3.1. Effect of temperature on odor emission
As shown in Fig. 2, the release characteristics of main sulfur-containing odors CS2, H2S, and SO2 with drying temperature were measured via the spectrophotometer method.
It shows that although the release of CS2 is not large during the drying process, the growth rate with increasing temperature is large because CS2 is produced by the reaction of CH4 and FeS and the reaction rate increases with increasing temperature; however, the peak temperature of CS2 formation is 500°C. In the temperature range of 150°C–350°C and 10 min, the release of CS2 increased from 17 to 27.2 μg/g, which is an increase of 62.5%. When the temperature is lower than 200°C, the release amount of H2S gas released is less than 50 μg/g, whereas when the temperature is higher than 200°C, the release amount of the gas released rapidly increases sharply, reaching 386.25 μg/g at 250°C. This is because of the large amount of organic acids produced in this temperature range, which transforms inorganic sulfides into H2S or decomposes a large amount of organic sulfides[19]. When the temperature rises above 250°C, the growth of H2S becomes more gradual and, eventually, constant. According to the influence of drying temperatures on SO2 emission, when the temperature is less than 250°C, SO2 release is slow and less intense. When the temperature reaches 300°C, SO2 emission increases from 88.74 to 307.81 μg/g, an increase of ~300%. This is because a large amount of SO2 is produced by the decomposition of sulfur-containing aliphatic compounds at 300°C, and then, with the increase in temperature, the release amount of SO2 increases growth is extremely slow.
To conclude, controlling the drying temperature of sludge should be controlled below 250°C reduces the release of the three gases is less.
3.2. Effect of time on odor emission
It can be seen from Fig. 3 that the release characteristics of the main sulfur gases CS2, H2S, and SO2 with drying time were measured via the spectrophotometer method. The figure shows that as time passes, the amount of the three gases released increases continuously, with the gas release amount of gas released increasing significantly at 30 s and tending to be constant at 10 min, and there is no gas was released afterward. This is due to heat transfer between the materials as the sludge is stacked in the three beakers. It takes a certain amount of time to heat the sludge to be heated from room temperature to 250°C. The sludge release rate is low during this time. When the sludge temperature reaches 250°C, the gas release rate increases continuously, releasing a large amount of gas. Fig. 3 shows that the release of CS2 and H2S is mainly concentrated in the first 3 min, with the release of C2S and H2S being 20.84 and 339.42 μg/g, respectively, accounting for 93.2% and 93.4% of the total release. At this temperature for CS2, most CH4 produced by sludge pyrolysis reacts with FeS within 3 min and is consumed completely. For H2S, this is mainly due to the complete decomposition of most sulfur-containing organic acids. When the time is 2 min, the SO2 release amount of SO2 released can reach 77.58 μg/g, accounting for 92.4% of the total release, and then, the release rate rapidly decreases. This is because the sulfur-containing aliphatic group that can be decomposed in the sludge decomposes almost completely in 2 min at a temperature of 250°C.
3.3. CBP inhibits the release of sulfur gas
Fig. 4 shows the variation of H2S concentration during sludge drying with different CBP contents. The concentration of H2S in the waste gas decreases significantly as CBP content increases. The concentration of H2S increases with increasing temperature. The concentration of H2S in the gas produced by the drying of pure sludge is 1038.62 mg/m3 at a temperature of 250°C. When 10 wt% CBP was added, the concentration of H2S was reduced by 85%; when the mass ratio of CBP to sludge was 1:5, the concentration of H2S was only 59.81 mg/m3, which was 94% lower than that of dry sludge gas (1038.62 mg/m3) without CBP. Additionally, the mixed CBP and pH value of sludge were determined. When the amount of CBP was 10 wt% (CBP:sludge = 1:10), the pH value of sludge increased from 6.5 to 11.1, whereas the concentration of H2S in the tail gas decreased significantly decreased. The concentration of H2S was 50.57 mg/m3 when dried at 200°C.
Previous studies have shown that heat decomposed aliphatic sulfur and aromatic sulfur in sludge, and then, C–S bonds were broken, producing sulfur-containing gas [20]. Additionally, alkali can inhibit the release of sulfur-containing gas during sludge drying, and the stronger the alkalinity, the better the effect [16,21]. This is because, at a certain temperature, alkali can promote the oxidation of aliphatic sulfur and aromatic sulfur to sulfoxide and sulfone in sludge, respectively, and eventually sulfonic acid may be produced [6,22]. Moreover, sulfoxide and sulfone almost do not produce sulfur-containing gas during the drying process, and their properties are more stable. Fig. 5 shows that R–S–R uniformly represents aliphatic sulfur and aromatic sulfur. Sulfoxide, sulfone, and sulfonate can be labeled as R–SO–R, R–SO2–R, and R–SO3–R, respectively. Because of the low bond dissociation energy of the C–S bond in R–S–R compounds, the C–S bond is easy to cleave at low temperatures to form sh-radical and then form H2S. Following the addition of alkali, a series of reactions, as shown in Fig. 5, occur because of the action of OH−. Finally, hexavalent sulfur sulfonic acids are formed through nucleophilic addition, which greatly inhibits the release of H2S and other gases. Some scholars have proposed that the active components of the conditioner added in the sludge can react with some free groups in the sludge to form precipitation, so that most of the sulfur elements are fixed in various solid compounds, and the relative ratio of sulfates and inorganic sulfides is rapidly increased sharply [23]. Thus, the addition of CaO can cause a series of complex physical and chemical reactions of sulfur-containing substances in sludge, forming chelates with high stability and nonpolar calcium salts.
Additionally, alkalinity inhibits the growth of sulfate-reducing bacteria (SRB). SRB are anaerobic bacteria that can reduce elemental sulfur or sulfate to H2S and other sulfur-containing gases. Thus, SRB activity is crucial for sulfate, sulfite, and organic sulfide in sludge to produce sulfur-containing gas [24]. It is found [25]that the optimal PH value for SRB growth is 7.0. Hence, the release of H2S and SO2 can be inhibited by controlling the pH value of the sludge drying process and inhibiting the growth of SRB. To conclude, the addition of calcium-based ultrafine powders, on one hand, uses the strong oxidizing hydroxyl group to change the proportion of all types of organic sulfur in the sludge; on the other hand, it provides an alkaline environment and changes its internal biochemical conditions, thereby inhibiting the release of sulfur-containing gases.
3.4. Absorption and adsorption of sulfur-containing gas by CBP
Some characteristic gases collected in the airbag are shown in Tables 2 and 3. Tables 2 and 3 show that a large number of alkanes, alkenes, alkynes, CH compounds, alkanes, and alkanols are produced when the sludge is decomposed at 300°C. According to the chemical composition of gases in Table 2, the sludge is decomposed without any treatment to produce various amino acids, organic compounds containing nitrogen and sulfur, and various sulfur-containing gases, such as H2S, CS2, and COS. Comparing the gas composition in Table 3, there are almost no sulfur-containing organic compounds and sulfur-containing gases. After CBP adsorption, the types of gas collected in the airbag did not significantly change much, but the substances containing S and N in Table 3 were reduced compared with those in Table 2, indicating that the adsorption of sulfur-containing gas in sludge using calcium-based ultrafine powder was very obvious, which helped control the emission of odor gas. The odor of sludge particles is obvious, but when the particles are completely coated by CBP, the odor is hardly emitted. This is because the odor is covered by a large amount of powder on the surface of the sludge particles, preventing it from passing through the surface fly ash layer.
Calcium-based desulfurizers commonly used in the market now include CaO, Ca (OH)2, and CaCO3 [26], and the main component of CBP is CaCO3. CaCO3 can produce CaO particles with high specific surface area and high porosity during calcination, which can absorb sulfur gases such as H2S and SO2. Second, CaO reacts with SO2 to form CaSO4 to realize desulfurization and sulfur fixation . The chemical reactions that occur are as follows:
CaCO3 → CaO + CO2
|
(1-1)
|
Ca(OH)2 → CaO + H2O
|
(1-2)
|
SO2+CaO → Ca SO3
|
(1-3)
|
SO2+H2O →H2SO3
|
(1-4)
|
Ca(OH)2+ H2SO3 →CaSO3+2 H2O
|
(1-5)
|
CaSO3 + 1/2O2 → CaSO4
|
(1-6)
|
Because of the evaporation of aliphatic compounds in the process of sludge drying at 200°C–450°C, water in the sludge drying process is not easily lost. The produced H2S can be partially ionized in the presence of water, and H+ and HS− can be generated by one-step ionization, and a small amount of HS− can be ionized in two steps to form S2−; with CaO, the amount of H2S can be ionized to form S2-, and by increasing its content, it reacts with water to release heat and form Ca (OH)2. The alkalinity in the system increases gradually, neutralizing more H+, thus promoting the ionization of H2S and producing more S2−, which can react with CaO to produce CaS or react with hydrated Ca(OH)2 to form CaS; SO2 produced from organic sulfur decomposition will also react with Ca(OH)2 to form CaSO3, and CaSO4 is more stable under oxidation conditions [27]. Other experimental results [6]also showed that when the sludge was dewatered with CaO as a conditioning agent, alkaline CaO promoted the conversion of most H2S and SO2 to CaS and CaSO4, resulting in a rapid increase in the relative ratio of sulfate to inorganic sulfide. In particular, the addition of CBP, on one hand, produces sludge in an alkaline environment, and the acid-free H2S and SO2 molecules can react with CaO and Ca(OH)2 to form stable calcium salt, thus reducing the inorganic sulfur release process in the subsequent drying process; on the other hand, the alkaline calcium materials can absorb the release of sulfur-containing organic matter during the sludge drying process. Acid gases such as H2S and SO2 can further reduce the sulfur released during the drying process.
Generally, the chemical process of absorption must be accompanied by the adsorption process. Calcium-based superfine powder can absorb and adsorb sulfur-containing gas at the same time. The main components of CBP powder are SiO2 and CaCO3, and it also contains a lot of amorphous SiO2 and Al2O3, which can be regarded as a type of pozzolanic ash. In the presence of water at normal temperature, properly crushed pozzolan can react with an alkali metal and alkaline earth metal hydrate.
These newly formed hydrated aluminosilicates are usually incomplete crystals, mostly fibrous, with large specific surface area and high-water holding capacity [28]. The results show that the reaction of (2-1)–(2-4) changes the surface structure of the powder, increases the specific surface area, improves the pore structure, and improves the pore structure and the gas adsorption effect, whereas the high-water holding capacity increases the humidity of the powder particles and accelerates the reaction on the surface [29]. CBP powder may also play a catalytic role, especially the high content of silicon, iron, magnesium, and aluminum, and some trace elements can also promote the absorption of gas [30]. Additionally, a large number of studies have shown that the potentially active powder and CaO can be digested to form calcium silicate hydrates and form loose porous structure, thus greatly improving the adsorption of sulfur-containing gas.
Ca(OH)2 + SiO2 + H2O → (CaO)x(SiO2)y(H2O)z
|
(2-1)
|
Ca(OH)2 + A12O3 +H2O → (CaO)x(A12O3)y(H2O)z
|
(2-2)
|
Ca(OH)2 + A12O3 + SiO2 + H2O → (CaO)x(A12O3)y(SiO2)z(H2O)w
|
(2-3)
|
Ca(OH)2 + A12O3 + SO3 + H2O → (CaO)x(A12O3)y(CaSO3)z(H2O)w
|
(2-4)
|
Table 2. Representative gas released by sludge at 300°C
Number
|
Type
|
Appearance time (min)
|
Name
|
Chemical
formula
|
Molecular weight
|
1
|
Hydrocarbon organic matter
|
0.392
|
Ketene
|
C2H2O
|
42.011
|
0.815
|
Oxalic acid
|
C2H2O4
|
89.995
|
2.258
|
Methanol-D4
|
CD4O
|
36.051
|
11.729
|
(R,R)-Tartaric acid
|
C4H6O6
|
150.016
|
11.729
|
Butanedioic acid, 2,3-dihydroxy-, [S-(R*,R*)]-
|
C4H6O6
|
150.016
|
2
|
Nitrogenous compounds
|
0.620
|
Diazirine
|
CH2N2
|
42.022
|
0.620
|
Pyrazine, methoxy-, 1-oxide
|
C5H6N2O2
|
126.043
|
0.815
|
Dimethylamine
|
C2H7N
|
45.058
|
0.815
|
Epinephrine
|
C9H13NO3
|
183.09
|
0.815
|
2-Hexanamine, 4-methyl-
|
C7H17N
|
115.136
|
0.815
|
2-Propanamine, 1-methoxy-
|
C4H11NO
|
89.084
|
3.798
|
2-Amino-1,3-propanediol
|
C3H9NO2
|
91.063
|
6.214
|
Methyl isocyanide
|
C2H3N
|
41.027
|
10.133
|
Butane, 1-isocyano-
|
C5H9N
|
83.073
|
3
|
Sulfur-containing substances
|
0.620
|
2-Pyrrolidinethione
|
C4H7NS
|
101.03
|
2.258
|
Hydrogen sulfide
|
H2S
|
33.988
|
3.798
|
Carbonyl sulfide
|
COS
|
59.967
|
3.798
|
Ethanone, 1-(5-methylfur-2-yl)-, Thiosemicarbazone
|
C8H11N3OS
|
197.062
|
11.729
|
Carbon disulfide
|
CS2
|
42.011
|
11.729
|
4,4'-Diisothiocyanatostilbene-2,2'-disulfonic acid
|
C16H10N2O6S4
|
453.942
|
11.729
|
Thiourea
|
CH4N2S
|
76.01
|
11.729
|
Monoethyl carbonotrithioate
|
C3H6S3
|
137.963
|
11.729
|
Mono-sec-butyl carbonotrithioate
|
C5H10S3
|
165.994
|
11.729
|
Mecysteine
|
C4H10ClNO2S
|
151.979
|
22.729
|
Monoisopropyl carbonotrithioate
|
C4H8S3
|
151.979
|
Table 3. Representative gas components collected by sludge 300°C heating sampling bag
Number
|
Type
|
Appearance time (min)
|
Name
|
Chemical
formula
|
Molecular weight
|
1
|
Hydrocarbon organic matter
|
22.153
|
Cyclobutanol
|
C4H8O
|
72.058
|
22.379
|
Phthalan
|
C8H8O
|
120.058
|
Benzene, (butoxymethyl)-
|
C11H16O
|
164.12
|
2
|
Nitrogen oxide organic matter
|
22.379
|
2-Propanamine, 1-methoxy-
|
C4H11NO
|
89.084
|
22.153
|
l-Guanidinosuccinimide
|
C5H7N3O2
|
141.054
|
l-Alanyl-l-alanyl-l-alanine methyl ester
|
C10H19N3O4
|
245.138
|
dl-Alanine ethyl ester
|
C5H11NO2
|
117.079
|
Acetic acid, hydroxy[(1-oxo-2-propenyl) amino]-
|
C5H7NO4
|
145.038
|
L-Alanine, methyl ester
|
C4H9NO2
|
103.063
|
Cathinone
|
C9H11NO
|
149.084
|
Benzenemethanol, 3-hydroxy-.alpha.-[(methylamino) methyl]-, (R)-
|
C9H13NO2
|
167.095
|
3
|
Nitrogenous compounds
|
22.153
|
n-Hexylmethylamine
|
C7H17N
|
115.136
|
Amphetamine-3-methyl
|
C10H15N
|
149.12
|
N-Dodecylmethylamine
|
C13H29N
|
199.23
|
2-Heptanamine, 5-methyl-
|
C8H19N
|
129.152
|
1-Octadecanamine, N-methyl-
|
C19H41N
|
283.324
|
22.397
|
1-Propanamine, N,2-dimethyl-
|
C5H13N
|
87.105
|
sec-Butylamine
|
C4H11N
|
73.089
|
|
Benzeneethanamine, N-methyl-
|
C9H13N
|
135.105
|