A Novel Highly Stable Biomass Gel Foam Based on Double Cross-Linked Structure for Inhibiting Coal Spontaneous Combustion

Abstract

To enhance the stability of biomass gel foam used for inhibiting coal spontaneous combustion (CSC), a novel highly stable biomass gel foam (SA-Ca²⁺@TA-GF) based on a double cross-linked structure was prepared by introducing tannic acid (TA) into a gel form (sodium alginate/calcium L-lactate/composite foaming agent). FT-IR confirmed the formation of the double cross-linked structure. The effects of TA concentration on the performance of SA-Ca²⁺@TA-GF were analyzed, considering gelation time, half-life, film microstructure, and strength. With the addition of 1.6 wt% TA, SA-Ca²⁺@TA-GF forms a dense foam structure with a gelation time of 10 min. The half-life of the gel foam improves from 0.4 to 30 days and the strength increases by 72.9% compared to that of foam without TA. The inhibition experiments show that SA-Ca²⁺@TA-GF can asphyxiate coal, thus effectively inhibiting coal oxidation. Additionally, it can increase the temperature of coal at the rapid oxidation stage by 60 °C, and the CO inhibition rate is up to 79.6% at 200 °C. The fire-fighting experiment shows that SA-Ca²⁺@TA-GF can effectively cool coal and quickly extinguish fires. This study provides a simple method to prepare highly stable biomass gel foams, which is useful for improving the efficiency of gel foams in inhibiting CSC.

1. Introduction

Coal is the second largest global energy source after oil, and it is very important for global economic development [1,2,3]. However, coal spontaneous combustion (CSC) can cause massive coal losses, casualties and environmental pollution. At present, a variety of fire prevention and extinguishing techniques have been developed, including grouting [4], foam [5,6], inert gas injection [7,8] and gel [9,10]. However, these techniques have their limits. For instance, during grouting, the slurry only flows along the low ground level [11]. Foam has poor stability and water retention [12]. Inert gases easily escape and are diffused [13]. Gel shows weak fluidity and low coverage range [14]. Gel foam technology has attracted wide attention because it can effectively seal air leakage, it can rapidly reduce coal temperature, and it has excellent water retention performance [15,16]. For instance, Lu et al. [11] prepared a new fire protection gel foam by adding polyacrylic acid, proanthocyanidin and organic bentonite to sodium silicate solution. The results showed that the gel foam could effectively inhibit coal spontaneous combustion; the inhibition rate was as high as 68.7%. Wang et al. [17] prepared a new gel foam using a composite foaming agent and a highly water absorbent gel (soluble starch and acrylic acid). The results show that this gel foam can isolate coal from oxygen while cooling the coal, with a CO inhibition rate of up to 64.07%. Gel foam technology has therefore been widely used in coal mine fire prevention processes. However, the materials of these gel foams, such as sodium silicate, acrylic acid and acrylamide, are mainly derived from chemical products, can pollute groundwater and can produce toxic and harmful gases such as NH₄ during CSC [16,18]. There is therefore an imperative to develop gel foams with environmentally friendly performance.

Sodium alginate (SA) is a water-soluble polysaccharide derived from brown algae [19], and it has a strong water fixation ability [20]. In addition, The Na⁺ in the –COONa of SA is easily exchanged with Ca²⁺ to form SA gel [21]. Therefore, gel foams based on SA have attracted much attention. For example, Lu et al. [3] prepared a single network of gel foam (CPSF) based on SA, fly ash and aqueous foam. The results showed that CPSF had excellent performance in inhibiting the process by which coal oxidation produces CO. However, the maximum half-life of CPSF was only 26 h. It cannot meet the needs of long-term CSC prevention in coal mines. Therefore, gel foams with organic/inorganic multiple networks based on SA have been developed. As an example, Qi et al. [22] used SA, WG, polyvinyl alcohol (PVA), calcium carbide slag, and foaming agents to prepare a multiple network gel foam. The results showed that the strength of the gel foam’s film was increased and the height in the beaker had only decreased by 17.3% on the seventh day. Unfortunately, these multiple network gel foams suffer from gel material compatibility problems which limit the significant improvement of their stability [12]. Therefore, there is an imperative to develop a new method to further improve the stability of SA gel foams.

Tannic acid (TA) is an organic polyphenol derived from tree bark or fruit. It is the second most abundant agricultural byproduct after lignin [23,24]. TA contains a large number of catechol and pyrogallol groups, thus, it is easy to couple with other materials by means of hydrogen bonding, etc., to form cross-linked structures [25,26]. For example, Hu et al. [27] prepared a double cross-linked hydrogel (PAA-CS@TA-Al³⁺) by using TA to form hydrogen bonds with chitosan and polyacrylic acid. The results showed that the elongation at break of PAA-CS@TA-Al³⁺ reached 1700% with an increasing cross-linked structure. Therefore, it is hoped that the stability of the gel foam can be substantially improved using this technique.

In this paper, a novel highly stable biomass gel foam (SA-Ca²⁺@TA-GF) based on a double cross-linked structure was prepared by introducing TA into a gel form (sodium alginate/calcium L-lactate/composite foaming agent). The gelation and stabilization mechanism of SA-Ca²⁺@TA-GF was investigated by use of the Fourier transform infrared spectra (FT-IR). Meanwhile, by testing gelation time, half-life, film microstructure, and strength, the effects of TA concentration on the performance of SA-Ca²⁺@TA-GF were analyzed. The inhibition of coal low-temperature oxidation performance and high-temperature fire extinguishing performance of SA-Ca²⁺@TA-GF were also comprehensively analyzed. Furthermore, we expect to provide a simple and versatile method to prepare novel highly stable biomass gel foams, a method which would be useful for improving the efficiency of gel foams in inhibiting coal spontaneous combustion.

2. Materials and Methods

2.1. Materials

Tannic acid (TA) and Calcium L-lactate (CL) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., (Shanghai, China). Sodium alginate (SA) was provided by Shanghai Maclean Biochemical Co., Ltd., (Shanghai, China). Composite foaming agent (CFA) was made in the laboratory and mainly included tea saponins and alkyl glycosides [27]. The best foaming property of CFA was achieved when the complex ratio of tea saponin and alkyl glycosides was 1:2.

2.2. Preparation of Gel Foam

First, the SA solution was prepared in advance in order to ensure that the SA was fully dissolved. Then TA and CFA were added to the SA solution, and the components were fully dissolved. After this, CL was added and the mixture was formed by mechanical stirring. The detailed information for all samples is shown in

Table 1. Gelation time and half-life with different TA concentrations.

SA/wt%	CL/wt%	CFA/wt%	TA/wt%	Gelation Time/min	Half-Life/d
0.4	0.05	0.3	0	No gelation	0.4
	0.05	0.3	0.8	20	20
	0.05	0.3	1.6	10	30
	0.05	0.3	2.4	1	35

.

2.3. The Process of Coating Coal with Gel Foam

The freshly prepared gel foam has a certain degree of fluidity before gelling and can be transported to the target area via a transport pipeline. Then, the gel foam can penetrate into the coal bodies through the voids within the coal. After gelling, the gel foam loses its fluidity and can adhere to the surface of the coal, forming a stable barrier.

2.4. Performance Test of Gel Foam

2.4.1. FT-IR Analysis

An IRA ffinity-1 spectrometer (Shimadzu, Japan) was used to analyze the FT-IR of SA, TA gel foams. The wavenumber range employed was 4500 to 500 cm⁻¹.

2.4.2. Gelation Time and Half-Life

The gelation time was determined by the bottle experiment [14]. The time at the point at which fluidity was lost was recorded as the gelation time.

The stability of each gel foam was studied by observing the relationship between gel foam volume and time. The time at which the volume was reduced by half is the half-life (t_1/2) of the gel foam.

2.4.3. Micro-Morphology of Gel Foams

The micro-morphology of the gel foam was visualized by use of an XSP-300 microscope under 40× magnification.

2.4.4. Strength of Gel Foams

The SH-10N digital push-pull meter (Yueqing Eidelberg Instruments Co., Ltd., Wenzhou, China) was used to test the strength of gel foams. The maximum force value after which the surface of gel foam ruptured was recorded as the strength of the gel foam.

2.4.5. The Inhibition Performance of SA-Ca²⁺@TA-GF in Coal Room Temperature Oxidation

A quantity of coal weighing 3 kg was loaded into a coal storage tank at room temperature, and then covered by 0.5 kg of fresh SA-Ca²⁺@TA-GF. The inhibition performance of SA-Ca²⁺@TA-GF in the coal’s room temperature oxidation was investigated by continuous analysis of the O₂ and CO concentrations in the device for 48 h by using gas chromatography.

2.4.6. The Inhibition Performance of SA-Ca²⁺@TA-GF in the Coal Temperature-Programmed Oxidation

As in previous studies [28,29], the CO concentration was recorded. Then the inhibition rate of SA-Ca²⁺@TA-GF was calculated by using the CO concentration.

2.4.7. The Fire Extinguishing Performance of SA-Ca²⁺@TA-GF

The fire extinguishing performance of SA-Ca²⁺@TA-GF was studied in the combustion experiment device. The combustion experiment device consisted of a temperature sensor and a coal combustion device.

The quantity of coal tested was 3 kg. When the coal was fully burned, 1 kg SA-Ca²⁺@TA-GF was slowly injected into the coal combustion device, and the temperature change of the coal was recorded.

2.4.8. XPS Analysis

The characteristic functional groups of the coal were measured by an ESCALAB 250Xi X-ray photoelectron spectrometer (XPS, ThermoFisher Scientific, Waltham, MA, USA). The coal samples were obtained from the temperature-programmed and oxidation experiment, and the temperatures of the coal samples were 30, 110 and 200 °C, respectively. The C1s peak was at 284.6 eV. In addition, aromatic or aliphatic (C-C/C-H), vacancy defects on functional groups (C*-C*), ethers or hydroxyl groups (C-O), carbonyl groups (C=O) and carboxylic acid groups (COO–) were at 284.6, 285.3, 286.3, 287.6 and 289.1 eV, respectively [30].

3. Results

3.1. FT-IR Analysis

FT-IR spectra of SA, TA, SA-Ca²⁺-GF (the gel foam without TA) and SA-Ca²⁺@TA-GF are shown in Figure 1. In the SA spectra, a broad band at 3460 cm⁻¹ is related to the –OH groups. The peaks at 1616 and 1419 cm⁻¹ are associated with the –COO– groups [31], and the absorption band at 892 cm⁻¹ corresponds to the –OH of –COOH groups [32]. It has been established that the intensity of the –COO– and the –OH in SA-Ca²⁺-GF decreases in comparison to SA, an occurrence which can be attributed to the formation of the gelation structure. This is because Ca²⁺ reacts with the carboxylate of SA (Figure 2), forming a cross-linked structure based on a metal ion coordination bond [3,21], and, as a result, the gelation structure is formed. TA shows an absorption band at 1029 cm⁻¹ associated with the phenolic hydroxyl groups [24,33,34]. SA-Ca²⁺-GF has an absorption peak at 3454 cm⁻¹ attributed to the –OH groups, and shows an absorption band at 1419 cm⁻¹ corresponding to the –COO– groups. Compared with TA and SA-Ca²⁺-GF, the phenolic hydroxyl peak of SA-Ca²⁺@TA-GF moves from 1029 to 1031 cm⁻¹, and the –OH peak of SA-Ca²⁺@TA-GF is significantly red-shifted from 3454 to 3420 cm⁻¹. Meanwhile, the –COO– peak moves from 1419 to 1448 cm⁻¹. All of these changes indicate that a hydrogen bond is formed between the carboxyl group of SA and the phenolic hydroxyl group of TA [23,25]. It can be concluded that it has double cross-linked structures in SA-Ca²⁺@TA-GF. The first cross-linked structure is formed by a metal ion coordination bond, and the second cross-linked structure is formed by the hydrogen bond.

3.2. Stability and Micro-Morphology Analysis of SA-Ca²⁺@TA-GF

The stability of SA-Ca²⁺@TA-GF was studied, as shown in Table 1. With no TA, the foam (SA-Ca²⁺-GF) cannot be fully gelled. The foam structure formed is sparse, and the foam’s film is flimsy (Figure 3a). The half-life of SA-Ca²⁺-GF is only 0.4 days, and it has very poor stability. With the addition of the TA (0.8 wt%), the foam loses its fluidity after 20 min and forms the gel foam (SA-Ca²⁺@TA-GF). The half-life is 20 days, which is greatly improved compared to that of SA-Ca²⁺-GF. As the TA concentration increases to 1.6 wt%, SA-Ca²⁺@TA-GF gels after 10 min and forms a denser foam structure. The foam’s film thickens (Figure 3b), and the half-life is up to 30 days, which is 75 times that of SA-Ca²⁺-GF. When the TA concentration is up to 2.4 wt%, the half-life of SA-Ca²⁺@TA-GF reaches up to 35 days. Although it has a good stability, its gelation time is just 1 min, which is not suitable for use in coal mines to inhibit the coal spontaneous combustion. The optimum concentration of TA is therefore 1.6 wt%. The combustibility of SA-Ca²⁺@TA-GF was further studied, as shown in Figure 4. SA-Ca²⁺@TA-GF was ignited for 10 s. During the ignition, no burning phenomenon was observed, demonstrating that SA-Ca²⁺@TA-GF is non-combustible, and can safely be used in coal mines.

3.3. Mechanical Strength Analysis of SA-Ca²⁺@TA-GF

The mechanical strength of SA-Ca²⁺@TA-GF after gelation was evaluated, as shown in Figure 5. Without TA, the strength of the gel foam is only 0.402 N. When the TA concentration is 1.6 wt%, the strength of SA-Ca²⁺@TA-GF increases to 0.695 N, increased by 72.9% compared to that of foam without TA. As the TA concentration further increases to 2.4 wt%, the strength of the SA-Ca²⁺@TA-GF is further increased to 0.967 N. It can be observed from Figure 5 that the surface of SA-Ca²⁺@TA-GF can retain excellent stability when 100 g of force is applied to the surface. The above results further demonstrate that the SA-Ca²⁺@TA-GF with double cross-linked structures has good mechanical strength, a quality which is useful for improving the stability of the gel foam.

3.4. The Inhibition Performance of SA-Ca²⁺@TA-GF in Coal Room Temperature Oxidation

During the oxidation process, coal usually consumes O₂ and produces CO. Figure 6 shows temperature-time curves of O₂ and CO concentration in the process of the room temperature coal oxidation experiment. As seen in the CO and O₂ concentration curves of the raw coal (Figure 6a), the coal oxidation in the confined space is divided into two stages, namely, oxygen-enriched oxidation and oxygen-lean oxidation. The oxygen-enriched oxidation stage occurs in the first 23 h, during which time the CO concentration rises rapidly to 6414.4 ppm, and the O₂ concentration drops to 3.09%. The oxygen-lean oxidation occurs after 23 h, during which time the CO and O₂ concentrations tend to stabilize. This indicates that the coal can easily consume O₂ to produce CO under conditions with sufficient O₂. When the device is injected with SA-Ca²⁺@TA-GF, the O₂ concentration in the experiment’s device is always maintained near 20.95% and no CO is produced during the 48 h experiment (Figure 6b). This indicates that covering the coal with SA-Ca²⁺@TA-GF can asphyxiate the coal and thus effectively inhibit coal’s room temperature oxidation.

3.5. The Inhibition Performance of SA-Ca²⁺@TA-GF in Coal Temperature-Programmed Oxidation

In the process of the temperature-programmed oxidation of coal, the index gases such as CO also increase sharply as the coal enters the rapid oxidation stage. Figure 7 shows the variation curves of CO concentration and temperature in the coal sample. For the raw coal, the CO concentration starts to increase rapidly at 120 °C, indicating that the coal is beginning to enter the rapid oxidation stage. At 200 °C, the CO concentration is up to 19,574 ppm. For the coal treated with SA-Ca²⁺@TA-GF, the CO concentration starts to increase rapidly at 180 °C; therefore, SA-Ca²⁺@TA-GF increases the rapid oxidation temperature of the coal to 180 °C. At 200 °C, the CO concentration is only 3991.03 ppm. Thus, the inhibition rate of coal by SA-Ca²⁺@TA-GF at 200 °C is as high as 79.60%. The above results show that SA-Ca²⁺@TA -GF can effectively prevent the coal from entering the accelerated oxidation stage and inhibit the coal’s oxidation.

3.6. Fire Extinguishing Performances Analysis of SA-Ca²⁺@TA-GF

Figure 8 shows the temperature-time curves of coal samples. When the burning coal is covered by SA-Ca²⁺@TA-GF, the temperature of the coal drops rapidly from 965 to 98.8 °C within 20 min (Figure 8b). The temperature is decreased by 89.8%, as compared to the initial temperature. 120 min later the temperature drops to 30 °C. In contrast, the temperature of the raw coal only drops by 55 °C in 20 min (Figure 8a). This is a decrease of 5.7%, compared to the initial temperature. At 120 min, the temperature is still high at 716 °C. It eventually drops to 30 °C after 540 min, which is 420 min slower than in the coal treated with SA-Ca²⁺@TA-GF. Thus, SA-Ca²⁺@TA-GF can cool the coal quickly enough to achieve effective extinguishment of coal fires.

3.7. XPS Analysis

Figure 9 shows the C1s sub-peak for coal. The group contents calculated from the peak areas of the C1s subpeaks are shown in

Table 2. Fraction of C1s on raw coal and the coal samples treated with SA-Ca²⁺@TA-GF surfaces.

Samples	Temperature/°C	Carbon Forms (Content/%)
		C-C/C-H	C*-C*	C-O	C=O	–COO
Raw Coal	30	62.21	12.74	8.23	4.23	12.59
	110	61.09	14.09	8.37	5.61	10.84
	200	58.95	14.33	8.64	6.47	11.62
Coal/SA-Ca2+@TA-GF	30	47.11	12	19.78	6.73	14.37
	110	51.62	13.53	18.08	5.20	11.56
	200	52.14	14.97	17.51	5.08	10.3

. At temperatures between 30 and 200 °C, for raw coal, the content of C-C/C-H decreases from 62.21 to 58.95% and the content of carbon dioxide increases from 25.05 to 26.73%. In contrast, for the coal treated with SA-Ca²⁺@TA-GF, the content of C-C/C-H increases from 47.11 to 52.14% and the total content of carbon dioxide decreases from 40.88 to 32.89%. The carbon dioxide content of the treated coal is significantly higher than that of raw coal. This is because the SA-Ca²⁺@TA-GF contains carbon dioxide, and, of note, the TA contains a large number of phenolic hydroxyl groups [3,34]. As can be seen from Figure 10, during the heating of raw coal, carbon radicals are formed by breaking the alkyl side chains of the coal, and these carbon radicals subsequently react with oxygen to form peroxy radicals. The peroxy radicals are further oxidized to form carboxyl radicals and carbonyl radicals. Then, these radicals produce CO₂, CO and new carbon radicals [35]. The above oxidation reactions continue in cycles due to the formation of new carbon radicals, etc. [36,37]. As a result, the content of C-C/C-H is significantly reduced and the content of carbon dioxide is elevated. In the process of SA-Ca²⁺@TA-GF to inhibit coal spontaneous combustion, SA-Ca²⁺@TA-GF can effectively isolate coal from oxygen, inhibiting the formation of carbon and oxygen compounds. The water in SA-Ca²⁺@TA-GF can cool the coal. Meanwhile, TA in SA-Ca²⁺@TA-GF has a strong free radical scavenging ability, and it blocks the chain cycle reaction of coal’s free radicals by transferring hydrogen atoms to peroxy radicals on the surface of the coal [33,38]. At the same time, a stable ether structure is formed by TA conjugation with the phenol structure in coal [39]. Therefore, the relative content of carbon dioxide decreases significantly and the relative content of the C-C/C-H increases gradually. These findings indicate that SA-Ca²⁺@TA-GF can effectively inhibit coal spontaneous combustion by isolating oxygen, cooling the coal, and scavenging free radicals from the coal surface.

4. Conclusions

In summary, a novel highly stable biomass gel foam, SA-Ca²⁺@TA-GF based on double cross-linked structure, was prepared by introducing TA into a gel form (sodium alginate/calcium L-lactate/composite foaming agent). The foam performance and coal spontaneous combustion inhibition performance of SA-Ca²⁺@TA-GF were studied. The following conclusions were obtained.

With the addition of 0.4 wt% SA, 0.05 wt% CL, 0.3 wt% CFA and 1.6 wt% TA, SA-Ca²⁺@TA-GF forms a dense foam structure with a gelation time of 10 min. The half-life of this gel foam increases from 0.4 to 30 days and the strength increases by 72.9% compared to that without TA. This is due to the fact that TA can form hydrogen bonds with SA, and SA can also form metal ion coordination bonds with Ca²⁺, thereby constructing a double cross-linked structure of SA-Ca²⁺@TA-GF.

The inhibition experiments show that SA-Ca²⁺@TA-GF can effectively asphyxiate the coal at room temperature, thus reducing the CO concentration from 7556.8 to 0 ppm, as compared to the raw coal. Besides, SA-Ca²⁺@TA-GF can increase the temperature of coal at the rapid oxidation stage by 60 °C, and the inhibition rate reaches 79.6% at 200 °C. The fire-fighting experiment shows that SA-Ca²⁺@TA-GF can rapidly cool the coal to achieve quick fire extinguishment.

SA-Ca²⁺@TA-GF has excellent stability, can asphyxiate coal, and the water contained in it can effectively cool coal. Meanwhile, TA has the effect of scavenging reactive free radicals from the coal’s surface. Therefore, SA-Ca²⁺@TA-GF can effectively inhibit coal spontaneous combustion by isolating oxygen, cooling the coal, and scavenging free radicals from the coal’s surface.

The gel foam has obtained good results during laboratory tests. Although the amount of raw material added to this gel foam is about 3–5% of that of ordinary inorganic gel foam, the half-life of this gel foam is more than three times that of ordinary inorganic gel foam, making this gel foam more cost-effective than ordinary inorganic gel foam. Meanwhile, the gel foam can use the same preparation and delivery systems as ordinary inorganic gel foam, avoiding the need for equipment modifications. However, an adequate supply of air and water needs to be ensured in order to use this gel foam.