There is a worldwide trend in the cement industry towards the utilisation of alternative fuels. However, the variable composition of these fuels can have undesired impacts on the efficiency and process stability of a cement plant.
The reliable control of the primary combustion process (CO and O2), as well as the measurement of other relevant flue gas components (NOX, SO2 and HCl), can be achieved by means of process gas analysis at the inlet of the rotary kiln. The continuous analysis of the gas atmosphere in the kiln inlet allows an effective source of relevant process data, which can be used for a closed loop control of the process. Modern process photometers with hot extractive probe sampling and conditioning can cope with the extremely high demands at this measuring point, even when alternative fuels are involved.
Current challenges in the cement industry
The substitution of gas, oil or coal with alternative fuels has a significant impact on the process gas analysis in cement clinker production, as these fuels cause the amount of chlorides and sulfur in the flue gas to increase dramatically in comparison to traditional fuels.
Typically, the measurement of the O2, CO and NO concentrations in the gas at the kiln inlet is sufficient for process control and monitoring of clinker quality. These compounds have been commonly measured using cold-dry analysis systems.
Only rarely was the measurement of minor components (e.g. SO2 and HCl) conducted and this usually came with great difficulties due to the harsh conditions of this measurement point (900 °C – 1400 °C, up to 2000 g/m3 dust). Measuring SO2 and HCl gives an indication of the risk for the formation of salts and clogging in the preheater cyclones/riser ducts. Additionally, these measured values allow the chlorine bypass to be quickly monitored.
During sampling in these systems, the temperature of the sampled gas inevitably drops in the sample line causing acid condensation, which leads to clogging, fouling and corrosion of both the sample system and the analyser – particularly in the case of plants burning alternative fuels due to the elevated sulfur and chloride levels. In order to avoid such issues, the entire sampling line should be heated as much as possible. For these reasons, measuring at this location using traditional cold-dry sampling is hardly applicable when alternative fuels are used.
Systems based on the hot-wet measurement cope well with aggressive gas mixtures at the extreme kiln inlet conditions of cement plants. The basic idea behind the hot sampling technology is rather simple: keep all parts in contact with the gas sample well above the acid and water dew points to avoid any condensation and therefore any corrosion or salt formation. Using this technique, the kiln inlet gas is kept above acid dewpoint in the sample system and is transferred into the analyser without any further gas conditioning or drying treatment.
Benefits of the hot-wet measurement technique
Combustion gases at kiln inlet are mainly composed of O2 (2 – 4%), CO2 (12 – 15%), H2O (8 – 10%) and N2. Additionally, combustion gases at the kiln inlet sometimes contain SO2 and HCl. As condensation of these elements in the measuring chamber will lead to major damage to the analyser, most of the systems sold currently in the market for the kiln inlet apply the cold-dry sampling system – eliminating SO2 on HCl from the gas matrix in order to save the analyser from corrosion. Unfortunately, the removal of SO2 and HCl also results in removing H2O.
H2O exists in significant concentrations in the gas matrix. Eliminating it from the sampled kiln inlet gas (by means of the cooler), results in an increase in the concentration of the remaining elements, thus falsifying the measured O2, CO and NO values. The hot-wet analysis method does not require cooling, thereby allowing a more reliable and representative analysis of the kiln inlet gas and ultimately a more efficient control of the combustion process.
Chlorine (Cl) and sulfur (S)-containing compounds are converted to HCl and SO2 upon combustion. These are both very reactive compounds with a strong affinity to alkali elements. When these compounds reach the upper (and colder) stages of the preheater tower, they combine first with the alkalis, forming the corresponding chlorides and sulfates. These compounds mainly condense on the incoming meal and are sent again into the kiln zone where the non-volatile sulfates leave the system with the final clinker product. The volatile compounds, however, re-evaporate and reach the preheater tower where they condense on the meals and circulate back again. This is an equilibrium process generally called the internal cycle of circulating elements and it typically refers to Cl and S content in fuels.
Excessive input of Cl and S can cause the condensation of some chloride and/or sulfate salts on the walls of the preheater cyclones, riser ducts and dip tubes, leading to buildup formation that will eventually cause cyclone blockages or duct restrictions. Usually, buildups are automatically removed by shock air blowers, where applicable, or manually by selected and specialised plant personnel. However, this requires special protection and procedures necessary to conduct this job safely. Normally, buildup formation is avoided by controlling the S and Cl content (as well as other components) in the material entering the kiln, known as the hot meal. However, hot meal sampling is quite complex and dangerous if no special automatic hot meal samplers are in place. Alternatively, the measurement of SO2 and HCl concentrations in the kiln inlet gas matrix indirectly provides the same information, but in a continuous and much faster manner. In fact it is now believed that there is a chemical equilibrium between the Cl and S compounds contained in the hot meal and HCl and SO2 concentrations in the kiln inlet gas matrix. When this information is provided on a continuous base, the kiln operator can effectively manage buildup formations. Cold-dry based analyser systems would not be able to supply this information, as the HCl and SO2 (as well as H2O) would be absorbed in the cooling system of such instruments.
Read part 2 here.
Written by Dan Kietzer, SICK Process Automation, USA, and Siegfried Andraess, SICK MAIHAK GmbH, Germany. This article appears in the April 2014 issue of World Cement. Subscribers can view the full issue by logging in.
Read the article online at: https://www.worldcement.com/the-americas/03042014/reliable_kiln_inlet_gas_analysis_part_1_990/