Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
pubs.acs.org/EF
Impact of Torrefaction on the Fuel Properties and Combustion Characteristics of Compost of Food Waste and Sawdust Haoran Yuan,†,‡,§,∥ Qiu Yang,†,‡,§,∥ Yazhuo Wang,†,§,∥ Jing Gu,*,†,§,∥ Mingyang He,‡ and Fu’an Sun‡ †
Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, People’s Republic of China ‡ Changzhou University, Changzhou, Jiangsu 213164, People’s Republic of China § Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, People’s Republic of China ∥ Guangzhou Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, People’s Republic of China ABSTRACT: With rapid global development, the amount of food waste is increasing, which has seriously affected the environment. Usually, food waste is composted with sawdust to prevent environmental pollution as a result of the loss of N and S. Unfortunately, the quality of the compost of food waste and sawdust (CFS) is poor, and the material is hard to handle. Torrefaction, a process of slow pyrolysis under anoxic conditions, can be used to improve the properties of a fuel sample. This study investigated the fuel properties of CFS samples subjected to torrefaction at five different temperatures (250, 300, 350, 400, and 450 °C) and a residence time of 30 min. Physicochemical analyses of the samples were carried out according to standard methods, and the combustion characteristics of the samples were studied by thermogravimetric analysis. Torrefaction has great impact on proximate and ultimate analyses, chlorine contents, energy and mass yields, and grindability and combustion characteristics. The grindability and combustion properties of CFS were also improved by torrefaction. At a torrefaction temperature (300 °C), the higher heating value reached its maximum value (19 334 kJ/kg), meaning that the torrefied CFS contained up to 63.89% of its original energy content and the combustion characteristics were optimal at this torrefaction temperature.
1. INTRODUCTION The rate at which the Chinese economy is growing has accelerated the process of urbanization, improved the standard of living for most residents, and brought a series of environmental problems. According to the China Statistical Yearbook,1 the yield of China’s municipal solid waste in 2010 was 158 million tons, and that value is constantly increasing; it reached 191 million tons in 2015. Therefore, the harmless reduction and resource reutilization of municipal solid waste has become the subject of much social concern. Food waste is a major component of municipal solid waste and accounts for 15.74−85.8%2 of the total municipal solid waste; the moisture content of the food waste is typically high, generally up to 54.5−89.09% of the total. Moreover, the problems associated with the high oil and salt contents and the complex composition of organic matter restrict the utilization of this organic matter because these issues can cause spoilage and bacterial growth. Because it is often used as livestock feed, food waste can spread infectious diseases; any garbage fed to pigs would be recycled onto people’s dining tables and cause serious food safety problems. The disposal of food waste has gradually become a significant energy issue, and people have built their careers around it. This issue has also been a subject of great concern in China. Food waste contains a large amount of organic matter and can be used for composting.3 Aerobic food waste composting is a natural degradation process, and in an aerobic environment, the complex organic matter in the food waste is decomposed © XXXX American Chemical Society
into simple organic matter by aerobic microorganisms that are common in nature. However, it takes a long time to compost food waste, and NH3 and H2S can easily be generated during composting;4 they are both air pollutants. The C/N ratio of food waste5 can be adjusted by the addition of sawdust to reduce the number of required composting cycles and the emissions of NH3 and H2S.6 However, unclassified food waste contains some impurities that are difficult to compost. These impurities can make the quality of the compost poor and decrease the N and P contents, and at the same time, a large number of these harmful substances can be absorbed into the soil. The salt content of compost is often high; therefore, compost that was intended to be fertilizer can cause soil compaction and salinization. As a result of the potential hazards of compost, compost of food waste and sawdust (CFS) is not desirable as a fertilizer treatment, but CFS has great potential as a source of raw material for thermal conversion. The total amount of fossil fuel is declining, and environmental pollution is deteriorating; human are searching for a clean, renewable, and viable energy that is an alternative to fossil fuel. Biomass, as the renewable and clean source, would become a viable substitution energy source in future energy. Thermal conversion of CFS can ease the energy crisis and solve the problem of food waste. In recent years, municipal solid waste Received: November 6, 2017 Revised: January 28, 2018 Published: January 29, 2018 A
DOI: 10.1021/acs.energyfuels.7b03421 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels incineration technologies,7 which can reduce the weight of waste by up to 75% and the volume by up to 90%, effectively decompose and fix harmful substances and achieve better energy recovery, and it is an effective way to realize the reduction and resource reutilization of municipal solid waste. Therefore, it is necessary to develop a harmless treatment to the fuel properties of CFS. Torrefaction,8−10 a promising thermal pretreatment, is the process of anoxic slow pyrolysis that produces torrefied solid products, condensable liquids, and non-condensable gases, and it has mainly been used to improve the higher heating value (HHV), grindability, fuel quality, and overall combustion characteristics of biomass. Torrefaction improved the fuel quality of the food waste by decomposing the macromolecular organics and removing moisture. However, high energy consumption of food waste during torrefaction would lead to low economic feasibility. After the compost of sawdust and food waste, the moisture contents of the composting product decrease, the porosity increases, the decomposition of organic matter accelerates, and the energy consumption of torrefaction declines. Sawdust is a renewable energy source, and the thermal conversion of the compost products into fuels can solve the problems of waste pollution. Poudel et al.11 discovered that torrefaction increased the HHV and fuel quality of food waste. Yuan et al.12 investigated the impact of torrefaction on the municipal solid waste and found that the contents of H and O decreased, the content of C increased, and the combustion characteristics were improved when municipal solid waste was torrefied at 450 °C instead of 250 °C. The content of Cl, the main pollutant, in torrefied municipal solid waste was lower; therefore, torrefaction may be an effective pretreatment method to curb the generation of dioxin precursors directly from the source and prevent secondary environmental pollution caused by the incineration of municipal solid waste. To determine the influences of torrefaction on the fuel characteristics of CFS, this study was conducted to investigate the impact of different torrefaction temperatures on the proximate and ultimate analyses, energy yield, and grindability and combustion characteristics of CFS.
2.2. Torrefaction. The torrefaction experiments were conducted in a horizontal tubular furnace, as shown in Figure 1.12 During torrefaction, approximately 10 g of unmilled CFS was put in a quartz tube, and the tube was filled with high-purity N2 at the flow rate of 100 mL/min for 30 min to make sure that there was no air in the tube. The temperature of the reactor was raised from room temperature to the desired temperatures of 250, 300, 350, 400, and 450 °C at a heating rate of 10 °C/min, and the reaction vessel was maintained at the given temperature for 30 min, which was optimal for torrefaction reaction from previous studies.9,13−15 After 30 min, the heating reactor was turned off but N2 continued to flow to cool the tube. After torrefaction, the final weight was determined to calculate the solid mass yield. Equations 1 and 2, as defined by Bergmen et al.,16 were adopted to calculate the mass and energy yields. The solid product was collected and ground for 25 min. The solid products obtained from torrefaction at 250, 300, 350, 400, and 450 °C are called T250, T300, T350, T400, and T450, respectively, and the liquid and gas products were also collected
X mass (mass yield) =
Xenergy (energy yield) = X mass
WCl‐absolute = WCl‐relativeX mass
20.42 68.54 11.04 40.56 5.62 28.01 3.66 0.27 1.46
(3)
where WCl‑absolute is the absolute content of insoluble Cl or soluble Cl (mg/g), WCl‑relative is the relative content of insoluble Cl or soluble Cl (mg/g), and Xmass is mass yield of T250, T300, T350, T400, or T450. 2.3.3. Analysis of HHV. The HHV of the samples was measured by a WZR-1T-CII and IKA C2000 microcomputer calorimeter using the ASTM D5865-03 method. 2.3.4. Grindability Analysis. To conduct this test, all of the samples were ground for 25 min in a PM0.4 L planetary ball mill. The particle size and surface morphology of samples after milling were observed by scanning electron microscopy (SEM). The particle size distribution of samples was determined by a laser diffraction particle analyzer. All of the samples were dispersed in deionized water before measurements were taken, and all measurements were conducted in triplicate. 2.4. Combustion Experiments. To study the combustion characteristics of torrefied CFS, combustion experiments were accomplished in a Nicolet 6700-Q50 thermogravimetric analyzer. Samples of approximately 10 mg were put into the furnace and heated to the desired temperature (900 °C) at the constant heating rate of 20 °C/min with the air influx rate of 30 mL/min. Finally, the thermogravimetry (TG) and derivative thermogravimetry (DTG) curves of the combustion of the samples were obtained.
Table 1. Proximate and Ultimate Analyses of Raw CFS mass ratio (%, dry basis)
HHV(torrefied CFS) × 100% HHV(raw CFS) (2)
2.1. Materials. CFS was collected from southern China as a raw material in this study. To reduce the influence of moisture on the torrefaction process and improve the homogeneity, the CFS was dried in a drying oven at the temperature of 105 °C for almost 24 h until a constant weight was reached, and then the material was ground for 25 min in a planetary ball mill. The proximate and ultimate analyses of the CFS (dry basis) are shown in Table 1, and the HHV of CFS was 17 525 kJ/kg.
ash volatile fixed carbon C H O N S Cl
(1)
where m(torrefied CFS) (g) represents the weight of torrefied CFS, m(raw CFS) (g) represents the weight of raw CFS, HHV(torrefied CFS) (kJ/kg) represents the higher heating value of torrefied CFS, and HHV(raw CFS) (kJ/kg) represents the higher heating value of raw CFS. 2.3. Physicochemical Analysis of the Samples. 2.3.1. Proximate Analysis. The volatile and ash contents of the samples were measured according to the ASTM D3175-11 and ASTM D3174-11 standards with a SX-G07123-type muffle furnace, and the fixed carbon content was gained by subtraction. 2.3.2. Ultimate Analysis. The contents of C, H, and N were obtained by an elemental analyzer (vario EL cube) using the ASTM D3175-11 standard, and the O content was calculated by subtraction. Cl in the sample was divided into insoluble Cl and soluble Cl, and the contents were measured separately. insoluble Cl and soluble Cl were analyzed in accordance with the elution analysis and the GB/ T3558-2014 national standard. Cl in the filtrate is soluble Cl; Cl in the filter residue is insoluble Cl; and the total Cl content is the sum of soluble Cl and insoluble Cl. On the basis of the mass yields and Cl contents of raw CFS and torrefied CFS, the absolute content of Cl was calculated according to the following equation:
2. EXPERIMENTAL SECTION
proximate and ultimate analyses
m(torrefied CFS) × 100% m(raw CFS)
B
DOI: 10.1021/acs.energyfuels.7b03421 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 1. Experimental setup: (1) nitrogen gas, (2) reducing valve, (3) flow meter, (4) rubber stopper, (5) tube furnace, (6) temperature controller, (7) material, (8) pipeline, (9) quartz tube, (10) tar collection system, (11) ice, (12) gas analyzer. During combustion, the combustion characteristics are generally characterized by the combustible index (Cn) and the burnout index (Sn).17 If the values of Cn and Sn are higher, the combustible characteristics of the material are better. The values of Cn and Sn were calculated according to following equations:
Cn = Wmax /Ti 2 Sn =
gas products increased from 11.98 and 6.95 to 22.21 and 12.69%, respectively (Table 2). The mass yield decreased dramatically because the major components (volatiles and moisture) were released.19 When the torrefaction temperature was more than 300 °C, the mass yield of liquid and gas products increased dramatically, which would lower the quality of solid products. 3.2. Fuel Characteristics of Torrefied CFS. Table 3 shows the proximate analysis results of the samples. In comparison to raw CFS, the fixed carbon and ash contents in the torrefied CFS increased with an increasing temperature (from 14.63 to 28.51% and from 24.94 to 46.24%, respectively), while the volatile content substantially decreased with an increasing temperature (from 60.63 to 25.25%). Moving from T250 to T300, the volatile content decreased noticeably; however, the fixed carbon content increased as a result of the intensity of the torrefaction reaction between 250 and 300 °C. During this stage, a large number of volatiles were released20 and some organic compounds21 underwent decarboxylation and dehydroxylation reactions and condensed a small amount of semi-coke.13,15,22,23 The formation of semi-coke led to the increase of the fixed carbon content. The volatile content of T300 was 48.53% lower, and the fixed carbon content of T300 was 20.02% relative to T250. Ultimate analysis of the torrefied CFS is listed in Table 3. Increasing torrefaction temperatures impacted the C, H, and O contents of the samples. Despite the loss of mass, the C content of torrefied samples (from T250 to T300 to T350) was increased significantly. The largest percentage of C was 43.57% in T300. The O contents decreased rapidly with increased temperature because the oxygen-containing functional groups were easy to crack or recombine to generate carbon monoxide (CO) and carbon dioxide (CO2).24 Torrefaction of CFS caused a decrease in the H content as a result of the dehydroxylation reactions and the release of hydrocarbons (such as CH4).10 In addition, all of the changes suggest that torrefaction can improve the HHVs of torrefied CFS from Figure 4. The content of S was very low; therefore, it was not shown in Table 2. However, it was easy to determine that the N contents of torrefied CFS changed only slightly with an increasing temperature, which indicates that torrefaction had little effect on the N contents. When the torrefaction temperature was 300 °C, the protein decomposition temperature, the absolute value of N decreased significantly. Torrefaction led to the slight decrease of the N content at increasing temperatures, which reduced the emission of NOx during the combustion process.
(4)
WmaxWmean Ti 2Th
(5)
where Ti represents the ignition temperature (°C), Th represents the burnout temperature (°C), Wmax represents the maximum rate of weight loss (mg min−1), and Wmean represents the average rate of weight loss (mg min−1).
3. RESULTS AND DISCUSSION 3.1. Characteristics of Torrefaction. Torrefaction is a slow pyrolysis process, and the whole torrefaction process18 can be divided into four stages, namely, heating, drying, torrefaction, and cooling. During the heating stage, the temperature of the sample is increased and moisture is beginning to be released. During the drying stage, the temperature in the prophase is 100 °C and free water is evaporating. In the middle and later stages, the sample is heated to 200 °C, water that is physical bound within the sample is evaporated, and some light organics are also starting to liberate; therefore, there continues to be some mass loss. In the torrefaction stage, the sample is torrefied and the temperature is above 200 °C. Substantial weight loss occurs during this stage as a result of the hydrolysis of carbonation. In the cooling stage, the torrefaction product is cooled from 200 °C to room temperature, and no further mass loss occurs. Table 2 shows the torrefied product distribution of CFS at different torrefaction temperatures. In this study, the Table 2. Product Yields of Torrefied CFS temperature (°C)
solid (%)
liquid (%)
gas (%)
250 300 350 400 450
81.07 65.1 55.13 48.28 45.18
11.98 22.21 31.58 36.17 30.92
6.95 12.69 13.29 15.55 23.9
torrefaction temperature played the major role in determining the mass yield of CFS. The mass yield of solid products decreased with the increase of the torrefaction temperature as a result of the degradation of hemicellulose, while the liquid and gas products were increased. When the torrefaction temperature increased from 250 to 300 °C, the mass yield of torrefied solid CFS dropped from 81.07 to 65.1%, while the liquid and C
DOI: 10.1021/acs.energyfuels.7b03421 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 3. Mass Yield and Proximate and Ultimate Analysis Results of Raw CFS and Torrefied CFS ultimate analysis (%) temperature (°C) 250 300 350 400 450
C 43.54 43.57 42.39 39.35 34.72
± ± ± ± ±
H 0.18 0.24 0.21 0.41 0.27
5.17 4.35 3.86 3.16 2.17
± ± ± ± ±
proximate analysis (%)
O 0.09 0.07 0.11 0.1 0.05
20.49 14.4 10.9 9.48 10.69
± ± ± ± ±
N 0.21 0.12 0.44 0.52 0.52
4.14 4.03 3.71 3.47 3.27
± ± ± ± ±
ash 0.06 0.07 0.1 0.06 0.06
24.94 31.45 36.48 41.99 46.24
± ± ± ± ±
volatile 0.05 0.08 0.05 0.06 0.03
60.43 48.53 39.6 32.19 25.25
± ± ± ± ±
0.29 0.25 0.14 0.3 0.08
fixed carbon 14.63 20.02 23.92 25.82 28.51
± ± ± ± ±
0.24 0.17 0.19 0.24 0.11
CFS, soluble Cl was mainly from the edible salts and insoluble Cl was mainly from organic compounds. The absolute contents of insoluble Cl and soluble Cl decreased with increasing torrefaction temperatures, and the decrease in the absolute content of total Cl was most notable at 300 °C (11.6 mg/g). The release of Cl was mainly due to the reactions of Cl with oxygen-containing functional groups, and Cl combined with free radicals to generate HCl, CH3Cl, or Cl tar.26−29 Rahim et al.27 loaded NaCl into the cellulose and found that Cl released as HCl at temperatures of 350 °C, Cl would be released in the form of Cl tar. However, the absolute content of Cl increased at 350 °C, which indicates that some liberated HCl may have reacted with the pyrolysis char, leading to the capture of Cl.26 The reduction in the Cl content in torrefied CFS indicates that torrefaction released Cl of the CFS, and the low Cl content of torrefied CFS indicated that torrefaction could be used as an effective pretreatment to prevent the formation of toxic substances [polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran (PCDD/F), HCl, etc.] in the process of CFS combustion. The HHVs and energy yields of the samples are presented in Figure 4. The HHVs of the torrefied CFS initially increased as
Figure 2 shows molar ratios of O/C and H/C of the samples. As the torrefaction temperature increased, the O/C and H/C
Figure 2. O/C and H/C molar ratio of torrefied CFS and raw CFS in a Van Krevelen diagram.
molar ratios decreased. When the torrefaction temperature was 300 °C, the HHVs decreased gradually as a result of the pyrolysis reaction starting11 at a high temperature. The HHVs represent the energy released from volatile and fixed carbon combustion and depend upon the carbon content. When the torrefaction temperature increased from 250 to 300 °C, the increase in deoxygenation led to an increase in the C content.30 The higher C content caused the HHV of T300 to be higher than that of T250, and the fuel properties of the T300 CFS were superior. Figure 4 shows the energy yields of CFS at various torrefaction temperatures. The energy yields of torrefied CFS corresponded to the mass yield; therefore, the energy yields
Figure 3. Absolute content of Cl in torrefied CFS and raw CFS. D
DOI: 10.1021/acs.energyfuels.7b03421 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 5. SEM images of samples. E
DOI: 10.1021/acs.energyfuels.7b03421 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels decreased gradually as the torrefaction temperatures increased. When the torrefaction temperatures were
