Determination of TOC in Ash, Slag and Filter Dust Application Note · TOC Solid Analyzer

Introduction

Fly ash or flue ash is typically a coal combustion product that is composed of fine particles that are driven out of the boilers of power plants together with the flue gases. Ash that falls to the bottom of the boiler‘s combustion chamber is called bottom ash. In modern coal-fired power plants, fly ash is generally captured by electrostatic precipitators or other particle filtration equipment before the flue gases reach the chimneys. Depending upon the source and composition of the coal being burned, the components of fly ash vary considerably, but always includes substantial amounts of silicon dioxide (SiO2), aluminum oxide (Al2O3) and calcium oxide (CaO) from the main mineral compounds in mined coal. Minor constituents are various heavy metals and soot or unburnt carbon.

In the case of waste incineration, e.g. in a waste-to-energy facility, the ash may contain higher levels of contaminants than coal ash. In that case the ash produced is often classified as hazardous waste.

The fly ash collected by emission control equipment is either dumped in landfills or recycled, e.g. used as a pozzolan to produce hydraulic cement or plaster or as a partial replacement for Portland cement in concrete production.

Clinker/ slag are terms used for residues and waste from industrial processes like incineration of fossil fuels for power and energy production or metal smelting processes. Clinker is often of a glassy look because of the formation of molten silica compounds in the processes. It is often used as cheap construction material for foodpaths or as an additive in concrete in combination with Portland cement. The parameter total organic carbon (TOC) is one of the important factors in clarifying the issue of recycling or disposal of fly ash or slag. The basics for the determination of the parameter TOC in solids are described in the DIN EN 15936 (also: EN 13137). Various techniques can be used, but all are based on a treatment of the sample with an inorganic non oxidizing acid as one step of the complete procedure. The carbonates react with the acid and form CO2, which is either quantitatively determined using the approach of the so-called indirect method (TOC = TC - TIC) or the CO2 is just eliminated from the sample with the aim to measure the TOC directly afterwards.