Conversion of Municipal Solid Wastes into Biochar through Hydrothermal Carbonization

In this study, the hydrothermal treatment of municipal solid wastes (MSWs) for the pro- duction of biochar as a renewable solid fuel was investigated. The properties of surrogate MSWs and mixtures of newspaper and vegetables were greatly improved by hydro- thermal treatment and were similar to those of coal-like fuel substances. Hydrothermal treatment increased the calorific value, the fixed carbon, and carbon contents. The com - position of the major biomass components of MSW was found to affect the alternation of their physical and chemical properties significantly. These characteristic changes in pure cellulose, hemicellulose, and lignin were similar to those of coalification at the hydrother mal reaction temperature range of 150–280°C. The treated products became a solid fuel substance, the characteristics of which corresponded with fuel between lignite and sub-bituminous coal. The results of this study indicate that hydrothermal treatment can be used as an effective means to generate highly energy-efficient renewable fuel resources using MSWs.


Introduction
Recently, to address increasing energy consumption and the steady depletion of fossil fuel reserve, many investigations have been conducted to develop alternative and renewable energy resources using biomass and waste [1,2]. Because rapid urbanization is occurring almost everywhere in the world, substantially increasing municipal solid waste (MSW), which is of great global concern. As an effective means for treating MSW, the mechanical biological treatment (MBT) system has received interest in Korea. MBT consists of two stages: mechanical treatment (MT) and biological treatment (BT). The MBT system was developed in Germany for waste processing [3]. In the first stage of this process (MT), MT is a size-based process that functions to remove all individual elements (regardless of size) that can be used for the production of refuse-derived fuel (RDF) (e.g., metal, plastic, paper, glass, and biodegradable materials). The larger MSW components are collected separately as combustible matter and then separated further to harvest materials for the production of refuse-derived fuel (RDF), which is used in energy generation. After the MT stage, the residual MSW is used for the production of organic fertilizers and biogas (CH 4 ) in the BT stage. However, the BT stage has many problems, such as a long treatment period requirement, (e.g., more than 1 week or 1 month) and the emission of unpleasant odors [1,3,4].
Thermo-treatment concept is the main conversion technology to develop the BT stage, such as carbonization (400-500°C), pyrolysis (500-600°C), gasification (600-1000°C), and combustion (800-1000°C) to produce carbon-neutral energy from several kinds of biomass wastes. Hydrothermal treatment, which is a thermo-chemical conversion process that employs subcritical water (water heated to any temperature less than its critical temperature of 373°C under sufficient pressure to maintain the liquid state), functions by hydrolyzing biomass components that contribute greatly to the decomposition of structural biomass compound, the major constituents of biomass contained in MSW [5][6][7][8][9]. Therefore, the properties and drying performance of biomass as an energy resource can be improved significantly in a short time [7,10,11].
In the current research, we employed a pilot-scale hydrothermal treatment system to generate alternative solid fuel products from MSW using subcritical water (200°C, 1.6 MPa). The MSW samples tested in this study were collected from an MSW treatment facility in Korea. The samples were at the MT stage of the MBT system and were mainly composed of food residue (40-50%) and paper (30-40%)-Chung et al. [12]. They had a high moisture content of ~50-60% due to the food residue. The physical and chemical characteristics of the MT residue needed to be altered (dehydrated, compacted, and upgraded) for it to be used as a solid fuel, such as RDF. Thus, cellulose, hemicellulose, and lignin were used as surrogate MSWs for food as well as paper wastes. Then, the effects of hydrothermal treatment on the conversion of the biomass comprising the MSW samples were examined by varying the reaction temperatures in the range of 150-280°C, and the changes in the biomass characteristics were investigated.

Materials
Food residue and paper content, which comprised the highest proportions in the composition of the MT residue obtained from an MBT system in Mokpo city, Korea, were evaluated because the composition of the MSW varies according to each season (e.g., food waste and paper components can fluctuate between 70 and 80% of total MSW) [12], surrogate MSW (SM) residues, which were prepared using newspaper and Korean Kimchi instead of paper and food waste, were mixed at two different ratios (SM 1 and SM 2) after the crushing process: SM 1 = 5:5 and SM 2 = 3:7 (waste paper:Kimchi (wet, w/w)). Table 1 shows the properties of these surrogate MSWs, including the results of proximate and ultimate analysis and the calorific values. In addition, pure cellulose (α-cellulose-fiberform, Nacalai Tesque Inc., Kyoto, Japan), xylan (Beechwood, SIGMA) which are the main components of hemicellulose [13], and lignin (Kanto Chemical Co., Inc., Japan) were also tested to investigate the effects of hydrothermal conversion on these materials.

Pilot-scale hydrothermal treatment system
Hydrothermal treatment experiments were performed using a 200 L pilot-scale reactor (Figure 1(a)). The reactor consists of a steam boiler and a steam condenser. For all of the experiments, 20 kg of surrogate MSW was supplied to the reactor. The operating temperature of the hydrothermal treatment was set at 200°C, with a pressure of 1.6 MPa, and the reaction was carried out for 60 min. After the hydrothermal reaction was completed, the residual steam was discharged, and the products were collected from the reactor.

Lab-scale hydrothermal treatment reactor
A laboratory-scale hydrothermal treatment reactor was used to investigate the effects of hydrothermal treatment on the characteristic changes in pure cellulose, hemicellulose, and lignin. The experiments were performed using a 500 mL autoclave reactor (Figure 1(b)) consisting of a reactor body, heater, and steam condenser and operated under N 2 gas. A compound sample (20 g) was mixed with an equal amount of water and loaded into the reactor. The operating temperatures and pressures ranged from 150 to 280°C and 1.3 to 5.5 MPa, respectively, and the reaction time was 30 min. The components in the reactor were mixed vigorously using an agitator rotating at 200 rpm.

Analytical procedures
Elemental composition analysis of surrogate MSWs, cellulose, hemicellulose, lignin, and their solid products were carried out using a PerkinElmer 2400 Series II CHN organic elemental where NDF is neutral detergent fiber, ADF is acid detergent fiber, and ADL is acid detergent lignin. Figure 2 shows the biochar products from MSW. Furthermore, Table 1 and    [13,17,18]. After the hydrothermal treatment, the volatile matter and oxygen contents decreased, whereas the fixed carbon content increased from 6.0 to 15.0% for SM 1 and from 8.9 to 13.3% for SM 2 via hydrolysis reactions (Figure 3a and b) shows the calorific values of the surrogate MSWs (SM 1 and SM 2) before and after the hydrothermal treatment. from 15.1 to 17.2 MJ/kg and 14.9 to 16.1 MJ/kg, respectively, which resulted in an increase of fixed carbon content. The increase in the calorific value of SM 1 was greater than that of SM 2 due to the higher paper content in the MSWs. This result suggests that the hydrothermal treatment is more effective at producing an upgraded solid fuel from MSW with high paper content.

Effect of hydrothermal treatment on upgrading of biomass in MSW
The surrogate MSW was prepared by blending newspaper and Kimchi. The content of the three major biomass components (cellulose, hemicellulose, and lignin) in the MSW samples was considered to be an important factor affecting the performance of the hydrothermal treatment. The characteristic changes of these components were analyzed. Additionally, the effects of hydrothermal treatment on the calorific values of cellulose, hemicellulose, and lignin were also examined.    Figure 4 shows the composition of the biomass components comprising the surrogate MSW. Dried newspaper was composed of 57.2 cellulose, 12.3 lignin, and 6.9% hemicellulose. Dried Kimchi was composed of 16.8 cellulose, 4.6 lignin, and 0.5% hemicellulose. Therefore, the total cellulose, lignin, and hemicellulose content of the newspaper and Kimchi were 76.5 and 29.1%, respectively. The total cellulose, lignin, and hemicellulose content in the biomass of the SM 1 and SM 2 samples were 49.4 and 38.4%, respectively. Specifically, the SM 1 and SM 2 samples contained 37.1 and 29.0% cellulose, 8.5 and 6.9% lignin, and 3.8 and 2.5% hemicellulose, respectively (Figure 4). The composition of biomass components (cellulose, hemicellulose, and lignin) of the MSW was found to influence the properties of the hydrothermal products.

Changes in the properties of biomass components
Hydrothermal treatment changed the properties of cellulose, hemicellulose, and lignin.  Along with the results of the ultimate analysis, Figure 5 and Table 2 show the results of the proximate analysis of the biomass components by varying the hydrothermal reaction temperature. The fixed carbon content of cellulose increased from 6.1 to 35.0% in response to hydrothermal treatment at 220°C ( Table 2 and Figure 5(a)). This result suggests that the cellulose began to decompose at 220°C. When xylan was used as a hemicellulose, the fixed carbon and carbon contents increased from 15.2 to 33.5% and from 41.9 to 61.3%, respectively, at 180°C (Figure 5(b)). Below 180°C, the compositions of these products were not different from those of the raw material. This result is not surprising because the hydrolysis of hemicellulose occurs at 180°C [6,11]. As the fixed carbon content and carbon contents increased due to the hydrothermal treatment, the calorific value increased. However, the results for lignin were different from those for cellulose and hemicellulose. Lignin started to decompose at temperatures exceeding 250°C (Figure 5(c)). This can most likely be attributed to the decomposition or pyrolysis of lignin at temperatures slightly below 250°C.
As a result, the increase in the fixed carbon content of the surrogate MSWs (SM 1 and SM 2) was influenced by the increase in the fixed carbon content of cellulose and hemicellulose (Figure 5(d)).
Additionally, the ash content of lignin was higher than that of cellulose, indicating that lignin possesses higher ash content.   Engineering Applications of Biochar chemical dehydration and decarboxylation (i.e., removal of CO 2 ) [7,[18][19][20][21]. Therefore, in hydrothermal reactions, an optimum temperature should be maintained to maximize the energy recovery efficiency (ERE) [22][23][24]. The ERE is an important parameter for evaluating the effect of hydrothermal treatment on solid fuel production (Figure 6(b)). The ERE is defined by Eq.

Mechanism of hydrothermal treatment for upgrading solid products
The results indicated that the hydrothermal treatment induce hydrolysis, chemical dehydration, and decarboxylation reactions (Figure 7). Hydrolytic reactions caused by 1 mol of water cleave cellulose and hemicellulose at ester and ether bonds of cellulose and hemicellulose into smaller molecules [6,7,11,25]. This hydrolysis reaction can complete the conversion of biomass within a few reaction cycles. Furthermore, the fuel properties of the products generated   [6,7,11,26]. Along with a loss in weight, these reactions caused a decrease in volatile matter and an increase in carbon content in the biomass products compared with the raw materials. These effects can be utilized for the drying and carbonization of biomass into an alternative fuel.

Coalification band of hydrothermal products
Hydrothermal treatment can upgrade the properties of the biomass components of MSWs in a manner similar to the coalification process. The coalification bands of raw and treated surrogate MSWs (SM 1 and SM 2) were compared with the coalification band of pure cellulose, hemicellulose, lignin, and various types of coal (Figure 8). MSW is known to have high H/C and O/C ratios, which is similar to those of cellulose and other biomass materials [6,17,18,21]. The H/C and O/C ratios of SM 1, SM 2, cellulose, hemicellulose, and lignin decreased with the coalification status between lignite and sub-bituminous coal. This occurs when the biomass components of MSW are converted into carbonaceous products by chemical dehydration reactions during hydrothermal treatment (4(C 6 H 10 O 5 )n ⇔ 2(C 12 H 10 O 5 )n + 10H 2 O) [6,10,11,18,20]. Due to dewatering, dehydration, and decarboxylation, hydrothermal reactions can enhance biomass properties by reducing the hydrogen and oxygen contents of reaction products, resulting in increased calorific values of biomass products.

Conclusion
The effects of hydrothermal treatment on the properties of MSWs were investigated for their conversion into fuel products with high-energy efficiencies. After the hydrothermal treatment, the surrogate MSWs containing high paper content demonstrated significant increases in their carbon content and calorific values. Therefore, cellulose, hemicellulose, and lignin that constitute the MSWs as biomass were used to investigate the effects of the reaction temperature. The optimum reaction temperature for a mixture of cellulose, hemicellulose, and lignin was found to be approximately 200°C. As a result, the status of the treated products corresponded with solid fuels between lignite and sub-bituminous coal.