Reaction Behavior of Biochar Composite Briquette Under H2-N2 Atmosphere: Experimental Study

Author Contributions

Conceptualization, H.T.; methodology, H.T.; software, T.Z. and H.T.; validation, T.Z. and Y.L.; formal analysis, T.Z. and Y.L.; investigation, T.Z. and Y.L.; resources, T.Z. and Y.L.; data curation, T.Z. and Y.L.; writing—T.Z.; writing—review and editing, H.T.; visualization, T.Z. and Y.L.; supervision, H.T.; project administration, H.T.; funding acquisition, H.T. All authors have read and agreed to the published version of the manuscript.

Figure 1.
Image of the obtained biochar.

Figure 2.
Photos of the prepared BCB.

Figure 3.
Experimental device: (1, 2) mass flow controller, (3) mixing chamber, (4) mass-loss sensor, (5) furnace, (6) MoSi₂ heating elements, (7) hot temperature zone, (8) sample holder, (9) reaction tube, (10) alumina ball, (11) thermocouple, (12) computer, (13) IR heater, (14) suspending wire, and (15) water sealing.

Figure 4.
Comparison between model-predicted and experimental mass-loss curves under different scenarios.

Figure 5.
Comparison between model-predicted and experimental final iron oxide reduction fractions and biochar gasification fractions under different scenarios.

Figure 6.
Influence of temperature on iron oxide reduction (a) and biochar gasification (b) in BCB reaction.

Figure 7.
Changes in gas generating rates of H₂, H₂O, CO, and CO₂ with time under different temperatures.

Figure 8.
Influences of H₂ content in the atmosphere on iron oxide reduction (a) and biochar gasification (b) in the BCB reaction.

Figure 9.
Changes of gas generation rates of H₂, CO, CO_2, and H₂O with time under different atmospheres.

Figure 10.
Changes of radial distribution of local reduction fraction (a) and local gasification fraction (b) with time under scenario II.

Figure 11.
XRD patterns at different times under scenario II: (a) 0 min, (b) 10 min, (c) 20 min, and (d) 30 min.

Figure 12.
SEM images near the BCB center at different reaction times under scenario II: (a) 0 min, (b) 10 min, (c) 20 min, and (d) 30 min.

Table 1.
Properties of prepared biochar fines (wt.%).

Proximate Analysis (ar)				Elemental Analysis (daf)
M	A	V	FC	C	H	O	N	S
2.42	4.63	3.60	89.35	89.82	0.93	8.67	0.58	0.03

Table 2.
BCB phase composition (wt.%).

Fe₃O₄	FeO	Fe	C	Gangue
52.57	24.54	0.98	13.16	8.75

Table 3.
Experimental scenarios.

No.	Temperature/K	Atmosphere (H₂:N₂ (Vol.))
I	1173	50:50
II	1273	50:50
III	1373	50:50
IV	1273	25:75
V	1273	75:25

Table 4.
Reactions involved in the model.

No	Reaction	Reaction Rate	Refs.
(R1)	${3 Fe}_{2} O_{3} (s) + CO (g) {= 2 Fe}_{3} O_{4} (s) {+ CO}_{2} (g)$	$r_{i} = \frac{(P_{CO} - P_{{CO}_{2}} / K_{i}) / (R T)}{(K_{i} / (k_{i} (1 + K_{i}))} {(1 - f_{i})}^{2 / 3}) a_{gs}$ $k_{1} = \exp (- 1.445 - 6038.0 / T)$ $K_{1} = \exp (7.255 + 3720 / T)$ $k_{2} = \exp (- 2.515 - 4811.0 / T)$ $K_{2} = \exp (5.289 - 4711.0 / T)$ $k_{3} = \exp (0.805 - 7385 / T)$ $K_{3} = \exp (- 2.946 + 2744.63 / T)$	[19]
(R2)	${Fe}_{3} O_{4} (s) + CO (g) = 3 FeO (s) {+ CO}_{2} (g)$
(R3)	$FeO (s) + CO (g) = Fe (s) {+ CO}_{2} (g)$
(R4)	${3 Fe}_{2} O_{3} (s) {+ H}_{2} (g) {= 2 Fe}_{3} O_{4} (s) {+ H}_{2} O (g)$	$\begin{array}{l} r_{i} = \frac{(P_{H_{2}} - P_{H_{2} O} / K_{2}) / (R T)}{(K_{i} / (k_{i} (1 + K_{i}))} {(1 - f_{i})}^{2 / 3}) a_{gs} \\ (f_{4} = f_{1}, f_{5} = f_{2}, f_{6} = f_{3}) \end{array}$ $k_{4} = \exp (2.490 - 4017 / T)$ $K_{4} = \exp (10.32 + 362 / T)$ $k_{5} = \exp (4.70 - 6999.7 / T)$ $K_{5} = \exp (8.98 - 8580 / T)$ $k_{6} = \exp (4.97 - 6867.4 / T)$ $K_{6} = \exp (- 1.30 + 2070 / T);$	[19]
(R5)	${Fe}_{3} O_{4} (s) {+ H}_{2} (g) = 3 FeO (s) {+ H}_{2} O (g)$
(R6)	$FeO (s) {+ H}_{2} (g) = Fe (s) {+ H}_{2} O (g)$
(R7)	$C (s) + {CO}_{2} (g) = 2 CO (g)$	$\begin{array}{l} r_{7} = ρ_{C, 0} k_{3} {(1 - f_{7})}^{2 / 3} {(P_{{CO}_{2}} / 1.01 \times 10^{5})}^{0.38} / M_{C}, \\ k_{7} = 3.1 \times 10^{6} \exp (- 230000 / R T), f_{7} = f_{C} \end{array}$	[20]
(R8)	$C (s) + H_{2} O (g) = {CO (g) + H}_{2} (g)$	$\begin{array}{l} r_{8} = ρ_{C, 0} k_{4} {(1 - f_{8})}^{2 / 3} {(P_{H_{2} O} / 1.01 \times 10^{5})}^{0.55} / M_{C} \\ k_{8} = 6750 \exp (- 156000 / R T), f_{8} = f_{C} \end{array}$	[20]

Table 5.
Terms in Equation (2).

$p$	$D$	$S$
$p_{C O}$	$D_{C O – N_{2}}$	$R T r_{C O}, r_{CO} = r_{8} + 2.0 * r_{7} - r_{1} - r_{2} - r_{3}$
$p_{{CO}_{2}}$	$D_{{CO}_{2} {– N}_{2}}$	$R T r_{{CO}_{2}}, r_{{CO}_{2}} = r_{1} + r_{2} + r_{3} - r_{7}$
$p_{H_{2}}$	$D_{H_{2} {– N}_{2}}$	$R T r_{H_{2}}, r_{H_{2}} = r_{8} - r_{4} - r_{5} - r_{6}$
$p_{H_{2} O}$	$D_{H_{2} {O – N}_{2}}$	$R T r_{H_{2} O}, r_{H_{2} O} = r_{4} + r_{5} + r_{6} - r_{8}$

Table 6.
Terms in Equation (8).

$ρ$	$S$
$ρ_{{Fe}_{2} O_{3}}$	$- 3.0 * (r_{1} + r_{4}) * M_{{Fe}_{2} O_{3}}$
$ρ_{{Fe}_{3} O_{4}}$	$(2 * r_{1} - r_{2}) * M_{{Fe}_{3} O_{4}} + (2 * r_{4} - r_{5}) * M_{{Fe}_{3} O_{4}}$
$ρ_{FeO}$	$(3 r_{2} - r_{3}) * M_{FeO} + (3 * r_{5} - r_{6}) * M_{FeO}$
$ρ_{Fe}$	$r_{3} * M_{Fe} + r_{6} * M_{Fe}$
$ρ_{C}$	$- 1.0 * (r_{7} + r_{8}) * M_{C}$

Table 7.
RSM values under different a_gs.

a_gs/(m²·m⁻³)	5	10	15	20	25
RMS/-	0.13544	0.05898	0.04340	0.07914	0.08714

Table 8.
Distribution of iron-oxide oxygen removal across different pathways under different temperatures.

Item	1173 K	1273 K	1373 K
Iron-oxide oxygen removed by H₂ (%)	62	42	26
Iron-oxide oxygen removed by biochar (%)	29	56	72
Residual iron-oxide oxygen in BCB (%)	9	2	2

Table 9.
Ratios of iron-oxide oxygen in different pathways under different atmospheres.

Item	25 Vol.% H₂-N₂	50 Vol.% H₂-N₂	75 Vol.% H₂-N₂
Iron-oxide oxygen removed by H₂ (%)	37	42	45
Iron-oxide oxygen removed by biochar (%)	57	56	53
Residual iron-oxide oxygen in BCB (%)	6	2	2

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Ting Zhang www.mdpi.com

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Reaction Behavior of Biochar Composite Briquette Under H2-N2 Atmosphere: Experimental Study

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