Thermogravimetric Analysis on Co-Gasification Characteristics of Sludge and Straw under CO₂ Atmosphere

Abstract

To maximize the potential energy utilization of agricultural and forestry wastes and sludge, experimental studies were conducted on the co-gasification characteristics of two types of sludge (municipal sludge, MS; paper-mill sludge, PS) and a typical biomass straw (ST) under CO₂ atmosphere. In this paper, the main two stages of the gasification process, the pyrolysis in the low-temperature region and the CO₂-gasification in the high-temperature region, were separately studied and analyzed. The experimental results showed that biomass could effectively promote the pyrolysis of the sludge in the low-temperature region and improve the gasification in the high-temperature region. Due to the complex interactions between components, the characteristic parameters presented obvious nonlinear rules during the co-pyrolysis and co-gasification processes. For the MS-ST mixtures, when increasing the ST content, (i) in the pyrolysis process, the initial reaction temperature gradually decreased, but the final reaction temperature, the peak reaction rate and the corresponding temperature, and the pyrolysis index gradually increased; (ii) in the gasification process, the initial reaction temperature, the reaction final temperature, and the temperature corresponding to the peak gradually increased. Combined with the reaction kinetics analysis of the co-pyrolysis and the co-gasification processes, 25% may be a reasonable mixing ratio for ST for the MS-ST mixtures, which had a relatively lower reaction temperature, relatively high pyrolysis index and low activation energy (26.58 kJ·mol⁻¹ and 178.29 kJ·mol⁻¹ for the co-pyrolysis and co-gasification processes, respectively). For the PS-ST mixtures, when increasing the ST content, (i) in the pyrolysis process, the initial reaction temperature, the peak reaction rate, the temperature corresponding to the peak and the pyrolysis index gradually decreased, but the final reaction temperature gradually increased; (ii) in the gasification process, the initial and final reaction temperatures and the temperature corresponding to the peak gradually decreased, but the peak reaction rate gradually increased. Combined with the reaction kinetics analysis of the pyrolysis and the gasification processes, 25% may be a reasonable mixing ratio for ST for the PS-ST mixtures, which had a relatively lower reaction temperature, relatively high pyrolysis index and low activation energy (64.29 kJ·mol⁻¹ and 301.16 kJ·mol⁻¹ for the co-pyrolysis and co-gasification processes, respectively). These findings can provide useful information for the co-gasification of sludge and straw under CO₂ atmosphere.

1. Introduction

With the improvement of urbanization and sewage treatment level in China, the sludge generated during sewage treatment has increased sharply [1,2]. Sludge not only contains a large amount of renewable energy organics but also contains heavy metals, toxic organics, pathogenic microorganisms and other harmful substances. The reduction and harmless treatment of sludge have attracted widespread attention [3,4].

Thermochemical conversion technologies, such as combustion, pyrolysis and gasification, are effective methods in disposing of various solid wastes [5,6,7,8]. As one of the most promising thermochemical conversion technologies, gasification technology has the advantage of adapting to different processes with various feedstocks and has been applied widely into industrial operation [9]. Therefore, gasification is an effective thermal conversion pathway for sludge to obtain high-value-added products, and sludge can be converted to syngas (CO, H₂, and CH₄) [10]. Many studies show that adding biomass can effectively improve the gasification characteristics of sludge owing to the good reactive characteristics of biomass [11]. At the same time, biomass energy is zero-carbon energy and co-gasification of sludge and biomass contributes to achieving “carbon neutrality” in China. In general, sludge gasification contains two main reaction stages: the pyrolysis process in the low-temperature region and the char gasification process in the high-temperature region [12]. In the co-pyrolysis process, the addition of biomass enhances gas production, improves the bio-char structure and eliminates oxygenated compounds, water and concentrations of other pollutants in the bio-oil [3]. Wang et al. [13] investigated the effect of rice husk additives on the pyrolysis behavior of sewage sludge. They found that introducing rice husk could improve sewage sludge’s pyrolysis reactivity and CO₂ production but reduce the cumulation of H₂, CH₄ and C₂H₂. Wang et al. [14] studied the synergetic effect of sewage sludge and biomass co-pyrolysis and found the significantly synergetic effect during co-pyrolysis processes of sewage sludge and wheat straw accelerated pyrolysis reactions. In the co-gasification process, the synergy of biomass and sludge also enhances the gasification reaction behavior. Chen et al. [11] studied the co-gasification of sewage sludge and palm kernel shells. They found that a synergistic effect exists and that sewage sludge in blended fuel exceeding 30% will increase gas production yield. The activation energy for blended fuels is lower than that in pure feedstocks, indicating biomass’s improvement effect. Rosha et al. [15] investigated the co-gasification parametric effects evaluation on product yield and found that the synergy of biomass and sludge positively improved H₂ content. Lo et al. [16] investigated the hydrogen production and energy conversion from automobile shredder residues and paper-mill sludge co-gasified in a full-scale fluidized bed gasification plant, and found adding 5–15% of automobile shredder residues could maintain the regular operation of a full-scale commercial gasifier. Differing from traditional gasifying agents (steam, oxygen, air), CO₂ as a gasifying agent in gasification has some merits [17]: (1) the producer gases can be easily regulated, (2) no energy is needed for vaporization process, (3) the gasification efficiency can be improved owing to more volatiles producing in a reactive char, (4) high concentration CO₂ is suitable for direct recovery and recycling of CO₂, and (5) employing pure CO₂ as a gasifying agent in gasification is also advantageous for CO₂ reduction. However, there are less reports about the co-gasification of sludge and biomass under CO₂ atmosphere. This study aimed to obtain co-gasification behaviors of sludge and biomass under CO₂ atmosphere. In this paper, experimental studies are conducted on the co-gasification characteristics of two types of sludge (municipal sludge, MS; paper-mill sludge, PS) and a typical biomass straw (ST) under CO₂ atmosphere. The pyrolysis in the low-temperature region and CO₂ gasification in the high-temperature region are separately studied and analyzed. The results can provide useful information for the co-gasification of sludge and straw under CO₂ atmosphere.

2. Experiments

2.1. Samples Description

Two types of sludge are selected: municipal sludge (MS), paper-mill sludge (PS) and a typical biomass straw (ST). The proximate analysis and ultimate analysis are shown in

Table 1. Proximate analysis and ultimate analysis of samples.

Samples	Proximate Analysis (dry, wt%)			Ultimate Analysis (daf, wt%)
	Volatile Matters	Ash	Fixed Carbon	C	H	O	N	S
MS	27.70	61.00	11.30	47.10	4.80	37.70	8.80	1.60
PS	81.97	5.80	12.23	43.81	9.51	44.69	1.26	0.73
ST	75.90	10.75	13.35	48.22	6.62	33.05	1.06	0.31

. Three samples are dried, ground, crushed and sieved to <150 μm. Before the experiment, samples are dried at 105 °C for 24 h to eliminate all traces of moisture.

2.2. Experimental Apparatus, Methods and Kinetic Method

The experimental equipment for the co-gasification of sludge and biomass under CO₂ atmosphere is a German NETZSCH thermogravimetric analyzer, as shown in Figure 1a. During the experiment, a total flow of CO₂ gas controlled at 100 mL/min is introduced, and 10 ± 0.1 mg of samples are used for each experiment. The temperature range is from 50 °C to 1200 °C with a heating rate of 20 °C/min. The CO₂-gasification process contains two main stages: the pyrolysis in the low-temperature region and CO₂-gasification in the high-temperature region. In this paper, these two stages are separately studied and analyzed. For the pyrolysis stage in the low-temperature region, according to the thermogravimetric curve (TG) and the weight loss rate curve (DTG), the temperature when the DTG curve reaches −1 wt%/min at the reaction initial stage is defined as the pyrolysis reaction initial temperature T_P−i, the temperature when the DTG curve reaches −1 wt%/min at the reaction final stage is defined as the pyrolysis reaction final temperature T_P−f and the temperature when the DTG curve reaches the maximum is defined as the pyrolysis reaction peak temperature T_P−p [18]. For the gasification stage in the high-temperature region, due to the relatively low-gasification reaction rate of chars, the temperature when the DTG curve reaches –0.1 wt%/min at the reaction initial stage is defined as the gasification reaction initial temperature T_G−i, the temperature when the DTG curve reaches –0.1wt%/min at the reaction final stage is defined as the gasification reaction final temperature T_G−f and the temperature when the DTG curve reaches the maximum is defined as the gasification reaction peak temperature T_G−p [19]. The definition of characteristic temperatures is shown in Figure 1b. In addition, the average reaction rate DTG_mean at the pyrolysis and gasification stages are also introduced to compare the reaction behavior quantitatively.

At the pyrolysis stage, the characteristic reaction index C is introduced to evaluate the difficulty of pyrolysis, which is determined by the maximum mass loss rate and reaction duration. The larger the index C, the easier the pyrolysis reaction. The calculation formula is described in Equation (1) [20,21]:

$$C=\frac{{|\mathrm{DTG}}_{\max }|·{|\mathrm{DTG}}_{\mathrm{mean}}|·\Delta \mathrm{W}}{T_{P-i}·T_{P-p}·\Delta T_{0.5}}$$

(1)

where DTG_max represents the maximum reaction rate (%·min), DTG_mean represents the average reaction rate (%·min), △W represents the total weight loss percentage of the reaction, T_P−p represents the temperature (°C) at which the maximum reaction rate is applied and △T_0.5 represents the temperature range (°C) at which DTG/DTG_max = 0.5.

Moreover, to measure whether there is interaction during the co-reaction process of straw and sludge, a linear calculation value of the characteristic parameter is defined as: linear value = (characteristic parameter of ST) × (ST mixing ratio) + (characteristic parameter of sludge) · (1–ST mixing ratio).

This study uses Coats–Redfern integral method to analyze the kinetic parameters at the pyrolysis stage in the low-temperature region and the CO₂-gasification stage in the high-temperature region [22,23,24,25]. The kinetic equation can generally be written as follows:

$$\frac{d\alpha }{dt}=k(T)f(\alpha )$$

(2)

$$k(T)=A\exp (-\frac{E}{RT})$$

(3)

$$f(\alpha )={(1-\alpha )}^n$$

(4)

$$\alpha =(m_0-m_t)/(m_0-m_{\infty })$$

(5)

where α is the conversion degree; t(s) is time; T(K) is the absolute temperature; A (s⁻¹) is pre-exponential or frequency factor; E (kJ·mol⁻¹) is the activation energy; R (kJ·mol⁻¹·K⁻¹) is 8.3145 kJ·mol⁻¹·K^−1; m₀ is the initial mass of the sample; m_t is the mass of the sample at time t; m_∞ is the sample final mass after the reaction.

Equations (3) and (4) are inserted in Equation (2). The kinetic equation is as follows:

$$\frac{d\alpha }{dt}=A\exp (-\frac{E}{RT}){(1-\alpha )}^n$$

(6)

where β = dT/dt is the heating rate and n is the reaction order, Equation (6) could be obtained as follows:

$$\frac{d\alpha }{{(1-\alpha )}^n}=\frac{A}{\beta }\exp (-\frac{E}{RT})dT$$

(7)

By integration of Equation (7), Coats–Redfern equation can be transferred to this form:

$$n=1,\ln \left\{\frac{-\ln (1-\alpha )}{T^2}\right\}=\ln \left\{\frac{AR}{\beta E}\left[1-\frac{2RT}{E}\right]\right\}-\frac{E}{RT}$$

(8)

$$n\ne 1,\ln \left\{\frac{1-{(1-\alpha )}^{(1-n)}}{T^2(1-n)}\right\}=\ln \left\{\frac{AR}{\beta E}\left[1-\frac{2RT}{E}\right]\right\}-\frac{E}{RT}$$

(9)

For the reaction process $E/RT\ge 1\mathrm{and}1-2RT/E\approx 1$, the above equations can be further simplified as follows:

$$n=1,\ln \left\{\frac{-\ln (1-\alpha )}{T^2}\right\}=\ln \left\{\frac{AR}{\beta E}\right\}-\frac{E}{RT}$$

(10)

$$n\ne 1,\ln \left\{\frac{1-{(1-\alpha )}^{(1-n)}}{T^2(1-n)}\right\}=\ln \left\{\frac{AR}{\beta E}\right\}-\frac{E}{RT}$$

(11)

The reaction can be assumed as a first-order reaction, using the Coats–Redfern equation can calculate the apparent activation energy E and pre-exponential factor A. Combining the reaction curves and parameters, a plot of $\ln |-\ln (1-\alpha )/T^2|$ versus 1/T produces a straight line with a slope equal to −E/R for the first-order kinetics.

3. Results and Discussion

Figure 2 shows the TG and DTG curves of the co-gasification whole process of sludge (MS, PS), biomass straw (ST) and the mixture samples. It can be seen that there are two obvious peaks in the curves in the low-temperature and high-temperature regions. The former represents the devolatilization pyrolysis process and the latter represents the gasification process of the chars. Compared to the low-temperature pyrolysis reaction peak value, the chars gasification peak value in the high-temperature region on the DTG curve is relatively small, mainly due to the relatively low fixed carbon content.

3.1. Co-Pyrolysis Characteristics of Sludge and Straw in the Low-Temperature Region

Figure 3 shows the TG and DTG curves of co-pyrolysis of sludge (MS, PS) and straw (ST) in the low-temperature region. The pyrolysis processes of MS, PS, ST and their mixtures have similar trends. Significant peaks on the DTG curves are observed and all mixed sample curves lie between the curves of pure samples. For MS-ST or PS-ST mixtures, as the ST content increases, the DTG curves at the initial stage gradually move towards the lower-temperature zone and at the final stage gradually move towards the higher-temperature zone. Figure 4a shows the initial and final temperatures (T_P−i, T_P−f) of the pyrolysis. The initial temperatures are obviously different for these three pure samples, mainly caused by the difference in volatile content (PS > ST > MS). The initial temperatures (T_P−i) for ST, MS and PS are 213 °C, 243 °C and 252 °C, respectively, and the final temperatures (T_P−f) are 484 °C, 475 °C and 410 °C, respectively. As the ST content increases in the mixture samples, the initial pyrolysis temperatures of the mixed samples gradually decrease. The experimental values are lower than the linear values, indicating that adding biomass straw promotes the pyrolysis of sludge. Meanwhile, the final pyrolysis temperatures of the mixed samples gradually increase as the ST content increases and the experimental values are lower than the linear values. Moreover, the deviation between the experimental and linear values indicates a certain interaction between the components. The deviation is more obvious for the PS-ST mixtures than the MS-ST mixtures.

Figure 4b shows the peak reaction rates and the corresponding temperatures. The peak pyrolysis reaction rates of ST, MS and PS are 10.825%·min⁻¹, 2.515%·min⁻¹ and 24.06%·min⁻¹, respectively. The corresponding temperatures for the peak reaction rates are 332 °C, 312 °C and 362 °C, respectively. For the MS-ST mixtures, the peak pyrolysis reaction rate and the corresponding temperature of the mixed sample gradually increase with an increase in the ST content. However, differing from the MS-ST mixtures, the peak pyrolysis reaction rate and the corresponding peak temperature gradually decrease with the increase in the ST content for the PS-ST mixture. This phenomenon is mainly caused by the difference in volatile content of ST, MS and PS. Figure 3c shows the characteristic index and half-peak temperature. The individual pyrolysis index C of ST, MS and PS are 3.731 × 10⁻⁵, 0.058 × 10⁻⁵ and 36.32 × 10⁻⁵, respectively. With the increase in ST content, the pyrolysis index C gradually increases for the MS-ST mixtures. In contrast, the pyrolysis index C gradually decreases in the PS-ST mixture. Moreover, the deviation between the experimental and the linear value is more obvious for PS-ST mixture, indicating a more intense interaction between the components. An interaction during the co-gasification process has been also observed [11]. For the MS-ST mixtures, the half-peak temperature significantly decreases as the content of ST increases from 0% to 25%. As it further increases from 25% to 100%, the half-peak temperature gradually decreases. For the PS-ST mixture, the half-peak temperature gradually increases as the content of ST increases from 0% to 100%. From the perspective of the pyrolysis index, the deviation between the experimental and linear values is more obvious for the PS-ST mixture than for the MS-ST mixtures.

3.2. Kinetic Analysis of the Co-Pyrolysis Reaction of Sludge and Straw

The kinetic curves at the pyrolysis stage are shown in Figure 5. Due to great fluctuation (intense reaction) in the kinetic curves, the kinetic analysis curves are divided into several segments according to the inflection point on the curves to obtain relatively accurate kinetic parameters at the pyrolysis stage. The average activation energy E and the average pre-exponential factor lnA are calculated from the equation

E = ∑E_i·F_i

(12)

lnA = ∑lnA_i·F_i

(13)

where E_i and lnA_i are the activation energy and the pre-exponential factor for each stage, respectively, and F_i is the mass loss fraction for each stage.

Table 2. Kinetic parameters at the pyrolysis stage.

Item	T Zone(°C)	Fi (%)	Ei (kJ/mol)	E (kJ/mol)	lnAi (s−1)	lnA(s−1)	R2
(a) MS/ST
ST	213–362	84.64	60.13	51.97	15.63	16.86	0.99854
	362–484	15.36	7.03		23.58		0.99380
25%MS + 75%ST	219–359	74.03	44.20	35.07	18.48	19.78	0.98494
	359–476	25.97	9.05		23.49		0.99510
50%MS + 50%ST	227–356	73.57	36.96	30.08	19.78	20.74	0.97546
	356–474	26.43	10.91		23.41		0.99628
75%MS + 25%ST	232–354	65.42	33.83	26.58	20.35	21.38	0.98021
	354–473	34.58	12.86		23.33		0.99550
100%MS	243–350	56.85	28.16	23.86	21.41	22.06	0.99511
	350–472	43.15	18.19		22.91		0.99452
(b) PS/ST
ST	213–362	84.64	60.13	51.97	15.63	16.86	0.99854
	362–484	15.36	7.03		23.58		0.99380
25%PS + 75%ST	221–382	88.21	43.73	39.14	18.74	19.30	0.97465
	382–462	11.79	4.83		23.48		0.958133
50%PS + 50%ST	227–386	93.24	50.82	47.64	17.66	18.04	0.966465
	386–430	6.76	3.72		23.33		0.913562
75%PS + 25%ST	241–387	96.79	66.27	64.29	18.06	18.23	0.968190
	387–414	3.21	4.68		23.29		0.912561
100%PS	252–389	98.73	89.17	88.23	18.95	18.99	0.961830
	389–410	1.27	15.24		22.18		0.98222

shows the kinetic parameters at the pyrolysis stage. It can be seen that each linear quasi-correlation coefficient R² is relatively high, indicating a good fitting degree. The activation energy is the following as a rule: PS > ST > MS. The average activation energy also exhibits a nonlinear change rule with changing the ST mixing ratio, mainly caused by the complex interaction between component samples during the co-pyrolysis process. For the MS-ST mixtures, the average activation energy gradually increases as the content of ST increases from 0% to 75%, and as it further increases from 75% to 100%, the average activation energy significantly increases Figure 6 shows the activation energy at the pyrolysis stage. Combining the characteristic parameters and activation energy in the low-temperature region, 25% of ST may be a reasonable mixing ratio for the MS-ST mixtures, which has a relatively low reaction temperature, relatively high pyrolysis index and low activation energy. For the PS-ST mixture, the average activation energy significantly decreases as the content of ST increases from 0% to 25%; as it increases from 25% to 75%, the average activation energy gradually decreases; as it further increases to 100%, the average activation energy increases again. Combining the characteristic parameters and activation energy in the low-temperature region, 25% of ST may be a reasonable mixing ratio for the PS-ST mixture, which has a relatively low reaction temperature, relatively high pyrolysis index and low activation energy.

3.3. Co-Gasification Characteristics in the High-Temperature Region

Figure 7 shows the TG and DTG curves of chars gasification of sludge (MS, PS) mixed with biomass straw (ST). It can be seen that there are relatively obvious gasification reaction processes of chars. The DTG/DTG curves of the mixture sample are between the DTG/DTG curves of pure samples. The non-additive rule can also be observed, indicating complex interactions between the components in the high-temperature region.

Figure 8a shows the initial and final temperatures (T_G−i, T_G−f) of the gasification reaction of chars in the high-temperature region. For MS and PS, the initial and final temperatures of the carbon residue gasification reaction show completely different rules. As the mixing ratio of ST enhances, the initial temperature of the co-gasification reaction gradually increases for the MS-ST mixtures but gradually decreases for the PS-ST mixtures; the final temperature of the carbon residue gasification reaction gradually increases for the MS-ST mixtures while it gradually decreases for the PS-ST mixtures. From the above two perspectives, in order to facilitate the gasification reaction occurring at lower temperatures, the mixing ratio of biomass straw ST should not be too high for the MS-ST mixtures. Nonetheless, the mixing ratio of biomass straw ST should not be too low for the PS-ST mixtures. At the same time, it can also be seen that the initial and final temperatures of the gasification reaction deviate from the linear values, indicating that there is also a certain interaction between the components in the mixed sample. For the MS-ST mixtures, the initial reaction temperatures of the gasification are closed to the linear value under the three mixing ratios, and the gasification reaction final temperatures are significantly higher than the linear values. For the PS-ST mixtures, the initial reaction temperatures of gasification are significantly lower than the linear values under the three mixing ratios, and the gasification reaction final temperatures are lower than the linear values when the mixing ratios of ST are 25% and 50%, but slightly higher than the linear values when the mixing ratio of ST is 75%.

Figure 8b shows the maximum reaction rate and the corresponding temperature of the gasification reaction of chars in the high-temperature region. When increasing the mixing ratio of ST, the maximum reaction rate and the corresponding temperature of the co-gasification reaction of chars gradually increases for the MS-ST mixtures. The maximum reaction rate gradually increases but the corresponding temperature gradually decreases for the PS-ST mixtures. Meanwhile, the maximum reaction rate and the corresponding temperature also present the non-additive rule. For the MS-ST mixtures, the corresponding temperatures of the maximum reaction rate are lower than the linear values when the mixing ratios of ST are 25% and 50%, but higher than the linear values when the mixing ratios of ST is 75%. The maximum reaction rate is obviously lower than the linear values under three mixing ratios. For the PS-ST mixtures, the maximum reaction rate and the corresponding temperatures are lower than the linear values under three mixing ratios.

From the perspectives of the characteristic temperatures and the maximum reaction rate, (i) for the MS-ST mixtures, 25% of ST may be a suitable mixing ratio, the gasification initial and final temperatures are relatively low and the deviation between the maximum reaction rate and the linear value is relatively small; (ii) 25% of ST may be a suitable mixing ratio, the gasification initial and final temperatures are obviously lower the linear values and the deviation between the maximum reaction rate and the linear value is relatively small.

3.4. Kinetic Analysis of the Co-Gasification Reaction of Chars in the High-Temperature Region

The gasification kinetic curves of chars are shown in Figure 9. Due to the small fluctuation of the kinetic curves, the kinetic analysis curves are divided into only one segment to obtain the relatively accurate kinetic parameters at the gasification stage.

Table 3. Kinetic parameters of gasification reaction of chars in the high-temperature region.

	T Zone(°C)	E (kJ/mol)	lnA(s−1)	R2
(a) MS/ST
ST	754–997	188.91	20.66	0.98112
25%MS + 75%ST	736–1027	149.22	16.39	0.97226
50%MS + 50%ST	729–983	147.43	16.71	0.98654
75%MS + 25%ST	720–935	178.29	20.67	0.98135
100%MS	701–894	190.71	22.72	0.98493
(b) PS/ST
ST	754–997	188.91	20.66	0.98112
25%PS + 75%ST	771–1011	204.47	22.05	0.97399
50%PS + 50%ST	788–1005	228.59	24.49	0.98578
75%PS + 25%ST	829–997	301.16	31.88	0.99165
100%PS	888–1026	408.69	41.56	0.97802

shows the gasification kinetic parameters. It can be seen that each linear quasi-correlation coefficient R² is relatively high. During the gasification of chars of pure samples, the activation energy of ST and MS have no significant difference, which is significantly lower than that of PS. Figure 10 gives the activation energy at the gasification stage in the high-temperature region. For the MS-ST mixtures, the activation energy gradually decreases from 0% to 50% and increases from 50% to 100%. For the PS-ST mixtures, the activation energy gradually decreases from 0% to 100%. From the perspective of reactivity kinetics, the activation energy between different mixing ratios of ST is relatively small; 25% ST may also be reasonable and the activation energy is relatively low (178.29 kJ/mol), which is lower than the linear calculated value (190.26 kJ/mol). When PS is mixed with straw 25% ST, the activation energy (301.16 kJ/mol) is lower than the linear value (353.74 kJ/mol), which can meet the lower activation energy requirements and facilitate the gasification reaction. Combining the characteristic parameters and activation energy in the high-temperature region, 25% of ST may be a reasonable mixing ratio for MS-ST mixture and the PS-ST mixture, which have a relatively lower reaction temperature and low activation energy.

3.5. Fourier Transform Infrared Spectroscopy

Based on the reasonable mixing ratios for the MS-ST and PS-ST mixtures, the FTIR analysis and testing are performed. Figure 11 shows the FTIR absorption spectra of the produced gases evolved from the reaction process of 25%ST + 75%MS and 25%ST + 75%PS, respectively. The FTIR spectrum information of gas species is obtained by online monitoring at the whole reaction temperature range (50–1200 °C). The infrared spectral data under six characteristic temperatures (in the pyrolysis process: T_P−i, T_P−p, T_P−f; in the gasification process: T_G−i, T_G−p, T_G−f) is displayed. Similar trends for 25%ST + 75%MS and 75%ST + 25%PS under these temperatures are shown. In the CO₂ atmosphere, C=O bonds (726–586, 2400–2250 cm⁻¹), which are the main functional groups and represent CO₂, are clearly observed [11]. At the low-temperature pyrolysis stage, there exist some obvious peaks in the range of 3500–4000 cm⁻¹, which represents H₂O from the volatilizes. C=O bonds (about 2240–2060 cm⁻¹) can also be clearly observed, indicating a higher existence of CO species [26]. CO is produced by the low-temperature pyrolysis reaction and the high-temperature gasification reaction (Boudouard reaction CO₂ + C → CO). Notably, 75%ST + 25%PS presents more intense absorption spectra than 25%ST + 75%MS, indicating that more CO is produced in the 25%ST + 75%PS. This is because the 25%ST + 75%PS contains more volatile and fixed carbon contents in the samples.

4. Conclusions

This paper conducts experimental studies on the co-pyrolysis and chars gasification characteristics of two types of sludge (municipal sludge, MS; paper-mill sludge, PS) and a typical biomass straw (ST). Some conclusions can be obtained as follows: (1) Biomass can effectively promote the pyrolysis of the sludge in the low-temperature region and improve the gasification in the high-temperature region. Due to the complex interactions between components, the characteristic parameters present obvious nonlinear rules during the co-pyrolysis and co-gasification processes. (2) For the MS-ST mixtures, when increasing the ST content, (i) in the pyrolysis process, the initial reaction temperature gradually decreases, but the final reaction temperature, the peak reaction rate and the corresponding temperature and the pyrolysis index gradually increase; (ii) in the gasification process, the initial reaction temperature, the reaction final temperature and the temperature corresponding to the peak gradually increase. Combined with the reaction kinetics analysis of the co-pyrolysis and the co-gasification processes, 25% may be a reasonable mixing ratio for ST for the MS-ST mixtures. (3) For the PS-ST mixtures, when increasing the ST content, (i) in the pyrolysis process, the initial reaction temperature, the peak reaction rate, the temperature corresponding to the peak and the pyrolysis index gradually decrease, but the final reaction temperature gradually increases; (ii) in the gasification process, the initial and final reaction temperatures and the temperature corresponding to the peak gradually decrease, but the peak reaction rate gradually increases. Combined with the reaction kinetics analysis of the co-pyrolysis and the co-gasification processes, 25% may be a reasonable mixing ratio for ST for the PS-ST mixtures. (4) The FTIR absorption spectra experimental results show that in the produced gases evolved from the reaction process, 25%ST + 75%PS present more intense absorption spectra than 25%ST + 75%MS, indicting that more CO is produced in the 25%ST + 75%PS. In summary, it is suggested that adding 25% ST into the sludge/biomass mixtures is suggested in the practical operation. These findings can provide useful information for the co-gasification of sludge and straw under CO₂ atmosphere.