Fuelling The Future
Published by Emily Thomas,
Founded in 1963, Vassiliko Cement Works (Public Company Ltd) is one of the largest heavy industries companies in Cyprus. It serves its international customers and imports raw materials, benefitting from the Vassiliko port, which is positioned next to the plant. The company operates in the clinker and cement production sectors and manages four quarries for the extraction of raw materials used in its onsite cement production. The plant was upgraded in 2011 from 1.115 million tpy to 2 million tpy. This upgrade led to several significant efficiency improvements, primarily in terms of reduced emissions and production costs.
The Vassiliko plant has always been at the forefront of acquiring and implementing cutting-edge technologies; in 2014, new alternative fuels and raw materials feeding systems were installed. Its recent ‘green initiative’ saw the commissioning of an 8 MWp photovoltaic park in early 2020, making it one of a few plants in Europe to generate its own solar power. The Vassiliko Plant therefore, where possible, collaborates and works with its international technical associates in order to stay ahead of the curve. One such initiative has been to model its inline calciner for reducing NOx, co-fired with petcoke and a combination of alternative fuels. This article presents some results from the calciner modelling exercise spanning over five years.1 The model used was Cinar’s internally developed MI-CFD (mineral interactive computational fluid dynamics) model.2,3,4
The calciner is designed to burn a variety of fuels, including petcoke and oil, as well as certain types of alternative fuels. Approximately five seconds of total gas residence time is available within the calciner, including the time in the exit loop duct, which is sufficient to fully combust the petcoke and oxide CO. The NO formation/reduction in the calciner very much depends on the mixing of the tertiary air duct (TAD) and riser gas duct (RGD) – one of the modelling objectives. In the present design, a single ‘Venturi’ section was built at about half of the calciner’s height for enhancing the mixing of the TAD and RGD stream, as well as for accelerating the fuel burnout and calcinations of the meal particles.
In the calciner, combustion products from the kiln enter the riser duct at the bottom at a relatively higher temperature (~1200°C). Initially, there were two petcoke burners located at two elevations on the east and south sides , which were designed to be multi-channel, placed in a downward orientation with a 30° angle to the vertical. Additionally, tertiary air was supplied to the calciner via a duct connecting the calciner at the lower cone-section and the bottom of the mid-cylindrical section with a downward orientation and a 30° angle to the vertical. Two meal inlets were located above the conical section of the calciner.
The plant has been designed to comply with the EU CO and NOx emission limits. The mixing ensures lower CO emissions, while the NOx emissions are reduced through kiln-generated-NOx destruction in the calciner, as well as use of selective non-catalytic reduction (SNCR).
In summary, in order to deal with efficiency and emission targets, the process/flow and combustion interactions inside the kiln, calciner and cyclones must be analysed in detail. Generally, some of the KPIs (kiln performance indicators) are readily available (e.g., pressure, temperature and exit concentrations of gas species) but local phenomena, like near reaction-zone information which influences the variables of interest (e.g., meal calcination, CO/NO reduction/formation and fuel burnout) are difficult to quantify and require detailed analysis. The only practical and economical way forward is to perform analysis using advanced computer models, based on mineral interactive computational fluid dynamics (MI-CFD) which were used at various stages of plant improvement phases.
In most in-line calciners, kiln-generated CO and NOx are reduced more than 70%, provided that favourable conditions of calciner fuel, kiln combustion products and tertiary air mixing are maintained. Through modelling, one can identify all important air-fuel mixing and calcination reaction zones which are practically impossible to access, and can relocate fuel injection locations with respect to meal inlets. This approach prevents higher CAPEX calciner modifications frequently implemented during the calciner upgrade, in which gas residence time is increased – an inefficient and expensive option, as reactions away from the fuel burning zone (i.e., of the order of 1 – 2 m) are extremely slow and hardly increase burnout and calcination beyond 1 – 2%.
The length of the RGD prior to the entry location of TAD air, TAD air inlet angle and velocities are the main parameters influencing calciner mixing and reaction zones in the Vasilliko calciner. In each calciner, fuel burnout and calcination levels are influenced by the ignition of fuel-released volatiles and suppression of temperature peaks through the calcination of meal particles.
All of these are accomplished within a few milliseconds of the onset of ignition. Therefore, allowing a higher-than-necessary ignition delay or inhibition of combustion reaction through earlier mixing of the meal particles, results in lower fuel burnout. Once the fuel burnout decreases below 95%, an increase in the volatile cycles is observed, causing build-ups, leading to more air blasting and sometimes even resulting in plant stoppages.
The early mixing of tertiary air reduces any kiln-formed CO but increases the NOx formation. The TAD connection with the RGD is inclined at a 30° angle to the vertical, which diverts a portion of the tertiary air downward, thereby reducing the residence time of fuel volatiles within fuel-rich and oxygen deficient sections; these are necessary conditions for kiln-generated-NOx destruction, via the CHi radicals reactions. We can also note the diffusion of unreacted volatiles into the sheer layer between the tertiary air and riser gases, transporting the unreacted nitrogenous species. These conditions are found to be unfavourable for reducing NOx emissions but are able to oxide CO. In order to reduce NO, it was necessary to reduce the downward transgression of the tertiary air stream and also to complete the consumption of petcoke volatiles earlier and prior to tertiary air inlet.
The first and the most cost-effective approach was to spread the fuel more within the KG and thus increase the mixing of the fuel-released volatiles with NO from the kiln (increasing their destruction potential). This succeeds the combustion of the petcoke volatiles within the near burner region and reduces their upward flow where more oxygen is available in the vicinity of the tertiary air inlet. Two MI-CFD simulations with a combination of three and four burner combinations were carried out, and the results showed a 38 to 51% reduction in NOx and an increase of petcoke burnout from 83 to 99.3% and 99.99%, respectively. The four-burner combination was considered to be the optimum solution for 100% petcoke firing conditions, as it was achieving the optimum mixing of the fuel within the KGR.
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1. SKOURIDES I., PETROU S., AKRITOPOULOS M. P. and ABBAS T., ‘Calciner optimisation for reducing NOx and increasing AF substitution rate’, Proceedings of 14th Global Cemfuels, Cyprus, 19 – 20 February, 2020.
2. AKHTAR, S.S., GOETZ, J., ABBAS, T, and KANDAMBY, N.H., ‘A Calciner at its Best’, Presented at 61st Cement Industry Technical Conference, IEEE-IAS/PCA, St. Louis, Missouri, April 28 – May 2, 2019.
3. AKRITOPOULOS M. P. and ABBAS T., ‘Calciner Challenges’, World Cement, November 2018, pp. 35 – 42.
4. ABBAS, T., AKRITOPOULOS, M. and AKHTAR, S.S., ‘Adapting Calciners Operating under CO2 enrichment for CCS’, Proceeding of 59th Cement Industry Technical Conference, IEEE-IAS/PCA, Calgary, May 21 – 25, 2017.
Read the article online at: https://www.worldcement.com/special-reports/07072020/fuelling-the-future/
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