Influence of modified biomass char on releases characteristics of volatiles during pyrolysis of cotton stalk

Abstract: The biomass char has an important influence on the release of volatile during the fast pyrolysis processes due to its complex structure features and the presence of inorganic minerals. Furthermore, as inherent catalysts, the large content of salts in biomass promotes the char to react with volatile. Thus this study aims to investigate the influence of char and inorganic salts on the volatile-char interactions. Experiments were conducted on a two-stage system with the same temperature of 500 ℃. Char pretreatments i.e. acid washing and salts (NaCl, KCl, MgCl2 and FeCl3) impregnating were introduced. Volatile derived from cotton stalk pyrolysis got reaction under the presence of fresh char with different mass. The condensed bio-oil obtained from the pyrolysis with and without the catalysts (cotton stalk char) was characterized by the GC-MS (gas chromatograph - mass spectrometer) analyses. The non-condensed pyrolysis gas via the filter and drier was characterized by the GC. The results indicated that: with the increasing of the mass of cotton stalk char, its ability of promoting the secondary cracking of pyrolysis volatile became stronger. As a result, the liquid bio-oil yield decreased from 56.70% to 51.32%, while the yield of small molecular gases continued to increase from 17.88% to 22.13%. When the dosage of the cotton stalk char was 2.0 g, the highest gas yield of 22.1% was obtained. When the cotton stalk char was added in the pyrolysis, the acids, aldehydes and lipid content declined sharply. It is deduced that the cotton stalk char plays an inhibiting effect in the formation of them. At the same time, this inhibiting effect is conducive to the formation of more phenols. After deliming, semi-coke group structure will weaken the inhibition of the formation of acids in the volatile. Likewise, the capacity of phenolic enrichment is also reduced. The cotton stalk loaded with metal chloride enhanced the semi-coke's effects on the volatile catalytic cracking and restructuring, generating the gas with smaller molecule. When loaded with KCl, the capacity of cotton stalk char to the secondary catalysis of volatile was enhanced. Compared with the cotton stalk char with pickling, the gas yield was increased under the pyrolysis with the fresh cotton stalk. It is deduced that the metal ions play more significant effects on the volatile reaction during the catalytic reforming. The metal ions could promote the conversion of CO2 and CH4. Meanwhile, Fe played the best promoting effect on the hydrogen yield, and the hydrogen content was up to12.96%. When the cotton stalks char were loaded with the KCl and MgCl2, the content of CO contained in the non-condensed gas was increasing sharply. In the respect of the capacity of volatile cracking, the metal ions were stronger than simply increasing the quality of cotton stalk char in the lower catalytic section. The order of metal ions to the enrichment of phenolic was depicted as follows: K+>Na+>Fe2+>Mg2+. For deliming cotton stalk char, it also benefitted the forming of the phenol, although most metal ions had been removed. It is concluded that the skeleton structure of cotton stalk char plays an effect on the volatile cracking. The fresh cotton stalk inhibited the formation of furans. However, the cotton stalk char promoted the yield of furans to increase, reaching 14.26%. Picking and loading with different metal ions played a different role in the yield of bio-oil. The catalysis and the structure both worked in the pyrolysis. The presence of metal ions also promoted the decomposition of acids, esters and aldehydes.