Alkali Salt Ash Formation During Black Liquor Combustion at Kraft Recovery Boilers

Abstract

Recovery boiler is an essential part of paper pulping process, where waste sludge called black liquor is burned for chemical recovery and energy production. This study was carried out at an operating industrial recovery boiler in Finland.

Measurement of aerosol particles was carried out at bullnose level of furnace, at boiler exit, and at outlet of electrostatic precipitator (ESP). Aerosol mass size distributions in size range 0.02–50 pm were measured with Berner type low pressure impactor (BLPI) operated with precyclone. BLPI samples were further analysed with ion chromatography for water soluble Na, K, SO₄, and Cl. Particle morphology was studied with scanning electron microscopy (SEM). Phase composition of crystalline salts was measured with X-ray diffraction (XRD). Particles larger than 1 ttm were analysed with computer controlled scanning electron microscopy (CCSEM) to derive particle composition classes.

At ESP inlet mass size distribution was bimodal with a major mode at about 1.2 tun and a minor mode at about 5 µm (aerodynamic diameter). At ESP outlet the mass size distribution showed only one peak at about 1.2 µm. Both submicron and supermicron particles were agglomerates formed from 0.3 to 0.5 µm spherical primary particles.

XRD analyses indicated that particles were crystalline with two phases of Na₂SO₄ (thenardite and sodium sulphate) and K₃Na(SO₄)₂. CCSEM results of individual particles larger than 1 pm showed that 79 to 88 volume percent of particles contained mainly Na and S, 7 to 10 volume percent Na, K, and S with minor amount of particles containing Na, S, and Ca.