Release characteristics of alkali metal in biomass pyrolysis under turbulent condition

As a clean and renewable energy, biomass has great significance in optimizing energy structure and reducing carbon emissions. As a large agricultural country, China has abundant biomass resources. These biomass materials have high alkali metal content relative to coal, causing the problem of slagging, corrosion and damage to boiler equipment. To investigate the alkali metal release characteristics, the pyrolysis experiments of 6 mm spherical lotus wood particles were carried out. Research was conducted in a turbulent environment from 200 ℃ to 900 ℃ using a four-fan opposed furnace. SEM−EDS, ICP−OES, IC and other detection methods were adopted to investigate the alkali metal accumulation, release and transformation mechanisms. The results showed that Na accounted for 86% of the alkali metal content in the wood, with water-soluble Na dominating, followed by Ca, K, and Mg. During the pyrolysis process, NaNO₃ was released firstly and reached the maximum release at 400 ℃. With the temperature further increasing, NaCl and Na₂CO₃ were released, and the release peaks were positively correlated with their melting points. The ammonia-soluble Na would be converted to water-soluble Na during the release of water-soluble Na. The total Na release proportion were 58.25%, 65.15% and 71.13% at 600 ℃, 800 ℃ and 900 ℃, respectively. Comparing the Na release under natural convection and turbulent pyrolysis, it was found that the Na release rates under turbulent pyrolysis were always higher than those under natural convection. It was attributed to the fact that the faster particle heating and significant increase in porosity under turbulent pyrolysis conditions. However, the final release rate of Na in both turbulent and natural convective environments at the same temperature exhibited no significant difference, indicating reaction temperature is the determining factor for the final release rate of alkali metals.