Abstract
Fast pyrolysis of hydrolysis lignin was studied in fluidized bed units. Hydrolysis lignin, a bioproduct from the lignocellulosic ethanol production process (St1 Cellunolix), was processed in bench scale bubbling fluidized bed (BFB) and pilot scale circulating fluidized bed (CFB) units. Utilization of steam and ethanol as hydrogen sources was tested in a BFB unit. Major technical challenges identified were related to slow reaction rates of lignin degradation and the rapid secondary reactions in the vapor phase resulting in deposit formation and pressure buildup in product gas lines. The carbohydrate content of hydrolysis lignin had a clear correlation to its processability. More challenges with clogging and bed agglomeration were observed with lignin feedstock having lower carbohydrate content. The challenges with the bed agglomeration in the BFB unit were overcome by adding a rapidly rotating mixer in the reactor to break the agglomerates. With the CFB unit, bed agglomeration was not a problem, due to high gas velocities and forces applied to sand and lignin particles. In the BFB unit, the screw feeder was cooled and no significant melting problems were observed. In the CFB unit, melting problems were avoided by feeding the raw material in the cold section of reactor. However, severe increases in the pressure buildup and deposit formation rates were observed in both units. Steam and ethanol was tested, separately, in the BFB unit, to provide excess hydrogen in the system. Based on the product analyses both added hydrogen into the system, but hydrogen ended up mostly in the gas phase. To enhance the hydrogen transfer to the organic liquid, a catalyst active in hydrogen transfer is probably needed.
Highlights
Lignin is a amorphous and branched polymer with a highly aromatic nature, consisting of three main precursors: coniferyl alcohol (G), sinapyl alcohol (S), and p-coumaryl alcohol (H)
For the carbohydrate and lignin composition, the samples were hydrolyzed with sulfuric acid at two stages and the resulting monosaccharides were determined by high-performance anion-exchange chromatography (HPAEC) with pulse amperometric detection (Dionex ICS 3000A equipped with CarboPac PA1 column).[41]
When experiments were continued with lignins C and D, severe bed agglomeration and defluidization were confronted and no stable runs could be made without the mixer
Summary
Lignin is a amorphous and branched polymer with a highly aromatic nature, consisting of three main precursors: coniferyl alcohol (G), sinapyl alcohol (S), and p-coumaryl alcohol (H). Precursors link to each other by several different carbon− carbon and carbon−oxygen bonds, such as α-O-4, β-O-4, 4-O5, β-5, and β-β, and form complex polymer structures. Lignin can bind with the plant polysaccharides adding more complexity to the system.[1,2] Native lignin in plants is, significantly different compared to the technical lignins extracted from lignocellulosic feedstocks during industrial processes. Current industrial processes are heavily carbohydrate focused, and typically the main goal is to extract high quality cellulose or cellulose derivatives. The aim is to solubilize the lignin and leave the cellulose intact, whereas the aim in ethanol production is to degrade and dissolve the cellulose and hemicellulose to fermentable sugars while lignin is collected as a solid precipitate.[3,5] Due to these fundamental differences, lignins differ chemically and structurally. The residual carbohydrate content of hydrolysis lignin is higher than in kraft lignin.[9]
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