Composition of coal combustion by-products: The importance of combustion technology

• Fly ash and bottom ash from pulverized fuel boiler and hybrid boiler were studied.
• Ashes from both boilers differ in mineral and chemical composition.
• Combustion technology influences the composition of combustion by-products.
• Material from the bed influences the composition of ash from hybrid boiler.

The combustion by-products (fly ash and bottom ash) from a pulverized fuel boiler and hybrid boiler (pulverized fuel burners and fluidized bed) were compared, where both boilers were fired with predominantly the same coal. The mineral (phase) composition was determined using an optical and scanning electron microscopy and X-ray diffraction; the chemical composition of particles by using energy dispersive spectrometry. The chemical composition of the bulk samples was also determined. The fly ash and bottom ash from the pulverized boiler were relatively enriched in glass and mullite, while the fly ash from the hybrid boiler also contained glass and mullite, but the bottom ash from the hybrid boiler was devoid of these high-temperature phases. The difference in ash chemical composition from both boilers was also noted. Differences between the combustion products from both boilers are related to the combustion technology (mostly temperature), and partly due to the addition of bed material.