Article pubs.acs.org/EF
Ash Melting Behavior during Co-gasification of Biomass and Polyethylene Tianhua Yang,† Jing Ma,† Rundong Li,*,† Xingping Kai,† Fei Liu,† Yang Sun,† and Lianjun Pei‡ †
Liaoning Key Laboratory of Clean Energy and School of Energy and Environment, Shenyang Aerospace University, Shenyang 110136, China ‡ Tianjin Energy Investment Holding Co., Ltd., Tianjin 300100, China ABSTRACT: In the present study, cogasification of straw and polyethylene was investigated in a fixed-bed gasification system. The fusion characteristics of ash during cogasification of two different kind of straw and polyethylene were investigated by AFT (Ash Fusion Test), XRD (X-ray diffraction), XRF (X-ray fluorescence), and SEM-EDX (scanning electron microscopy and energy dispersive X-ray). The results showed that the addition of polyethylene had little influence on the mineral species in the gasified ash and only affected the deformation temperature (DT) of gasified ash. The degree of disintegration of gasified ash was positively correlated to the amount of polyethylene, which was beneficial for the gasification process. The changes in relative contents of acidic oxides (SiO2+P2O5+Al2O3+SO3) and basic oxides (Na2O+K2O+CaO+MgO+Fe2O3) showed a consistent trend in influencing DT of the gasified ashes of two kinds of mixtures while showed different trends in influencing the softening temperatures (ST). PE addition reduces the amount of KCl formed in gasification of RS. tion.15 Dong et al.16 demonstrated that it was highly likely that addition of PE affected the formation and characteristics of ash, when mixed with the biomass during gasification. However, little work has been done on the melting behavior of gasification ash of biomass and PE, which need to be investigated thoroughly. In this study, the behavior of ash during cogasification of biomass with PE was explored, focusing particularly on the influence of different quantities of PE in mixtures on the fusion characteristics of gasified ash. The fixed-bed gasification system was applied to study the deposition behavior of the cogasified ash of PE with rice straw or cornstalk. The fusion characteristics of the cogasified ash of biomass with different proportions of PE were investigated by XRF (X-ray fluorescence), AFT (ash fusion test), SEM-EDX (scanning electron miscroscopy energy dispersive X-ray), and XRD (X-ray diffraction).
1. INTRODUCTION As an organic fuel available in abundance, biomass is clean and renewable, and the development and utilization of biomass energy is increasingly valued worldwide.1 Gasification of biomass is an advanced conversion mode for biomass energy. During gasification, the solid fuel could be converted to gaseous mixtures effectively. However, this process results in a series of problems because of a low content of hydrogen in the biomass, high CO2 content, and low calorific value of produced gas, all of which limit the development of gasification technology for biomass. PE (polyethylene) plastic forms a large part of household garbage and has a high content of H2 along with a high calorific value. It also produces a low content of CO in its gasified product.2,3 The quality of syngas is improved upon biomass cogasification with PE.4,5 However, a high concentration of alkali metals (K, Na) in some biomass results in the fouling of the heating surface and furnace slag or agglomeration of bed materials during heating.6−9 Slagging and fouling not only reduce heat transfer but also may be cause corrosion and erosion problems, which reduce the lifetime of the equipment and increase the probability of shutdown for maintenance and cleaning and affect the products of biomass gasification.10 Fryda et al.11,12 explored the sintering properties of three different biomass fuels distributed in the south Mediterranean in a fluidized bed gasifier and found that rich potassium (K) content in bulrush and sugar cane could easily result in serious slagging when the bed material was made of quartz sand or olivine and the bed temperature was above 800 °C. Lang et al.13 believed that the ash became adhesive and agglomerated when the biomass fuels (rice straw and wheat straw) are burnt with a bed temperature above 700 °C. When the bed temperature is lower than 400 °C, alkali metals are quickly precipitated in KCl and K2SO4 due to pyrolysis.14 The unprecipitated metals remain in the biomass and react in the incinerator to generate a translucent glassy substance after high-temperature combus© 2014 American Chemical Society
2. METHODS 2.1. Materials. Two commonly available biomass variety of cornstalk (CS) and rice straw (RS), procured from the Shenyang suburb in Liaoning province were smashed, grinded and sieved with size below 0.15 mm by pulverizer (Retsch SM 2000, Germany). The polyethylene (PE) came from Rongsheng Company in Dongguan city, Guangdong province of China. The size was below 0.15 mm. The biomass powder was premixed with PE and the content of PE was 0, 40, and 80%. The fuels composition is listed in Table 1. 2.2. Experimental Equipment and Process. The schematic representation of the fixed-bed gasification system adopted in this study is shown in Figure 1. Received: October 17, 2013 Revised: March 14, 2014 Published: April 25, 2014 3096
dx.doi.org/10.1021/ef4020789 | Energy Fuels 2014, 28, 3096−3101
Energy & Fuels
Article
XRD analysis was conducted with a PW3040/60 X-ray diffractometer (Philips, Holland). The scanning angle was 10∼70° and the scanning speed was set at 2°/min. The radiation utilized was Cu Kα (1.5406 Å). SEM energy dispersive analysis was conducted with Quanta 600 scanning electron microscope equipment (FEI, U.S.A.) with an OXFORD INCA energy dispersive X-ray detector (Oxford, U.K.). XRF-1800 (SHIMADZU, Japan) was used for XRF analysis. 5E-AFIII intelligent ash fusion tester (Kaiyuan, Changsha, Hunan, China) was employed for ash fusion point test (ASTM standard D1857).
Table 1. Fuels Composition rice straw C H N S Oa Mad Aad Vad FCad SiO2 K2O CaO MgO P2O5 SO3 Cl Al2O3 Fe2O3 Na2O
cornstalk
elemental analysis wt (%) 39.77 41.44 5.53 5.31 0.82 0.84 0.24 0.14 46.16 48.23 proximate analysis wt (%)b 5.52 5.99 12.38 15.84 74.61 71.55 7.49 6.62 ash composition wt (%) 50.06 49.06 17.78 16.52 12.88 9.48 5.52 4.23 1.45 2.09 1.60 2.82 3.19 2.01 0.19 0.31 1.26 0.84 4.68 3.27
polyethylene 85.39 14.38 0.16 0.00 0.07 0.18 0.16 99.66 0.00
3. RESULTS AND DISCUSSION 3.1. Characterization of the Fusion Characteristic Temperature and Elemental Constitution of Cogasified Ash. Under a weak reducing atmosphere, the 5E-AFIII intelligent ash fusion tester was used to determine the ash fusion points of the gasified ashes of two mixtures. The heating rate was 5 °C/min. Since ash fusion temperatures were the main factor, which restricted the highest technique during thermochemical processes, the four conventional ash fusion temperatures (deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), flowing temperature (FT)) of all samples were compared in Table 2. The reproducibility of the duplicated test run was good with the standard deviation of each product
