Figures (10)  Tables (8)

    • Figure 1. 

      Diagram of comparison between microwave pyrolysis and conventional pyrolysis[19,20].

    • Figure 2. 

      Diagram of microwave-assisted pyrolysis experimental equipment: (1) microwave oven, (2) quartz reactor, (3) temperature sensor, (4) K-type thermocouple, (5) adapter connecting pipe, (6) vacuum pump, and (7) electric meter.

    • Figure 3. 

      Two-step activation experimental process.

    • Figure 4. 

      Nitrogen adsorption and desorption isotherms for different samples.

    • Figure 5. 

      Pore size distribution of different samples.

    • Figure 6. 

      Mesoporous volume proportion of different samples.

    • Figure 7. 

      CO2 adsorption isotherms of different samples at 1 bar and 25 °C.

    • Figure 8. 

      The breakthrough curves of different samples at 25 °C.

    • Figure 9. 

      Adsorption kinetics curves for CO2 adsorption of different samples at 25 °C.

    • Figure 10. 

      The maximum adsorption capacity, average adsorption rate, and activation energy for CO2 adsorption of different samples.

    • Item Value
      Proximate analysis (wt.%) Moisture 5.11
      Ash 8.66
      Volatile 71.23
      Fixed carbon 15.00
      Ultimate analysis (wt.%) C 42.24
      H 5.78
      O 37.78
      N 0.39
      S 0.04

      Table 1. 

      Proximate analysis and ultimate analysis of corn straw

    • Sample KBC PKBC-1 PKBC-2 PKBC-3 PKBC-4
      H3PO4/biomass 0 1:1 2:1 3:1 4:1

      Table 2. 

      Experimental conditions of biochar activation

    • Sample SSA (m2/g) Vtotal (cm3/g) Vmicro (cm3/g) Vmeso (cm3/g) Vmeso/Vtotal (%) Average pore size (nm)
      KBC 1,338.74 0.614 0.512 0.102 16.61 1.64
      PKBC-1 2,088.10 0.898 0.653 0.345 27.28 1.82
      PKBC-2 2,993.07 1.638 1.021 0.617 37.67 1.98
      PKBC-3 3,038.92 1.891 1.089 0.802 42.41 1.93
      PKBC-4 1,709.26 0.699 0.319 0.380 54.42 2.11

      Table 3. 

      Pore structure parameters of biochar

    • Sample Langmuir parameters Freundich parameters
      qm
      (mmol/g)      		 KL
      (/bar)      		 R2 n KF
      (mmol/(g·bar1/n))      		 R2
      KBC 1.4715 0.1018 0.9993 1.0568 0.4121 0.9976
      PKBC-1 2.8623 0.1872 0.9998 1.1031 0.9314 0.9967
      PKBC-2 3.3387 0.2677 0.9999 1.1448 1.5635 0.9936
      PKBC-3 3.4341 0.3363 0.9929 1.1797 2.1513 0.9908
      PKBC-4 1.1255 0.0929 0.9981 1.0523 0.2665 0.9927

      Table 4. 

      Isotherms parameters of different samples

    • Sample qe (mmol/g) k1 (1/min) R2
      KBC 1.6615 0.1904 0.9946
      PKBC-1 2.1262 0.1837 0.9957
      PKBC-2 4.2337 0.3188 0.9709
      PKBC-3 4.6811 0.3901 0.9581
      PKBC-4 1.1883 0.2812 0.9809

      Table 5. 

      Kinetic parameters of CO2 adsorption of different samples fitted by Pseudo-first order model

    • Sample qe (mmol/g) k2 [g/(mmol·min)] R2
      KBC 5.3659 0.0172 0.9945
      PKBC-1 8.5141 0.0102 0.9946
      PKBC-2 11.6545 0.0169 0.9620
      PKBC-3 11.7385 0.0219 0.9440
      PKBC-4 1.7005 0.0811 0.9856

      Table 6. 

      Kinetic parameters of CO2 adsorption of different samples fitted by Pseudo-second order model

    • Sample qe (mmol/g) kA (1/min) n R2
      KBC 2.1881 0.1755 0.9725 0.9946
      PKBC-1 2.6693 0.2713 1.2194 0.9986
      PKBC-2 3.7394 0.4331 1.6640 0.9991
      PKBC-3 4.4478 0.4912 1.8423 0.9995
      PKBC-4 1.6798 0.0426 0.6701 0.9909

      Table 7. 

      Kinetic parameters of CO2 adsorption of different samples fitted by Avrami model

    • Feedstocks Conditions SSA (m2/g) Adsorption capacity (mmol/g) Ref.
      Corn straw KOH : biochar = 2:1, tube furnace, 700 °C, 60 min 1,631.77 3.06 at 25 °C [42]
      Corn straw KOH : biochar = 0.1:1, biomass gasifier, 450 °C, 120 min 365.11 2.88 [43]
      Corn straw TEPA, muffle furnace, 700 °C, 120 min 423 2.38 at 60 °C [44]
      Pine KOH : biochar = 2:1, tube furnace, 800 °C, 60 min 1,709.04 3.79 at 25 °C [42]
      Sugarcane bagasse TEPA, muffle furnace, 700 °C, 120 min 352 2.22 at 60 °C [44]
      Miscanthus TEPA, muffle furnace, 700 °C, 120 min 532 2.89 at 60 °C [44]
      Switchgrass TEPA, muffle furnace, 700 °C, 120 min 520 2.51 at 60 °C [44]
      Rambutan peel Tube furnace, 900 °C, 90 min 569.64 1.56 at 30 °C [45]
      Rice husk KOH : biochar = 1:1, 750 °C 1439 3.51 at 25 °C [46]
      Blue algal KOH : biochar = 0.5:1, tube furnace, 800 °C, 120 min 1,018.55 2.76 at 25 °C [47]
      Poplar wood chip K2C2O4, 800 °C, 120 min 1,418 3.82 at 25 °C [48]
      Corn straw H3PO4 + KOH, 750 °C, 60 min 3,038.92 3.43 at 25 °C This study

      Table 8. 

      SSAs and CO2 adsorption capacities of biochars