Article pubs.acs.org/EF
Combustion and Emission Characteristics of Coal Logs with Different Binders for Transportation in a Hydraulic Pipeline Wen J. Li,† Sheng Y. Lu,*,† Ru P. Wang,‡ Cheng Q. Lin,† Xiao D. Li,† and Jian H. Yan† †
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China Hangzhou Jinjiang Group, Hangzhou, Zhejiang 310005, People’s Republic of China
‡
ABSTRACT: Cationic emulsified asphalt (CEA) and poly(vinyl alcohol) (PVA) polymer binding agents may affect the combustion characteristics and pollutant emissions of coal logs shaped for transportation in hydraulic pipelines. Therefore, these characteristics were investigated using thermal analysis as well as a lab-scale fixed-bed combustor, together with those of raw coals. Three raw coals, anthracite, bitumite, and lignite, were selected to produce coal logs. The results of thermal analysis were compared to those of raw coals. Binders made the three coals more prone to be ignited. Binders improved the combustion of coal logs made of anthracite yet worsened the combustion performance of bitumite coal logs. CEA binder ameliorated the combustion performance of coal logs made of lignite. PVA binder slightly worsened the combustion performance of coal logs made of lignite. Different binders had dissimilar effects on the pollutant emissions when burning coal logs made of different coals.
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INTRODUCTION The coal log pipeline (CLP) technology is used for transporting coal. In this concept, coal particles are first compacted into cylindrical shapes called “coal log”, and later, these coal logs are transported by a liquid carrier, usually water, through a pipeline.1 In 1990, Liu and Marrero of the University of Missouri obtained a U.S. patent about CLP technology.2 Later, Liu et al. conducted a large number of experimental studies relating to the hydraulic mechanism, the design of a transportation pipeline, the compaction of coal logs, and wear tests of coal logs.3 In comparison to conventional coal transportation techniques, the CLP technique shows several advantages, such as less pollution, less energy consumption, larger throughput, less weather dependence, and lower coal transportation cost.3 CLP technology has achieved that coal logs are successfully transported: a CLP pilot plant was built in 2000.1 However, some practical problems about the subsequent use of coal logs still need discussion and investigation, such as the effects of binder addition on coal properties, the dewatering and crushing of coal logs after transportation, and the combustion and emission characteristics of coal logs. The main use of coal in China is combustion. Power generation accounted for more than 60% of total coal consumption, in 2012.4 Whether the combustion and emission characteristics of coal logs change will become a public concern when developing CLP technology in China. CLP requires water-resistant and wear-resistant coal logs.5 A successful method of making coal logs is mixing binders and pulverized coal and then compacting the mixture into coal logs. The binders used should have adequate adhesion and hydrophobicity. Emulsified asphalt and polymer binders meet these requirements.3,6,7 Emulsified asphalt and polymer binders are organic binders, however. They could change the combustion and emission characteristics of coal logs after they are added to coal logs. Altun et al. studied the combustion of lignite briquettes, adding several organic binders (carboxyl methyl cellulose, Peridur © 2016 American Chemical Society
XC3, and Peridur C10). Peridur XC3 and Peridur C10 were two types of Peridur binders, which were formed by chemical deformation of natural cellulose polymer. They concluded that binder type and amount as well as water addition significantly affected the thermal behavior and combustion efficiency of coal briquettes.8 Zhang et al. studied the combustion of a mixture of lignite and asphalt using thermogravimetric and derivative thermogravimetric (TG−DTG) analysis and concluded that lignite with more asphalt added burned more intensely and that the combustion reactivity of lignite was reduced.9 However, the combustion and emission characteristics of biomass coal briquettes spurred much interest.10−15 Therefore, in China, such characteristics need to be further studied for coal logs that are suitable for transportation in a pipeline. This paper explored these combustion and emission characteristics of coal logs, compared these to those of raw coals, and analyzed the effects of adding different binders on the combustion and emission characteristics of coal. Therefore, thermogravimetric analysis with differential scanning calorimetry (TG−DSC) was used to analyze the combustion characteristics of coal logs with different binders. The pollutant emissions of coal logs during combustion were also investigated on a relatively large-scale tubular furnace.
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EXPERIMENTAL SECTION
Sample Preparation. Anthracite, bitumite, and lignite, which are widely used in China, are three types of raw coals classified on the basis of the coal rank level. Anthracite is a high-rank coal; lignite is a low-rank coal; and bitumite is in between. They were used as the raw materials to compact coal logs. The proximate and ultimate analyses on an air-dried basis of the three coals were shown in Table 1. Cationic emulsified asphalt (CEA) and poly(vinyl alcohol) (PVA) solution were selected as representative binders. Received: September 13, 2015 Revised: January 26, 2016 Published: January 26, 2016 1335
DOI: 10.1021/acs.energyfuels.5b02075 Energy Fuels 2016, 30, 1335−1340
Article
Energy & Fuels Table 1. Proximate and Ultimate Analyses of Raw Coals proximate analysis (wt %)
a
ultimate analysis (wt %)
coal
Mada
Aad
Vad
FCad
Cad
Had
Nad
Sad
Oad
LHVb (MJ/kg)
anthracite bitumite lignite
1.46 15.33 30.06
12.09 9.06 14.94
7.87 28.9 26.12
78.58 46.71 28.88
81.71 62.29 39.16
3.21 3.49 2.35
0.49 0.7 0.52
0.39 0.26 0.34
0.65 8.87 12.63
29.95 23.22 20.39
ad = air-dried basis. bLHV = lower heating value. was burned, producing pollutant emissions. The flue gas analyzer (MG5/Vario Plus, manufactured by Germany MRU Company) measured the pollutant emissions every 5 s. After burning was finished, the porcelain crucible was taken out and another crucible was fed again. Each experiment was repeated 3 times, and the resulting pollutant emissions were the average of the three experiments.
The diameter of one cylindrical coal log was 45 mm, and the length of it was 1.5 times the diameter. When coal logs were produced, raw coal was ground to pulverized coal and then mixed evenly with CEA or PVA binder, according to the optimal mixing ratios in Table 2. Then,
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Table 2. Optimal Mixing Ratios of Coals with Different Binders sample number
sample
mass ratio
1 2 3 4 5 6
anthracite/PVA anthracite/CEA bitumite/CEA bitumite/PVA lignite/CEA lignite/PVA
1:0.03 1:0.03 1:0.1 1:0.08 1:0.2 1:0.15
RESULTS AND DISCUSSION Component Analysis of Coal Logs. The proximate and ultimate analysis results of coal logs are presented in Table 3. In comparison to Table 1, adding binders somewhat changes the composition of anthracite, bitumite, and lignite. The moisture, ash, and volatile matter contents of anthracite increase after adding binders, while the fixed carbon content decreases. For bitumite, binders raise the ash content and decrease the volatile matter content. The fixed carbon content grows after adding CEA and shrinks after adding PVA. For lignite, binders diminish the ash and moisture contents and inflate the contents of volatile matter and fixed carbon. Adding PVA reduces the lower heating values (LHVs) of coals, while CEA increases their LHVs. The effect of PVA is related to its decomposition when the temperature attains >200 °C. It affects the testing of LHV. When the LHV of CEA is higher than the LHV of coal, the LHV of their mixture will be raised. The contents of nitrogen and sulfur surge by adding binders, maybe inflating the emission of SO2 and NOx during combustion. CEA is based on petroleum pitch, high in nitrogen and sulfur.16 PVA is a polymer of carbon, hydrogen, and oxygen with chemical formula [CH2CH(OH)]n. The increase of N and S indicates that PVA is mixed with impurities during generation. This affects the properties of coal logs. Combustion Characteristics of Coal Logs. The TG, DTG, and DSC profiles of coal logs and raw coals are shown in Figures 2−4. The downward peaks on DSC curves represent exothermic processes. The standard error (SE) of three experiments of one sample is also shown in Table 5. Figures 2−4 show the combustion processes of coal logs and raw coals. Moisture evaporates from room temperature to 100 °C.17 Because the samples were dried already before the thermal analysis, their masses of samples change little in this temperature range. Then, the mass increases a little as a result
the mixture was compacted by a hydraulic press, providing a pressure of 138−150 MPa on each coal log. A paper about the compaction experiments of coal logs and how to determine the mixing ratios in Table 2 was being prepared for publication. Thus, six types of coal logs were produced: the coal log made of anthracite with CEA, the coal log made of anthracite with PVA, the coal log made of bitumite with CEA, the coal log made of bitumite with PVA, the coal log made of lignite with CEA, and the coal log made of lignite with PVA. Anthracite, bitumite, and lignite without pressure treatment were crushed down to the particle size of
