Article pubs.acs.org/EF
Effect of the Particle Size on Co-combustion of Municipal Solid Waste and Biomass Briquette under N2/O2 and CO2/O2 Atmospheres Yongling Li,† Xianjun Xing,*,†,‡ Baojie Xu,§ Yongqiang Xing,† Xuefei Zhang,† Jing Yang,† and Jishou Xing§ †
School of Mechanical Engineering, Hefei University of Technology, Hefei, Anhui 230009, People’s Republic of China National Urban Energy Measurement Center (Anhui), Hefei, Anhui 230009, People’s Republic of China § Measurement and Control of Mechanical and Electrical System Key Laboratory of Beijing, Beijing Information Science and Technology University, Beijing 100192, People’s Republic of China ‡
ABSTRACT: The combustion characteristics and kinetics of municipal solid waste and biomass briquette blend were investigated at different particle sizes using thermogravimetric analysis. For the comparison between N2/O2 and CO2/O2 atmospheres, the mole fraction of oxygen was 20% and effects of the heating rate on the blend combustion behavior were also evaluated. Results showed that, in the range of less than 270 μm, the most vigorous combustion occurred at a particle size of 150−180 μm, regardless of the heating rate. With the increase of the heating rate, the combustion intensity of the blend could be improved and the reaction process was influenced to some extent. The replacement of N2 with CO2 reduced the combustion reactivity and had a significant effect at the particle sizes of less than 106 μm. The maximum values of activation energy (E) occurred at 150−180 μm, which corresponded to the maximum comprehensive combustion characteristic index S. The E values under an 80CO2/20O2 atmosphere were close to those under an 80N2/20O2 atmosphere in the first and second reaction stages, but the differences of E values in the third stage were larger, with a maximum of 114.79 kJ/mol. The results obtained from this work can help to optimize the parameters of municipal solid waste and biomass briquette co-combustion equipment and provide reference for the blend combustion under oxy-fuel combustion technology.
1. INTRODUCTION The rapid growth of municipal solid waste (MSW) production has become one of the world’s most serious public nuisances as a result of the adverse environmental and human health impacts. Incineration is a promising approach to deal with MSW in making up for the shortage of fossil fuels, reducing the volume of waste generated and the secondary pollution problems.1−4 Additional fuel is necessary to be added in the process of MSW incineration as a result of the high moisture content and low calorific value of MSW in most parts of China. Because agricultural residues are abundant in China and have a substantial effect on CO2 reduction, biomass is an effective substitute for fossil fuels.5,6 The biomass raw materials, which were compressed after drying and pulverization into hardpacked biomass briquettes or biomass pellets, could increase homogeneity and allow for a wider range of lignocellulosic materials to be used as fuel.7,8 Chirone et al.9 found that pelletization could extend biomass devolatilization times and improve evenness of volatile matter release in the combustion process compared to that of the loose raw material. The biomass briquettes were more applicable to be used in grate furnaces and fluidized-bed combustion with the advantage of easy storage and transport, lower pollution, lower dust levels, higher heating values, etc.10 Therefore, the co-combustion of MSW and biomass briquette is an attractive option to achieve the coordinated treatment of both wastes, but it is still lacking a systematic theoretical study. Thermogravimetric analysis (TGA) is a distinctly important and commonly used technique to investigate the solid-phase thermal degradation mechanisms © 2016 American Chemical Society
and obtain the kinetic parameters during the combustion process.2 Existing studies have investigated the combustion characteristics of MSW, raw biomass, or both combined firing with fossil fuels using the TGA method. However, the cocombustion characteristics of MSW and biomass briquette were rarely studied. Moreover, the particle size of fuels has vital effects on the internal heating rate and volatile release rate of particles and, thus, affects the whole combustion or pyrolysis process.11,12 However, the particle sizes in the abovementioned literature were different, ranging from 80 to 600 μm.13−16 For example, the combustion characteristics of MSW were investigated by Guo et al.17 with a particle size less than 80 μm. Ren et al.5 studied the co-pyrolysis of MSW with biomass using thermogravimetry (TG)−Fourier transform infrared spectroscopy (FTIR) at a particle size of 100 μm. Liu et al.18 gave the co-combustion characteristics of herbaceous biomass and bituminous coal with a particle size of 200−600 μm. According to the previous studies, the particle size also had important effects on the product yield and composition in the gasification process of the MSW mixture or representative components (plastic, kitchen garbage, and wood). By minimization of the particle size of materials, the H2 and CO yields were enhanced and also with less tar as well as the ash and carbon element contents in the char.19,20 Yao and Xu12 found that the influence of particle sizes on the mass loss, mass loss rate, initial release temperature of volatiles, and burnout Received: October 18, 2016 Revised: December 2, 2016 Published: December 6, 2016 932
DOI: 10.1021/acs.energyfuels.6b02705 Energy Fuels 2017, 31, 932−940
Article
Energy & Fuels Table 1. Ultimate and Proximate Analyses of MSW and CSB Samples on a Dried Basis proximate analysis (wt %)
ultimate analysis (wt %)
sample
ash
volatile matter
fixed carbon
C
H
O
N
S
MSW CSB
6.32 17.06
76.95 64.86
16.73 18.08
40.26 44.96
5.19 5.34
37.64 41.49
1.21 0.93
0.24 0.14
of adjusted MSW were mixed together one by one and then blended with CSB at the ratio of 5:5. Six different particle sizes of samples were uniformly blended by a mechanical mixing method and stored in desiccators until they were used. The 10 ± 0.5 mg samples were weighed for tests by the coning quartering method before the experiment began. 2.2. Experimental Method. The non-isothermal combustion experiments were conducted in a SETARAM thermogravimetric and simultaneous thermal analyzer Setsys Evo TG−DSC/DTA. The furnace is characterized by a vertical structure, to prevent the pollution coming from residual volatile matter. The alumina crucible is suspended in the center of the furnace chamber to avoid thermogravimetric error, resulting from a change in sample gravity position. Inlet gas that entered the chamber from the top of the furnace body could completely surround the samples in the crucible, which is an important point in the process of combustion. The temperature precision and microbalance sensitivity of this thermal analyzer is ±0.3 °C and ±0.023 μg, respectively. The mixture samples were heated in a crucible from ambient temperature up to 1000 °C at a heating rate of 10, 20, and 40 °C/min. The flow rate of 80N2/20O2 mixed gas with an oxygen mole fraction of 20% was maintained at 60 mL/min. To compare the effects of the atmosphere type on the particle size, some experiments under an 80CO2/20O2 atmosphere at the same oxygen concentration were also carried out at a heating rate of 20 °C/min. Blank experiments without samples were performed to eliminate the systematic errors caused by the buoyancy effect, temperature, and weight of the crucible. All of the tests were replicated at least twice to reduce test errors, and the results of reproducibility were quite good. 2.3. Kinetic Theory. The co-combustion process of MSW and CSB is generally considered as a typical solid heterogeneous reaction. According to the fundamental rate equation (Arrhenius equation), the reaction could be described by the following equation:24,25
temperature of corn cob in the pyrolysis process was complex. However, previous studies did not provide the effect of the particle size on the co-combustion characteristics and kinetics of MSW and biomass briquette by TGA. Therefore, the reference role of existing studies for the co-combustion equipment of MSW and biomass briquette was insufficient. The oxy-fuel combustion technology with the replacement of N2 by CO2 significantly increases the partial pressure of CO2 in the exhaust gases, which has been a clean and promising combustion technique in realizing the carbon capture and the reduction of NOx emissions.21 Duan et al.22 also found that the SO2 emission first increased and then decreased as the O2 fraction increased in the CO2/O2 combustion. The combustion behavior of materials under a CO2/O2 atmosphere is different from that of conventional air combustion, with the negative effects on the ignition, burnout, and other characteristics.23 Although some researchers have investigated the oxy-fuel combustion technology of MSW or raw biomass, little studies have rarely been published regarding the oxy-fuel cocombustion characteristics of MSW and biomass briquette, especially on the effect of the particle size under the CO2/O2 atmosphere. In this work, TGA was used to investigate the co-combustion characteristics and kinetics of MSW and biomass briquette at different particle sizes and heating rates. The comparison of the effect of the particle size between N2/O2 and CO2/O2 atmospheres was also evaluated. Activation energies and other kinetic parameters of the combustion process were calculated by two different kinetic methods: the Kissinger−Akahira− Sunose (KAS) and Coats−Redfern (C−R) methods. The results obtained from this work could help to optimize the parameters of MSW and biomass briquette co-combustion equipment and offer reference for the blend combustion under oxy-fuel combustion technology.
dα /dt = k(T )f (α) = A exp(− E /RT )f (α)
(1)
where α is the conversion degree of the sample, t is the time, f(α) is the reaction mechanism function, T is the reaction temperature, A is the pre-exponential factor, E is the activation energy, and R is the universal gas constant. The conversion degree of weight loss for the sample α is expressed as
2. METHODOLOGY 2.1. Raw Materials. The MSW mixture employed during the tests was collected from Hefei University of Technology according to the typical component of MSW in Hefei, China. The combustible substances, food waste, fruit peel, plastic, rubber, textile, paper, bamboo, and wood, were chosen from the MSW mixture as the test samples, with the removal of metal, glass, dust, etc. Cotton straw biomass briquette (CSB) was selected as the biomass briquette for the experiments. The CSB was produced by Anhui Haosheng Energy Technology Co., Ltd. (Anhui, China) with a compact density of 1.36 g/cm3. The results of the proximate and ultimate analyses of adjusted MSW and CSB are shown in Table 1. The proximate analysis was performed by a MAC-3000 fully auto-measuring industrial analyzer (Jiangyan Guochuang Analytical Instruments Co., Ltd., Jiangsu, China), while the ultimate analysis was carried out in a Vario El Cube elemental analyzer (Elementar, Germany). All of the components of MSW and CSB were dried at 105 °C for 12 h in a drying oven, then pulverized repeatedly, and sieved into six different size fractions:
