In earlier studies, NSCWG of model compounds and biomass without catalyst were widely investigated by some researchers. They indicated that high reaction temperature was required so that it could get more amount of gas fuel and achieve
0.0 0.1 0.2 0.3 0.4 0.5 0.6
450 550 Black liquor
Concentration / g⋅l-1 Concentration / g⋅l-1
Temperature / °C
Temperature / °C 2-methoxy-phenol
phenol
4-ethyl-2-methoxy-phenol m-cresol
4-ethyl-phenol 2,6-dimethoxy-phenol
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
450 550 2-methoxy-phenol
phenol
4-ethyl-2-methoxy-phenol m-cresol
4-ethyl-phenol 2,6-dimethoxy-phenol
Black liquor
(a)
(b) Fig. 13.21 Effect of
temperature on liquid products of 9.5 wt% black liquor gasification in supercritical water with microminiature continuous reactor (25 MPa, flow rate:
a5 kg/h,b7 kg/h)
the nearly complete gasification. However, the problems of high reactor cost and corrosion, etc., were caused by the high reaction temperature. Therefore, low reaction temperature was required. Osada et al. [66] identified three temperature regions in NSCWG: (1) Region I (500–700C supercritical water) biomass decomposed and AC catalyst was used to avoid char formation or alkali catalyst facilitated the water–gas shift reaction. (2) Region II (374–500C, supercritical water) biomass hydrolyzed and metal catalysts facilitated gasification. (3) Region III (below 374C, subcritical water) biomass hydrolysis was slow and catalysts were required for gas formation. In the low temperature region of Region II and Region III, the use of catalyst was necessary. It could not only reduce reaction temperature but also enhance the formation of gas products. So, the use of catalyst was very important for NSCWG of model compounds and biomass, and it might be also important for organic wastes gasification. The catalysts used in NSCWG could be mainly categorized into two types: One is the homogenous catalyst and the other is the heterogeneous catalyst [67].
13.6.1 Homogenous Catalysts
The alkali catalysts including NaOH, KOH, Na2CO3, and K2CO3[15,16,20,52, 68] are the main homogenous catalysts, and other catalysts such as trona (NaH- CO3Na2CO32H2O), nickel(II) acetylacetonate (Ni(acac)2), cobalt(II) acetylace- tonate (Co(acac)2), and iron(III) acetylacetonate (Fe(acac)3) are also used [69]. In organic waste gasification, the homogenous catalysts are widely used.
Jarana et al. [33] investigated the effect of KOH catalyst on oleaginous wastewater and alcohol distillery wastewater gasification in supercritical water.
They found that the addition of KOH catalyst increased the CODr and enhanced the formation of H2and CH4 at the same reaction condition. Nakhla et al. [34]
studied the effect of NaOH catalyst on hog manure gasification. They found that the NaOH catalyst could enhance the reduction of COD better than other catalysts, and the CODr achieved 81 %. Onwudili and Williams [35] investigated the effect of NaOH concentration on bio-diesel plant waste reforming at 380C and they found that the molar fraction of H2increased with the increase of concentration.
The maximum molar fraction of H2reached 90 % at 3 M NaOH concentration.
Analysis of gas yield and the products
distributuion of gas, liquid and solid
SEM, BET and FTIR analysis
TOC and GC/MS analysis
Fig. 13.22 Schematic diagram of the analysis process
Schmieder et al. [16] studied the effect of K2CO3catalyst on sewage sludge gasification at 450C with batch reactor. They found that the addition of K2CO3 catalyst enhanced the decrease of TOC and the TOC destruction efficiency reached 85.3 %. Yanik et al. [36] investigated the effect of K2CO3and trona catalysts on leather waste gasification in supercritical water. They found that the yield of H2 increased significantly when the two catalysts were used. The K2CO3catalyst was the best catalyst on the leather waste gasification. In SKLMF, Chen et al. [44]
studied the effect of NaOH, KOH, Na2CO3, and K2CO3on sewage sludge gasi- fication in fluidized bed reactor. They found that the addition of alkali catalysts was suitable for sewage sludge gasification in supercritical water with fluidized bed reactor and the addition of alkali catalysts enhanced the formation of H2better.
The maximum yield of H2reached 15.49 mol/kg by using KOH. The maximum value of GE and CE reached 53.22 and 46.39 %, respectively when K2CO3was used.
The effect of KOH on the gasification of black liquor with batch reactor (140 ml volume) was also studied in SKLMF. As shown in Fig.13.23, the yield of H2increased when KOH catalyst was used. The maximum yield of H2reached 11.15 mol/kg and the CODr reached 95 %.
13.6.2 Heterogeneous Catalysts
Compared with the homogenous catalysts, the heterogeneous catalysts have the advantages of high selectivity, recyclability, environment-friendly, etc. They mainly include AC, metal, and metal oxide catalyst. Metal catalysts, including noble metals such as Pt, Pd, Ru, Rh [70–73] and low-cost metal such as Ni [69,74–76], are widely
None KOH
0 5 10 15 20 25 30 35 40
CODr / %
Gas yield / mol⋅kg-1
CO2 CH4
CO H2
CODr
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Fig. 13.23 Effect of KOH catalyst on gas yield and CODr of 7.8 wt% black liquor gasification in supercritical water with batch reactor (500C, black liquor: 10 g, catalyst amount: 1 g, N2initial pressure: 4 MPa, residence time: 10 min)
used in NSCWG. Ca(OH)2, AC [41,42] and metal oxide [77,78] were also used as catalysts in these experiments. In organic waste gasification, the heterogeneous catalysts are widely used.
Nakhla et al. [50] studied the effect of Pd/AC, Ru/Al2O3, Ru/AC, and AC catalysts on hog manure gasification. The addition of catalysts enhanced the for- mation of H2and the Pd/AC catalyst produced the highest H2yield followed by Ru/Al2O3. The negligible CO yield during the Pd/AC, Ru/Al2O3, Ru/AC, and AC catalytic experiments was found. The use of them could get the similar CODr and the maximum value reached to 71 % when Ru/AC catalyst was used. Yanik et al.
[36] studied the effect of red mud (Fe-oxide containing residue from Al-produc- tion) on leather waste gasification. Although the catalytic efficiency of red mud is lower than other alkali catalysts, it also could enhance the formation of H2better and the yield of H2was almost doubled compared to that without catalysts.
Yamamura et al. [37] investigated the effect of RuO2 on paper sludge and sewage sludge gasification at 450C and 120 min. They found that RuO2could enhance the paper sludge gasification better and the yield of H2was higher. The increase amount of RuO2resulted in a decrease in the yield of H2and an increase in the yield of CH4, and they attributed this result to effect of RuO2catalysis on both the gasification reaction and the methanation reaction. Afif et al. [39]
investigated the effect of Raney nickel on activated sludge gasification in near- critical water. They found that the addition of Raney nickel had a positive effect on the total gas yield and it increased almost linearly from 3.5 to 33 mol/kg at a catalyst loading of 1.5 g/g after which the total yield reached a plateau level at 380C and 15 min. The yield of H2and CE also increased with the increase of catalyst loading and the maximum yield of H2 reached 14 mol/kg at a catalyst loading of 1.5 g/g and the maximum value of CE reached 70 % at a catalyst loading of 1.8 g/g. Xu et al. [41,42] studied the effect of AC on sewage sludge gasification in supercritical water with continuous reactor. They achieved nearly complete gasification of sewage sludge with a feed concentration of 2.8 wt% at 650C by using carbon catalyst and also achieved 77 % CE when the feed con- centration of sewage sludge is 2.8 wt% at 600C by using carbon catalyst and the maximum yield of H2reached 11 mol/kg. However, catalyst loading is about two- thirds of the heated zone volume of the reactor in their experiments. In SKLMF, the effect of Ru/C and various Raney catalysts on sewage sludge gasification in near- and supercritical water was investigated in batch reactor. The Raney cata- lysts included Raney-copper (RTH6110), Raney-cobalt (RTH7110), Raney-nickel 3110 (RTH3110 including Mo), Raney-nickel 3140 (RTH3140 including Mo), Raney-nickel 4110 (RTH4110), and Raney-nickel 5110 (RTH5110 including Fe).
As shown in Fig.13.24, the maximum yield of H2 was obtained when the RTH3140 was used whenever the reaction happened in near- or supercritical water. The catalytic activity for H2production in supercritical water was in the following order: RTH3140[RTH3110[Ru/C[RTH5110[RTH4110[ RTH6110[RTH7110, and the catalytic activity for H2production in near-critical water was in the following order: RTH3140[RTH5110[Ru/C[RTH3110[ RTH4110[RTH6110[RTH7110. They indicated that Raney-nickel catalysts
with metal components (Fe or Mo) were more helpful for the formation of H2in near- or supercritical water than other Raney catalysts. The maximum yield of H2 reached 20.38 mol/kg dry sludge in the presence of 1.57 g RTH3140 per gram dry sludge. In this case, the maximum GE and CE reached 91.31 and 88.20 %, respectively.
None Ru/C RTH3110 RTH3140 RTH4110 RTH5110 RTH6110 RTH7110 0
2 4 6 8 10 12 14 16 18 20
Gas yield / mol⋅kg-1 Gas yield / mol⋅kg-1
CO2 CH4
CO H2
None Ru/C RTH3110 RTH3140 RTH4110 RTH5110 RTH6110 RTH7110 0
1 2 3 4 5 6 7 8 9 10
CO2 CH4
CO H2
(a)
(b)
Fig. 13.24 Effect of catalyst type on sewage sludge gasification in near- and supercritical water:
feed concentration: 8.9 wt%, feed amount: 10 g, residence time: 20 min, catalyst loading (g catalyst /g dry sludge): 0.23 ga450C,b350C
The effect of various heterogeneous catalysts on the gasification of black liquor with batch reactor was also studied in SKLMF. These heterogeneous catalysts included Raney-nickel, Fe2O3, MnO2, Ru/C, and Ca(OH)2. As shown in Fig.13.25, the yield of H2increased obviously when the five catalysts are used.
The maximum yield of H2 reached 17.5 mol/kg when MnO2 is used, and the following yield of H2reached 17 mol/kg when Ru/C was used. The CODr reached 96.2, 97, 96.7, 98.2, and 96.2 % when Raney-nickel, Fe2O3, MnO2, Ru/C and Ca(OH)2were used, respectively.