The catalytic effect of inherent and adsorbed metals on the fast/flash pyrolysis of biomass: A review

Abstract

Relevant literature which deals with the presence of metals during the catalytic pyrolysis of biomass is reviewed. Only those conditions where the metal was in intimate contact with the biomass components are included. Cellulose, hemicellulose and lignin all fuse during the early stages of pyrolytic heating and the dehydration and decomposition processes begin during this transition. Hemicelluloses such as xylan are more labile and difficult to isolate, whereas both cellulose and lignin produce mostly bio-oils when demineralised and flash pyrolysed. The dominant primary products from 'pure' cellulose are anhydrosugars as well as smaller oxygenates. Lignin gives aromatics based on the syringol and guiacol molecules. The alkali and alkaline earth metals are found to curtail the yield of bio-oil and modify product distribution, even in the low concentrations naturally found in biomass. All other metals act to catalyse cross-linking reactions, with the nickel and zinc most studied. The electropositivity of the metal tends to correlate with the extent of catalytic activity. The presence of metals causes little change to the production of gases, but more char is formed at the expense of liquids. If the aim of pyrolysis is bio-oil for fuel, prewashing of biomass to remove metals is beneficial.

Effect of Metal Chlorides on the Pyrolysis of Wheat Straw

“…e chlorides of manganese, iron, nickel, cobalt, and copper increase significantly the formation of carbon-containing residue and decrease the pyrolysis temperature [25,26]. Moreover, the acid character of the transition metal chlorides significantly accelerates the pyrolysis reactions, leading to the changes in the gaseous product composition [27,28]. It should be noted that the data on the effect of these compounds are available for the individual components of plant biomass (hemicellulose, cellulose, and lignin).…”

Solar pyrolysis of heavy metal contaminated biomass for gas fuel production

“…Indeed, copper and nickel are used as efficient catalysts for tar decomposition reactions for producing H 2 and CO [26,37]. As found by other researchers, copper and nickel must be in the metal forms (Cu 0 and Ni 0 ) for resulting in a catalytic effect on biomass pyrolysis reactions [22]. At low temperature, sulfur and oxygen have a high affinity with Ni and Cu, respectively [36].…”

“…For nickel impregnated willow pyrolysis, the char yield decreases from 21% to 9.9% with temperature rising from 600 to 1600 C. According to Ref. [22] nickel could promote CeH and CeO bonds cleavage of char at high temperatures, thereby reducing the yield of char. The impact of heavy metal on pyrolysis liquid yield is two steps.…”

“…The impact of heavy metal on pyrolysis liquid yield is two steps. The copper and nickel firstly promote the depolymerization of cellulose and hemicellulose resulting in more levoglucosan formation [22]. Furthermore, they substantially catalyze the secondary reactions of levoglucosan when the pyrolysis temperature is high enough.…”

“…Besides, temperature and heating rate are known to affect HMs transformation during conventional pyrolysis of contaminated biomass [3]. Many heavy metals such as Cu and Ni have catalytic effect on biomass pyrolysis reactions [22]. In HMs contaminated biomass, HMs can work as in-situ catalysts during pyrolysis for enhancing products quality and value [23].…”