2.1. Materials
Dewatered SS with a moisture content of ~80 wt%, collected from a wastewater treatment plant in Jilin City, Jilin Province, China, was stored at 4 °C and used as the raw material for HTC. For characterization, SS was dried at 105 °C for 24 h, ground into fine powder, and sealed in a dry glass bottle for subsequent analysis. Table 1 lists the primary properties of raw SS. The high content volatiles were the main source of heat released during sludge thermochemical conversion, and the ash components were mainly SiO2, Al2O3, and Fe2O3.
Table 1. Main characteristics of the employed sludge.
Sample
|
Ultimate analysis (wt%, db*)
|
Proximate analysis (wt%, db)
|
HHV (kJ kg−1)
|
C
|
H
|
N
|
S
|
Volatiles
|
Fixed C
|
Ash
|
Sludge
|
19.68
|
3.33
|
2.99
|
0.62
|
32.88
|
5.78
|
61.34
|
8200
|
Ash (wt%)
|
SiO2
|
Al2O3
|
Fe2O3
|
P2O5
|
CaO
|
MgO
|
K2O
|
SO3
|
Na2O
|
|
41.90
|
18.10
|
10.40
|
7.03
|
4.76
|
2.53
|
2.40
|
1.58
|
1.21
|
*db = dry basis.
2.2. HTC experiments
HTC experiments were carried out in a 0.5-L stirred batch reactor (stainless steel 316 L, GCF-type, Dalian Controlled Plant, China). In each experiment, the reactor was charged with a mixture of SS (20 g) and deionized water (200 mL). In experiments involving acid catalysis, SS (20 g) was mixed with acetic acid solutions (200 mL) of different concentrations. The reactor was sealed, purged with N2 to remove residual air, heated to the predetermined temperature (160, 210, or 260 °C) using an electric heater, and held at this temperature for a certain time (30, 90, or 150 min). The reaction pressure (that of water alone at the respective temperature) ranged from 1.6 to 4.7 MPa. After the reaction, the slurry samples were collected and separated into filtrates and hydrochar by filtration. After 24-h drying at 60 °C, hydrochar was ground into fine powder and stored in an enclosed plastic pipe until analysis. Hydrochar samples were denoted as “HC-A-B-C,” where A is the HTC temperature, B is the HTC residence time, and C is the acetic acid concentration. Two indexes (hydrochar mass yield (Hy) and HHV) were selected as dependent variables representing responses to HTC condition variation.
Hy was estimated using Eq. (1), while HHV was estimated by bomb calorimetry (SDAC6000, Hunan Sundy Science and Technology Co., Ltd., China). The nitrogen, hydrogen, and carbon contents of hydrochar were determined by combustion at 950 °C employing an automatic elemental analyzer (EA3000, Euro Vector S.P.A., Italy).
Hy (%) = 100% × weight of dry hydrochar/weight of dry raw material. (1)
2.3. CO2-assisted gasification experiments
Hydrochar gasification was carried out in a micro-fluidized bed reaction analyzer coupled with a mass spectrometer (MFBRA-MS). The assembly mainly comprised a gas supply system, a gasification system, and an online gas monitoring and analysis system (Fig. 1).
Gasification was performed as follows. (1) The reactor was heated to 800 °C. (2) Hydrochar (10 mg) was placed into the feeding pipe. (3) The gasification agent (99.999 vol% CO2) was supplied to the reactor at a flow rate of 0.5 L min−1 to stabilize the gas baseline. (4) The pulse was turned on through the solenoid valve, and the sample was instantly sprayed into the reaction area. The gas was analyzed using an online mass spectrometer. (5) Procedures (2)–(4) were repeated until all experiments were completed.
The carbon conversion of hydrochar gasification (X) was calculated as
(2)
where t is the reaction time (s), t0 is the initial reaction time (s), td is the end reaction time, ψi is the volume fraction of component i in the produced gas (%), and qv is the flow rate (mol min−1).
To establish a relationship between HTC conditions and hydrochar gasification reactivity, we used RSM to optimize this reactivity and thus obtain optimal HTC conditions. The reaction index R0.9, employed to quantitatively characterize the overall gasification reactivity of hydrochar, was determined as
R0.9 = 0.9/tX = 0.9, (3)
where tX = 0.9 represents the gasification time (min) required for a carbon conversion of 0.9.
2.4. Experimental design for process optimization
RSM is a method of optimizing experimental conditions, which is suitable for solving the related problems of nonlinear data processing. By means of regression fitting and response surface drawing, the predicted optimal response value and corresponding experimental conditions can be found out.
Experiments were designed using the Box–Behnken method, a typical RSM technique that is usually used to optimize response-affecting process parameters.
To determine the optimum HTC conditions for hydrochar production, we investigated the effects of three factors (reaction temperature, residence time, and acetic acid concentration) and optimized them within the ranges of 160–260 °C, 30–150 min, and 0–3.0 M, respectively, to maximize the yield and HHV of SS hydrochar. The relationship between HTC conditions and gasification performance was investigated using the gasification activity index as the optimization goal under different HTC conditions.
The experimental design was carried out using Design-Expert.V8.0.6.1 data analysis software (Stat-Ease, Inc). The optimization conditions and objectives of HTC and gasification processes are presented in Tables 2 and 3.
Table 2. RSM design parameters and optimization objectives of HTC.
Run
|
A: HTC temperature (°C)
|
B: Residence time (min)
|
C: Acetic acid concentration (M)
|
1
|
260
|
90
|
3
|
2
|
210
|
30
|
0
|
3
|
210
|
90
|
1.5
|
4
|
160
|
90
|
0
|
5
|
210
|
90
|
1.5
|
6
|
260
|
150
|
1.5
|
7
|
210
|
150
|
0
|
8
|
160
|
90
|
3
|
9
|
210
|
90
|
1.5
|
10
|
210
|
90
|
1.5
|
11
|
210
|
150
|
3
|
12
|
160
|
30
|
1.5
|
13
|
160
|
150
|
1.5
|
14
|
260
|
90
|
0
|
15
|
210
|
90
|
1.5
|
16
|
260
|
30
|
1.5
|
17
|
210
|
30
|
3
|
Table 3. RSM design parameters and optimization objectives of SS hydrochar gasification.
Run
|
A: HTC temperature (°C)
|
B: Residence time (min)
|
C: Acetic acid concentration (M)
|
1
|
260
|
90
|
3
|
2
|
210
|
30
|
0
|
3
|
210
|
90
|
1.5
|
4
|
160
|
90
|
0
|
5
|
210
|
90
|
1.5
|
6
|
260
|
150
|
1.5
|
7
|
210
|
150
|
0
|
8
|
160
|
90
|
3
|
9
|
210
|
90
|
1.5
|
10
|
210
|
90
|
1.5
|
11
|
210
|
150
|
3
|
12
|
160
|
30
|
1.5
|
13
|
160
|
150
|
1.5
|
14
|
260
|
90
|
0
|
15
|
210
|
90
|
1.5
|
16
|
260
|
30
|
1.5
|
17
|
210
|
30
|
3
|