in enhancing the CO release during sludge combustion.
Fig. 6 Releasing rates (a) and yields (b) of CO during sludge combustion under different CO2 concentrations. |
Fig. 7 illustrates the influence of CO2 concentration on CH4 release. The release of CH4 gradually decreases with the concentration of CO2 increasing from 0 to 25 vol%. This may be attributed to the gasification effect, which results in the promotion of condensation polymerization reaction between semi-char and tar. Then, the methyl functional group is more likely to participate in the condensation polymerization reaction, preventing the release of CH4 from sludge reburning. In addition, the increasing gasification rate probably increases the specific surface area of the activated char, enhancing the catalytic cracking reaction of CH4, which further reduces the release of CH4.27 However, the release of CH4 increased when the concentration of CO2 raised to 35 vol%. This phenomenon may be attributed to the fact that excessive CO2 may weaken the gasification rate, which decreases the surface area of the char. Thus, the effective contact area between CH4 and activated char is fewer, which results in the weakening of the catalytic cracking reaction of CH4.33
Fig. 7 Releasing rates (a) and yields (b) of CH4 during sludge combustion under different CO2 concentrations. |
The releasing characteristic curves of NH3 at different CO2 concentrations are shown in Fig. 8. The releasing peak value and amount of NH3 decrease with the increase of CO2 concentration from 0 to 25 vol%. This may be attributed to the suppression effect of CO2 through the consumption of H-radical and nitrogen-containing functional groups.34 When the CO2 concentration further increases to 35 vol%, the amount of NH3 released is relatively stable. Although a comparatively higher CO2 concentration weakens the gasification rate and reduces the exfoliation of H radical of char. In combination with the previous analysis of sludge characteristic in Fig. 3, the release of NH3 mainly occurred at the initial stage of combustion. NH3 formation is not at the same stage as the gasification reaction. So, the weakening of gasification reaction does not affect the release of NH3. Moreover, a relatively higher concentration of CO2 reduces the local reaction temperature, which may reduce the suppression effect that unstable nitrogen-containing functional groups are consumed by CO2. Thus, NH3 release is relatively stable with a considerably higher CO2 concentration (25–35 vol%).
Fig. 8 Releasing rates (a) and yields (b) of NH3 during sludge combustion under different CO2 concentrations. |
The ratio of the amount of the nitrogen-containing gases released to the nitrogen content in sludge is defined as the yield. The N2 yield is calculated by eqn (1). The yields of the N-containing gaseous products are shown in Fig. 9.
(1) |
Fig. 9 The conversion ratio of nitrogen-containing gaseous products under different CO2 concentrations. |
With the increase of CO2 concentration from 0 to 25 vol%, the yield of N2 increases significantly from 47% to 55%. When the CO2 concentration further increases to 35 vol%, the yield of N2 is stable, while the release of NOX somewhat increases. When combustion temperature and O2 concentration do not change, NOX release is mainly affected by the reduction reaction of NO, which preliminarily indicates that a higher CO2 concentration (25–35%) leads to the decline of the reduction of NO.
(2) |
Fig. 10(a) shows the variation of NO concentration during NO reduction by sludge reburning under different CO2 concentrations. There are three stages in the reaction between sludge and NO with oxygen concentration of 3%. The first stage is that the NO concentration increase and exceeds 600 mg m−3. When sludge is added to the reaction tube at one time (completed in 2 s), volatiles and fixed carbon are burned to generate large amounts of NO under well-oxygenated conditions. The amount of NO generated exceeds the amount that of reduced by reducing gas and nascent char. Therefore, the initial stage is dominated by the NO oxidation. At the second stage, with the gradual release of reducing gases (HCN, CO, CH4 and NH3), O2 in atmosphere is depleted quickly and is correspondingly insufficient surrounding the reducing species. Therefore, the amount of NO reduced by reducing gases and char is more than the amount of NO generated. This stage is dominated by the NO reduction, which decreases the NO concentration to less than 600 mg m−3 at the reactor outlet. At the last stage, as the reaction continues, the residual volatiles and char gradually decreased. The rate of O2 consumption becomes correspondingly lower, and O2 is sufficient to oxidize the reducing gases and char. Thus, the rate of NO reduction by reducing gases and char is lower than that of NO formation. The NO concentration at the reactor outlet is more than 600 mg m−3 again. This stage is dominated by NO oxidation.35
NO reduction efficiency at the reduction stage (second stage) is displayed in Fig. 10(b). It is observed that CO2 concentration affects NO reduction. NO reduction efficiency follows a pattern of first increasing and then decreasing with the increase of CO2 concentration. When the CO2 concentration increased from 0 to 25 vol%, NO reduction efficiency showed a slightly increasing trend. As the CO2 concentration further increased (25–35 vol%), the NO reduction clearly decreased. The optimal NO reduction efficiency is 54% at a CO2 concentration of 25 vol% during sludge reburning. Results suggest that a comparatively higher NO reduction efficiency (more than 50%) could be acquired during sludge reburning with a CO2 concentration of 10–30 vol%.
As presented in Fig. 11, variation of O2 and CO2 concentrations during sludge combustion also have some effects on NO reduction. When the sludge is added downward into the reaction tube, O2 in the atmosphere is consumed quickly, and a large amount of CO2 is generated during the combustion process. Compared with Fig. 10, there is a valley value for O2 consumption and a peak value for CO2 generation when the NO reduction efficiency reaches the maximum value. At the same time, the combustion reaction of sludge is the most severe, which is consistent with the variation of NO concentration.
Fig. 12 TEM (a–c) images, nitrogen adsorption–desorption isotherms and the corresponding size distribution curves (inset) of (d) sludge char. |
Sample | SBET (m2 g−1) | Total volume (cm3 g−1) | Peak pore size (nm) | Optimal NO reduction ratio |
---|---|---|---|---|
Sludge char | 70.12 | 0.12 | 6.63 | 11% |
Dynamic properties of NO reduction by sludge char are shown in Fig. 13. The optimum NO reduction efficiency by char reburning is 11%. The reducing capacity of char may be associated with the specific surface area and the distribution of active sites. On the basis of the previous discussion, the reduction efficiency of NO by sludge reburning can reach 54% under the same conditions, which implies that the reducing gases have an important influence on the NO reduction efficiency. The results indicate that NO reduction is dominated by the gas–gas homogeneous reduction reactions during sludge reburning.
Fig. 13 Dynamic properties of NO reduction by CO in the presence of sludge char. Reaction conditions: 600 mg m−3 of NO, 25 vol% of CO2, 3 vol% of O2, and balanced N2, temperature = 900 °C. |
In order to characterize the gas–gas homogeneous reduction reactions, the dynamic processes of NO reduction by CO are investigated. CO exhibited negligible activity on the reduction of NO from the result of blank experiment. The addition of CO can improve NO reduction efficiency in the presence of char, which confirm that the NO reduction by CO requires the presence of catalysts.36 Compared with the case without CO, the NO reduction increased about 8.5% and 19% corresponding to the CO concentration of 600 and 2400 mg m−3, respectively. The increase of NO reduction efficiency over sludge char is attributed to the enhancement of solid–gas reaction between NO and char and its surface-catalyzed reaction of NO with CO.36 The results indicate that the reactions between CO, NO and char play an important role in NO reduction, especially under a relatively higher concentration ratio of CO to NO. The highest reduction ratio of 30% is achieved when the concentration ratio of CO to NO is 4:1.
NO can react with NH3 to produce the final product N2, which involves three reactions presented as eqn (3)–(5):14,18,37
NH3 + ˙O → ˙NH2 + ˙OH | (3) |
NH3 + ˙OH → ˙NH2 + H2O | (4) |
˙NH2 + NO → N2 + H2O | (5) |
When the moisture content of 0%, the gas–gas homogeneous phase reaction between NO and NH3 does not occur without moisture. Thus, it is reasonable to deduce that ˙OH and ˙O radicals involved in these reactions initially come from the thermal decomposition of H2O.14,15 To investigate the gas–gas homogeneous reduction reactions of NH3 to NO, a certain amount of water vapor needs to be added. The influence of NH3 concentration on NO reduction with an H2O concentration of 3 vol% is shown in Fig. 14(a). The NO reduction efficiency (52.3%) under a NH3 concentration of 1200 mg m−3 is much higher than that under a NH3 concentration of 300 mg m−3 (29.6%). The significantly improved NO reduction can be ascribed to the increase of molecular weight of NH3 in unit gas and the more sufficient reaction contact areas. When the concentration ratio of NH3 to NO continually increases to 4:1, the NO reduction efficiency only increases to 56%. Besides, the influence of CH4 concentration on NO reduction is presented in Fig. 14(b). When the concentration ratio of CH4 to NO is 4:1, the NO reduction efficiency is only 6%, which shows that CH4 exhibits negligible effect on NO reduction compared with CO and NH3. CH4 can be decomposed into CHi radicals, which react with NO to form HCN.14,19 Theoretically, the reduction of NO by CH4 will show high efficiency. However, the actual results are opposite. It is reasonable to deduce that when CHi radicals react with NO to form HCN. The generated HCN will be oxidized to NO as a precursor under the condition of a certain residence time (approximately 1.5 s), reaching a balance between the reduction and oxidation of NO.
Fig. 14 Efficiency of NO reduction by NH3 (a) and CH4 (b). Reaction conditions: 600 mg m−3 of NO, 3 vol% of O2, 25 vol% of CO2, 3 vol% of H2O, and balanced N2, temperature = 900 °C. |
(1) CO2 concentration plays an important role on NO reduction during sludge reburning. The maximum NO reduction efficiency of 54% can be achieved with a CO2 concentration of 25 vol%. It is feasible to use sludge as a reducing agent for NO reduction in the cement industry.
(2) The homogeneous and heterogeneous reduction reactions of NO occur simultaneously during sludge reburning. According to the reduction performance of NO by NH3, CO, CH4 and sludge char, CO and NH3 make greater contributions to the NO reduction than sludge char. So NO reduction is dominated by homogeneous reduction reactions.
(3) The main types of reducing gases produced during sludge reburning are HCN, NH3, CO and CH4. Among them, NH3 is mainly derived from low volatility components combustion, while HCN, CO and CH4 are produced from thermally unstable and stable functional groups. Meanwhile, CO2 concentration plays an important role in the release of HCN, CO and CH4. The influence of CO2 concentration on NO reduction is mainly attributed to the CO released.
This journal is © The Royal Society of Chemistry 2019 |