The effect of future recovery boiler operation conditions on ESP performance

Alkali salt ash particles with typical concentrations of 20-30 g/Nm³ in the flue gases are efficient sulphur capturers thus preventing SO₂ emissions, but simulta-neously they are a serious particle emission problem. Eventhough an electro-static precipitator (ESP) is an efficient emission control device for fly ash, the particle capture efficiency is the poorest for particles in a size range 0.1-1 μm. Furthermore, during ESP operation the particles are depositing onto the ESP collector plates and onto electrode surfaces, where the deposited particle layer may sinter and react with the passing flue gases. Accordingly, deposits may be difficult to remove via rapping and thus ESP performance may significantly deteriorate. However, not enough detailed information on the particle size, com-position and morphology is available to solve these problems when designing the ESP to collect recovery boiler fly ash particles.

The alkali salt fly ash particles were studied during extensive field studies at two industrial recovery boilers in Finland. A detailed characterisation of the particles in four operational conditions was carried out. The particle size was measured for a number size distribution with a differential mobility analyser connected to a condensation nucleus counter (DMA/CNC) and for a mass size distribution with a Berner type low pressure impactor (BLPI). The BLPI samples were fur-ther analysed with ion chromatography (IC) for a composition size distribution. The composition of the particles was further studied with an X-ray diffraction method (XRD) and an X-Ray absorption fine structure spectroscopy (XAFS). Individual particles were collected for analyses with a scanning electron micro-scope connected with an energy dispersive spectroscopy. Simultaneously with the particle characterisation the gas composition and the boiler operation was recorded.

In the measured high sulphidity, low K and Cl boilers particle mass size distri-bution is bimodal with the major peak at about 2 μm and the minor peak at about 20 μm. The number size distribution peaks at about 0.4 μm. The particles consist mainly of Na2SO4 with minor fraction of double salt (Na,K)SO₄. The Cl is present as NaCl. According to their morphology, four particle types are detected:
- spherical single particles about 0.5 µm in size,
- small agglomerates with primary particle size about 0.5 µm,
- large agglomerates with primary particle size about 0.5 µm, and
- spherical single particles some micrometers in size.

Three first types of particles consist of Na, K, S, and O. In the spherical individ-ual micrometer sized particles in addition to previous elements Si is detected.