Samenvatting

Limestone is a sedimentary rock that is composed mostly of calcium carbonate (CaCO3). Limestone is, among many applications, used as a raw material for the manufacture of quicklime. When limestone is heated, the stone calcinates and quicklime is formed. The primary use of quicklime is in the steel and building industries.
For quicklime production, process temperatures above 1000oC are required. Nowadays, to achieve this process temperature, fossil fuels such as coal, natural gas and oil are the main sources of energy. The disadvantage of using fossil fuels is the amount of CO2 that is released into the atmosphere. Beside the CO2 emissions due to burning fossil fuels, CO2 is also a product of limestone decomposition, which is released during the heating process. To reduce CO2 emissions, converting the process from using fossil fuels to biofuels is a promising possibility.
In existing kilns for quicklime production, fuel and combustion air are in direct contact with the stone. That means ash-forming elements in the fuel could possibly affect both the process and the product quality. The purpose of this study is to analyse how different ashes from the biomasses wheat straw, logging residues and DDGS (Distillers Dried Grain with Solubles) affect the product quality of the quicklime. Each biomass tested has a different ash composition, representing groups of biofuels with high potassium and silicon content, high calcium content, and high phosphorus content, respectively. These three biomasses are all from residual streams, which means that they don’t contribute to deforestation and food competition.
In this study, a tube furnace was used to simulate the conditions in an industrial rotary kiln. In the tube furnace, limestone was exposed to the ash from the biomasses for 25 minutes at two different temperatures, 1100oC and 1350oC. After the exposure, the quicklime samples were analysed with a SEM (Scanning Electron Microscope) with an EDS (Energy Dispersive Spectrometer). The ash infiltration, changes in microstructure and reactions from ash-forming elements in the exposed material were analysed. Besides the elemental analyses, changes in the microstructures were analysed with help of the programmes ImageJ and Matlab.
In all the samples, the ash of the biomass infiltrated in the quicklime. The wheat straw samples had the most infiltration, followed by the DDGS. The forest residue samples had the least ash infiltration. In both the wheat straw and forest residues, K, Ca and Si is found, possibly in the form of a K-Ca-silicate. The DDGS samples proved that the calcium and phosphorus are attracted to each other. Beside phosphorus, potassium had also infiltrated.
To analyse the microstructure in Matlab, binary images were provided with ImageJ. For all binary pictures, the functions ‘Smooth’ and ‘Auto Local Threshold; Otsu; Radius 20’ were used. The microstructure analyses verified that ash from the biomass affects the microstructure of the quicklime at the interface. In comparison with the reference samples, the quicklime samples exposed to biomass ash had a higher porosity at the interface. Between the samples exposed to the biomass ash, there is no visible difference between the 1100oC and the 1350oC quicklime samples. This is in contrast to the reference samples, whose structures differs significantly between the samples exposed at 1100oC and 1350oC.
A proposal which biomass should be the best option to substitute coal as fuel in the heating process of the quicklime manufacturing, can’t be made. For that more analyses are needed. Nevertheless, it can be concluded that all tested biomasses affect the microstructure at the interface of the quicklime samples, and the ash of the biomasses infiltrates at the interface of the quicklime and interactions between elements occurs.