Abstract
Abstract: Smooth cordgrass (Spartina alterniflora), a saltmarsh plant, has spread in intertidal flats of many regions of China since it was introduced from the USA in 1979. The application of S. alternilfora in energy has gained more attention due to its high production. However, the direct combustion of S. alternilfora was hindered due to its high potassium (K) and sodium (Na) contents. Co-pyrolysis of biomass and coal, a subject of much study in an effort to reduce greenhouse gases emission, was reported to be able to produce a synergetic effect mainly due to the catalytic function of alkali metals in biomass. S. alterniflora, rich in Na and K which are 22 683 mg/kg and 8 063 mg/kg, respectively, has great bioenergy potential as a co-pyrolysis material of coal. In order to to verify the interaction of S. alterniflora and lignite during pyrolysis, experiments were carried out with pure S. alterniflora, pure lignite, and their blends with mass ratio (S. alterniflora to lignite, S:L) of 1:4, 2:3, 3:2, and 4:1 by thermogravimetry coupled with a Fourier transform infrared spectroscopy (TG-FTIR).S. alternilfora used in the experiments was collected from Dafeng County of Jiangsu Province, China in October 2012. Lignite was from Shanxi Province, China. Na, K, volatile, H/C, O/C, and heating value of S. alterniflora were 16 064.3 mg/kg, 6 175.7 mg/kg, 75.40%, 0.12, 0.80, and 19.08 MJ/kg, respectively. Volatile content, H/C, O/C, and heating value of lignite were 33.92%, 0.07, 0.23, and 20.47 MJ/kg, respectively. TG tests were done under an N2 flow rate of 25 mL/min and at a heating rate of 10℃/min from 30℃ to 900℃. Infrared scanning resolution was set to 4cm-1, and scanning scope varied from 4 000 cm-1 to 500 cm-1.According to TG and DTG analysis, the process of co-pyrolysis can be divided into two stages at 385℃. The pyrolysis of S. alterniflora took place mainly in the first stage of 250℃ to 385℃. The pyrolysis of lignite and fixed carbon in S. alterniflora occurred in the second stage. TG analysis results showed that the activation energy (Ea) for co-pyrolysis decreased with the increase of S. alterniflora in the blends in the range of 385℃ to 510℃, especially for the blend with S:L of 4:1, whose Ea decreased to 13.34 kJ/mol compared to the 53.62 kJ/mol of pure lignite pyrolysis. At the same time, the reaction rate constant k for co-pyrolysis increased by one to two orders of magnitude compared to pyrolysis of lignite alone, although the frequency factor A of co-pyrolysis decreased. After heating the temperature over 385℃, obvious differences occurred between the calculated values and the test values of TG and DTG. This situation continued to 700℃. FTIR analysis of pyrolysis gas showed that co-pyrolysis improved the quality of pyrolysis gas by enhancing the yields of CO and CH4, especially for two blend samples with higher S. alterniflora content in which there were significant CO releasing peaks around 400℃. On the contrary, for the pyrolysis of S. alterniflora or lignite, no obvious CO releasing peak occurred. Nonetheless, FTIR results presented that co-pyrolysis promoted the production of acetic acid, especially for the blends with higher S. alterniflora content. In conclusion, co-pyrolysis of S. alterniflora and lignite can produce a synergetic effect by promoting the production of CH4 and CO, as well as by decreasing activation energy and increasing reaction rate constant of pyrolysis reaction. It should be emphasized that this synergetic effect is mainly reflected by the catalytic effect of S. alterniflora on lignite.
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