Acta Chim. Slov. 2003, 50, 789-798. 789 CHARACTERISATION OF CARBONIZATE PRODUCED FROM VELENJE LIGNITE IN LAB-SCALE REACTOR Alenka Zapušek* ERICo Velenje, Institute for Ecological Research, Koroška 58, 3320 Velenje, Slovenia Christian Wirtgen, Joerg Weigandt Aachen University, Coking, Briauetting and Thermal JVaste Treatment Group, JVuellnerstr. 2, 52056 Aachen, Germany Franci Lenart Karbon, d.o.o., Partizanska 78, 3320 Velenje, Slovenia Received 21-08-2003 Abstract In year 2002 laboratory carbonization of lignite samples was done in a rotary kiln reactor at five different temperatures (550, 600, 650, 700, 750°C), four different retention times (15, 30, 60 and 120 min) and three different particle size categories (4-10, 10-20 and 20-40 mm). Later on specific surface determination of these carbonizates were carried out, because use of active carbon in industrial and chemical application may be of great importance. From the results obtained, carbonization temperature seems to be the most important factor that influences the enlargement of specific surface area. Specific surface area of raw coal is 88 m2/g and in the process of carbonization becomes enlarged almost by factor 3. The same situation (enlargement of the surface area) can be observed with enlargement of particle size. Results also showed us that retention tirne of 60 min is the most favorable for enlargement of the specific surface area. Introduction Lignite, known as a high volatile coal is excavated in Velenje Coal Mine and used in Thermo Power Plant Šoštanj for electricity production. At coal combustion large amounts of gases and dust are emitted into the environment, which can cause harmful effects. Following the desire for the reduction of these impacts Velenje Coal Mine has joined the world trends in the development and application of technologies aimed at the reduction of the negative effects of production and consumption of coal. Due to lignite’s low energv content, it is possible to make lignite economicallv and ecologicallv viable with its transformation to carbonizate in rotarv kiln reactors. The term “carbonization” means to convert to carbon, which is achieved by pyrolysis in the mentioned process, in A. Zapušek, C. Wirtgen, J. Weigandt, F. Lenart: Characterisation of Carbonizate Produced From... 790 Acta Chim. Slov. 2003, 50, 789-798. which thermal depolymerization of macromolecules, like cellulose, semicellulose and lignin, occur in the absence of oxygen. A solid, black residue (carbonizate), noncondensable gases and a liquid/oily phase are produced. The carbonizate and the carbonizate briquettes can be used for household cooking and household and industrial heating. Moreover, the carbonizate can be used as active carbon in industrial and chemical applications, e.g. gas cleaning, waste and drinking water treatment and as reducing agent in metallurgical processes. Coals and their products are highly porous materials with most of their surface area enclosed in pores with diameter of less than 50 nm. Coal porosity plays a key role in practically ali aspects of its utilization: diffusion of methane and other gases from coal seams, gasification, production of metallurgical čoke, activated carbon, and carbon molecular sieves. ' Thermal decomposition of carbonaceous material followed by activation with chemicals, steam or carbon dioxide at elevated temperatures are usual procedures for activated carbon production. The majority of the volatiles are removed in the form of gaseous products and a carbonized intermediate product is obtained. The activation process involves essentially the removal of tarry carbonization products formed during pyrolysis from the spaces between elementary microcrystallites, thereby opening the pores. The very large porosity provides a large surface area, which results in exceptional adsorptive properties. Therefore, activated carbons can be used as adsorbents, filters, catalyst supports, reactants and pigments and in numerous other applications. Adsorption on porous carbons was described as early as 1550 b.c. in an ancient Egyptian papyrus and later by Hippocrates and Pliny the Elder, mainly for medical purposes. The adsorptive properties of charcoal were first observed near the end of the 18* century. It was discovered that charcoal was capable of decolorizing certain liquids. This discovery led to the first industrial use of charcoal in an English sugar refinery in 1794. Since 1938, when the BET (Brunauer-Emmet-Teller) equation was developed, surface area of porous adsorbents has been determined on a routine basis from nitrogen adsorption isotherms measured at 77 K. But later the observations showed that adsorption of N2 on coals and their products at 77 K involves activated diffusion into the internal pore structure, so it is generally accepted that this method does not measure the A. Zapušek, C. Wirtgen, J. Weigandt, F. Lenart: Characterisation of Carbonizate Produced From... Acta Chim. Slov. 2003, 50, 789-798. 791 total surface area of coals and their products. Surface areas of coals should be measured by adsorption of gases that have small molecular size and high critical temperature. CO2 meets these criteria: it is a small molecule (0.33 nm compared to 0.365 nm for N2 molecule), and has a critical temperature of 304.5 K. CO2 adsorption can be used for determination of the surface area of not only anthracites, but also highly oxygenated coals, for example, lignites. ' ' Experimental The lignite samples were carbonized in a continuously running electric laboratory-scale rotary kiln reactor at five different temperatures (550, 600, 650, 700, 750 °C), using various particle size categories (4-10, 10-20 and 20-40 mm) and retention times (15, 30, 60 and 120 min). Laboratory tests were executed in the laboratory of the Coking, Briquetting and Thermal Waste Treatment Group, Aachen University. The scheme of the laboratory rotary kiln reactor is shown in Figure 1. The coal was charged from the bunker (left) into the kiln by a screw conveyor. At the end of the kiln (right) the carbonized coal was collected in a drum. The temperature of carbonization is controlled by the electrical heating muffle. Hydrogen, carbon monoxide, carbon dioxide and methane form the main part (in average 80 vol%) of the permanent gas, which is released from coal during the pyrolysis. As the temperature increases, a larger amount of each component is released from the coal. Released gases were piped into a burner where they burnt up completely after passing a cooling system containing a membrane pump with a vacuum controller (-20 mbar). Gases can be combusted with an adequate surplus of oxygen, producing mainly CO2 and H2O and in minor amounts also SO2 and NO2. The emissions of CO during good combustion are negligible. A later industrial size kiln can use the generated heat by burning the permanent gas as well as the condensable part of the pyrolysis gas because a separation of the components for analysis is not imperative. The heat will be more than sufficient to reach thermal autarkic operating conditions. Only for start up an external source of energy (diesel, gas, kerosene, etc) is necessary. The advantages of this process are the 100 % combustion of the pyrolysis gases and the oxidation of organic sulphur compounds during the process. The final product that has a significantly higher heating value does not release any smoke or A. Zapušek, C. Wirtgen, J. Weigandt, F. Lenart: Characterisation of Carbonizate Produced From... 792 Acta Chim. Slov. 2003, 50, 789-798. unpleasant odours during combustion when used as fuel for household or industrial consumption. input ^7