Shaft Kiln Research Articles

ЕФЕКТИВНІСТЬ ВИКОРИСТАННЯ НИЗЬКОКАЛОРІЙНОГО ПАЛИВА В ПРОТИТОЧНИХ ВАПНЯНО-ВИПАЛЮВАЛЬНИХ ПЕЧАХ

Highly expensive energy carriers require revision of the existing engineering approaches in heating of industrial furnaces. The paper presents the results of research into the performance of a shaft kiln for lime-stone burning, which was fired with mixtures of natural gas and a fuel gas of a low calorific value. On the basis of studying various schemes of fuel distribution between the burners, the optimal operation mode of the kiln with reduced consumption of natural gas by 30% was identified. The lime quality indicators increase when the blast furnace gas is supplied to the central burner and improve when supplied to the peripheral burners. The joint supply of blast furnace gas in the calorific value section of the natural blast furnace mixture 9.4 ÷ 35 MJ/m3 allows keeping the performance of the furnace unchanged. At the same time, the coefficient of replacing natural gas with blast furnace gas is close to unity The proposed mode of heating the furnace with a capacity of 200 t/day with a natural blast furnace mixture provides 30% natural gas savings

Decomposition mechanism of lumpy dolomite particles

Cylindrical cm-sized dolomite particles were decomposed using the two steps decomposition scheme. Several studies on dolomite decomposition have been done using µm- to mm-sized samples over the years. However, these sizes are too small for industrial kiln application and ignore the influence of important parameters, e.g. thermal conductivity, on the decomposition process. In this work, the decomposition behavior of lumpy dolomite particles in each step was investigated. Experimentally measured sample temperature and conversion profiles in steps 1 and 2 are respectively similar to those obtained for the decomposition of pure magnesite and limestone. To measure the core temperature and conversion is to essentially analyze the decomposition behavior. The thermophysical material properties of the decomposition product in each step as a function of temperature was estimated using the shrinking core model. Finite difference method was employed in solving the 1D heat equation describing the process. The reaction rate coefficient in step 1 is shown to have a weak temperature dependence, the thermal conductivity of the product, MgO∙CaCO3, slightly increases with temperature but the permeability is not dependent. In step 2, the reaction coefficient and MgO∙CaO thermal conductivity and pore diffusivity do not show a clear temperature dependence. Finally, the sensitivity of the material and transport properties on the decomposition process was studied under shaft kiln condition. Thermal conductivity is observed to have the highest influence on decomposition with a value of up to 60 % at 1400 °C and for 120 mm particles.

Numerical simulation of gas-solid heat transfer and moisture evaporation process in preheating shaft kiln for ferromagnesium pellets

The arc furnace is an important equipment in the production of manganese ferroalloys. In the smelting process, it produces high temperature flue gas above 700 K, which has high utilization value. During the pelleting process, moisture exists on the surface and inside of the pellets, which, if directly fed into the arc furnace, will lower the temperature of the furnace, affecting the production, and will also cause the pellets to burst, resulting in pressure fluctuations in the furnace and other hazards. The preheating shaft kiln reduces the moisture content of the pellet while preheating the pellet by utilizing high temperature flue gas. In this paper, through the establishment of porous media and shrinking core coupling mathematical model, to achieve the prediction of the heating and drying process of the pellets of various particle sizes, which is in good agreement with the production monitoring data. The results show that 9~11 mm pellets have the best preheating effect in terms of tail gas temperature, pellet temperature and preheating time.

Shape-changing particles for locally-resolved particle geometry in DEM simulations

Taking into account the complex shape of particles in discrete element method (DEM) simulations of large-scale granular systems is computationally demanding due to the time-consuming contact detection algorithms for polyhedral particles. In this short communication, a novel approach that locally resolves the particle shapes where needed and uses a simplified representation elsewhere, to accelerate simulations without compromising accuracy, is presented. For this purpose, a method employing a smooth transition of the particle shape representation from analytical spheres to shape-resolving polyhedra is introduced in DEM. The feasibility and correct implementation of this approach are demonstrated through simulations of hopper discharge involving spherical and dodecahedral particles from a flat bottom silo or shaft kiln. The model capabilities, in terms of accuracy as well as reduction in computational effort, are quantified for a moving bed with continuous outflow.

Effect of the swirl angle on calcium carbide gas combustion characteristics of lower burner under annular shaft kiln

In order to solve the problem of high CO concentration in the emitted flue gas after adopting the flue gas recirculation technology in the annular shaft kiln, in this paper, the combustion characteristics of calcium carbide gas in swirl burners under the annular shaft kilns were studied by numerical simulation, the influence of fuel and primary air swirl intensity on combustion characteristics were studied, and the gas temperature distribution, pressure distribution and combustion products including CO, CO2 and NO was analyzed. The results show that the fuel gas swirl intensity has a great influence on the improvement of fuel burn-out ratio. The angle of the fuel gas blade increases, the maximum temperature of the combustion flame and the fuel combustion rate increase, and the CO content at the outlet of the burner decreases. When the angle of the fuel gas blade increased from 30o to 55o, swirl number increased from 0.41 to 1.02, the CO content at the inner sleeve position decreased from 8929 mg/m3 to 2467 mg/m3. The results also show that the effect of the primary air swirl intensity on reduce CO emissions and NOx formation was not obvious.

DEM-CFD coupled simulation of limestone calcination and fuel combustion in beam type lime shaft kiln

This paper presents the development and simulation analysis of a coupled DEM-CFD model of the gas–solid heat and mass transfer process in an industrial-scale beam lime shaft kiln, which uses Computational Fluid Dynamics (CFD) to simulate the combustion of gaseous fuels and the three-dimensional transport of mass, momentum and energy in the gas, and the Discrete Element Method (DEM) to simulate the movement of limestone particles and calcination reactions in the kiln are simulated using the Discrete Element Method. A beam lime kiln uses five burner beams staggered up and down to provide heat for the decomposition of limestone particles in the kiln, and natural gas is injected as fuel from the burners on the burner beams into the limestone bed of the kiln for combustion, while cooling air enters the kiln from the bottom air cap to cool the calcined products and then enters the calcination zone as secondary air to participate in the combustion. In this paper, a 250 m3 beam lime shaft kiln in normal production is used as the research object to simulate and analyze the data of temperature, gas and solidity field and limestone decomposition rate in the kiln. The simulation results show that the overall limestone calcination temperature in the kiln is low and the temperature distribution is uneven, and the decomposition degree of limestone near the kiln wall is higher than that in the center of the kiln. It's caused by the wall effect, the particle porosity is larger near wall, and combustion gas penetration is therefore enhanced in the vicinity. Meanwhile, the simulation results are in good agreement with the actual measured data, which verifies the reasonableness of the model for application in industrial-grade lime kiln.

Optimised processes for the production of performance concrete constituents based on agricultural wastes

In most parts of the world urban populations are growing. This inevitably requires resources for construction materials, which is a challenge to achieving global climate goals. Since there is no alternative to concrete as structural material, novel approaches are required to use cement in concrete more efficiently and to use as little Portland cement clinker in the binder for concrete. With novel approaches, Africa can spearhead green urban construction technologies globally, as the continent’s carbon emissions re marginal today in comparison to the rest of the world and the potential to use local resources are enormous. The authors of the paper develop conclusions on how bio-waste can be converted to organic and mineral concrete constituents that have a potential to create rural livelihoods and urban construction materials markets. A three-step solution including a hydrothermal process, a pyrolysis step and a co-fuelling process with brick production in a vertical shaft kiln is suggested to derive organic components, various by-products as well as bricks and pozzolanic cement replacements, respectively.

Numerical investigation of gas-solid heat transfer process and parameter optimization in shaft kiln for high-purity magnesia

As the core equipment for the production of high-purity magnesia, the determination of suitable structure and operating parameters has a significant impact on its efficient production. In present work, a three-dimensional numerical model of the high-purity magnesia shaft kiln is established to study the influence of different operating and structural parameters on outlet clinker temperature and outlet exhaust air temperature. The novelty is adding a convective term to the solid-phase equation of the energy model, as well as embedding the heat transfer coefficient and a modified Ergun’s correlation, obtained by a homemade experimental setup. Production data verify the accuracy and reliability of the model. The results indicate that with all other parameters being constant, the most significant impact on the outlet temperature of clinker and exhaust air is cooling air flow rate, followed by sintering gas flow rate, height below the burners and height above the burners. Finally, a combination of parameters for the 50,000 tons shaft kiln is obtained through orthogonal experiment and optimization, which is 2622 m3/h for sintering gas flow rate, 2422 m3/h for cooling air flow rate, 7.07 m for the height above the burners, and 12.14 m for the height below the burners.

Simulation of Reacting, Moving Granular Assemblies of Thermally Thick Particles by Discrete Element Method/Computational Fluid Dynamics

AbstractThe potential and limitations of the discrete element method (DEM) coupled with computational fluid mechanics (CFD) to simulate chemically reacting, moving granular material interacting with a fluid flow are summarized. A special focus is set on thermally thick particles, which requires to resolve the intraparticle transport and reaction processes. The aspect of complex particle shape is addressed, as shape may dominate the particle behavior in densely packed granular assemblies even more than the details of contact force laws. The fluid flow in the granular assembly is assumed to be a gas. The potential of DEM/CFD will be highlighted presenting three illustrative examples: a large‐scale lime shaft kiln with intermittent operation, an industrial‐size grate firing system for the incineration of municipal waste, and a small‐scale straw pellet stove.

Emission characteristics and inventory of volatile organic compounds from the Chinese cement industry based on field measurements

Volatile organic compounds (VOCs) are major precursors of ozone (O3) and secondary organic aerosols (SOA), which degrade air quality and pose a serious risk to human health and ecological systems. Previous studies on the emission characteristics of VOCs have predominantly focused on petrochemical and solvent-using sources, while localized studies on the cement industry are scarce in China. Field measurements for four cement plants were carried out in this study to investigate the emission levels, source profiles, and secondary pollutant generation potential of 98 VOCs species emitted from rotary and shaft kilns in China. Furthermore, a species-differentiated VOCs emission inventory was compiled for the Chinese cement industry in 2019. The results demonstrated that the mass concentration of VOCs emitted from shaft kiln was more than 20-fold higher than that emitted from rotary kilns, and the alkanes was the dominant species (56%) in shaft kilns, while oxygenated VOCs (OVOCs) and halocarbons were the main species in rotary kilns. Moreover, alkenes & alkyne were the dominant contributors to ozone formation potential (OFP) in shaft kilns, whereas alkenes & alkyne and OVOCs were comparable and prominent contributors in rotary kilns. In contrast, secondary organic aerosol potential (SOAP) for the two types of kilns was dominated by aromatics. In 2019, approximately 18.18 kt VOCs were emitted from cement production and were found to be largely concentrated in the southeast and central provinces of China. Considering the influence on environmental conditions, high OFP-contributing species in cement kilns are suggested to be a priority in the pollution mitigation of O3. This study provides a new, comprehensive, and reasonable cognition of the current VOCs emissions from both rotary and shaft kilns in China, which will aid in a better understanding of VOCs emission characteristics and guide future policy-making.

Investigation on mechanical and tribological properties of magnesite reinforced aluminium 6061 composites

During the sintering process in the shaft kiln of refractory factories, a considerable quantity of materials is separated and dumped as Waste Magnesite (WM). The other superior grades, Lightly-Calcined Magnesite (LCM) and Dead-Burned Magnesite (DBM), are separated at different temperatures from the shaft kiln. The WM materials still have 8% of magnesium with some sand and dust particles in huge ranges. These materials are primarily used in the applications of medicine and fertilizers and animal feed processing and additives. This work investigates the potential of WM particles as reinforcement materials in the Al 6061 alloy matrix. In order to compare the characteristics of WM-based composite, LCM and DBM powders were also considered as reinforcement particles. The WM, LCM, and DBM particles were mixed with a 15% weight ratio to Al 6061 alloy, and composites were fabricated using the Stir Casting method. The surface morphology investigations through Scanning Electron Microscopy (SEM) revealed that these particles were well distributed and dispersed within the alloy matrix and with good interfacial adhesion. It is noted that Al 6061/15% wt. LCM composite possesses a better tensile strength than Al 6061/15% wt. WM and Al 6061/15% wt.DBM composites. The impact value produced by the WM-based composite is better than the LCM and DBM composites. The Wear Rate and Coefficient of Friction (COF) were examined through a Pin-on-Disc apparatus. Al 6061/15% wt. WM composite tested a low wear rate (9.74 × 10−6 mm3 m)−1. The results show that Al 6061/15% wt. LCM composite achieved the least COF value of 0.681 at an applied load of 0.5 Kg and a sliding distance of 2826 m. The results prove that the wastage magnesite is good enough and the most robust reinforcement material for Aluminum 6061 alloy-based composite.

Study on Gas-Solid Heat Transfer and Decomposition Reaction of Calcination Process in an Annular Shaft Kiln Based on the Finite Volume Method

As an excellent reducing agent, lime has an important role in the steel production process. Annular Shaft Kiln (ASK) has been widely used in the lime production industry for its low cost, low footprint, high chemical activity, easy construction, and easy maintenance. Due to the high temperature generated inside ASK during operation, it is hard to observe the limestone decomposition process and the field distribution in the lime kiln. The simulation analysis of temperature field, velocity field and decomposition field in the limestone calcination process by CFD provides practical guidance for future lime product quality control, ASK design and operation parameters’ control. This study is based on an ASK that was put into production. Based on the finite volume method, this paper combines the porous medium model and the shrinking core model to establish a set of mathematical models that can describe the temperature and flow field distribution inside the ASK, as well as the limestone decomposition process and the heat and mass transfer process inside the ASK. According to the feedback from the production site, the mathematical model is in good agreement with the production results.

The CFD simulation of reactive flow in parallel flow regenerative shaft kilns using porous media model

Understanding the flow pattern of the gas jets in packed beds can have considerable significance in improving reactor design and process optimization. This study researches the fuel diffusion in the radial direction and the flame length in a packed bed of a parallel flow regenerative shaft kiln. This kiln is characterized that the fuel is injected vertically in the packed bed using a lot of lances in the cross-section while the combustion air is distributed continuously. Such a large, packed bed has to be approximated as a porous media. This assumption is used to model the reactive flow in the kilns. Using a box with 700 spheres of 52 mm spheres in body-centered cube arrangement the local concentrations of injected nitrogen in air-flow were measured. The measured values match approximately with those calculated with the porous media model. The studied parameters are the number of burners and burner arrangements. The radial mixing of fuel and air in a packed bed is relatively bad. Therefore, a lot of burners are needed for better temperature homogenization in the cross-section

Evaluation and analysis of exergoeconomic performance for the calcination process of green petroleum coke in vertical shaft kiln

The main objective of this paper is to establish a mathematical framework to analyze the complex thermal economic performance of the calcination process. To find the factors affecting exergy efficiency loss, different exergy destruction is investigated in detail. Furthermore, the exergy flow cost model for exergy cost saving has also been developed. The results show that the vertical shaft furnace is a self-sufficiency equipment without additional fuel required, but the overall exergy destruction accounts for 54.11% of the total exergy input. In addition, the energy efficiency of the waste heat recovery boiler and thermal deaerator are 83.52% and 96.40%, whereas the exergy efficiency of the two equipment are 65.98% and 94.27%. Furthermore, the import exergy flow cost of vertical shaft furnace, waste heat recovery boiler and thermal deaerator are 366.5197 RMB per MJ, 0.1426 RMB per MJ, and 0.0020 RMB per MJ, respectively. Based on the result, several suggestions were proposed to improve the exergoeconomic performance. Assessing the performance of suggested improvements, the total exergy destruction of vertical shaft furnace is reduced to 134.34 GJ per hours and the exergy efficiency of waste heat recovery boiler is raised up to 66.02%. Moreover, the import exergy flow cost of the three different equipment is reduced to 0.0329 RMB per MJ, 0.1304 RMB per MJ, and 0.0002 RMB per MJ, respectively.

Dem-Cfd Coupled Simulation of Limestone Calcination and Fuel Combustion in Beam Type Lime Shaft Kiln

Dem-Cfd Coupled Simulation of Limestone Calcination and Fuel Combustion in Beam Type Lime Shaft Kiln

Field measurements on emission characteristics, chemical profiles, and emission factors of size-segregated PM from cement plants in China

Cement manufacturing is a major contributor to ambient particulate matter (PM) pollution in China, threatening urban and regional air quality improvement. Here, we tested the typical outlets (kiln tail, kiln head, and coal mill) in one shaft kiln and three rotary kilns to investigate the mass concentrations, size distributions, and chemical compositions of size-segregated PM. Results show that the concentrations of PM in most samples are lower than the strict local emission standards (10 mg/Nm3). We show that the characteristic chemical compositions in PM for most tested outlets are Ca and Ca2+, while for shaft kiln are K, S, K+, and SO42−, and organic carbon. Elemental carbon accounts for a relatively high proportion of PM emitted from coal mills. Meanwhile, unstable and abnormal operating conditions and variations on feed coal compositions will cause high levels of NH4+ and Cl− in PM from the kiln tail/head. Besides, the emission factors (EFs) of PM2.5, PM10, and PM after air pollution control devices for typical outlets of cement plants are calculated, which fall in ranges of 0.16–2.48, 1.49–18.46, and 3.32–35.35 g/(t of clinker), respectively. It suggests that mass emission characteristics, source profiles, and EFs of PM have changed notably as emission standards become more stringent. We believe the newly detailed size-segregated PM EFs and chemical profiles will help update and compile the refined emission inventory for current cement production in China.

Determination of Key Parameters Required to Optimize Calcination Process in Ferrous Metallurgy Heating Plants

Lime is the product of calcination. Its formation is always related to removal of carbon dioxide generated in the course of carbonate decomposition. Ferrous metallurgy, construction material, chemical and food industry companies account for about 90 % of lime produced in the country. Ferrous metallurgy is the major consumer of commercial lime using up to 40 % of all produced lime. Currently, despite occurrence of new binding and artificially produced chemical compounds, lime remains the major chemical compound produced by the industry in terms of output. Various units (shaft, rotary tubular kilns and fluidized bed kilns) are used for calcination. Shaft kilns are used the most widely. Considering continuously growing demand for lime, the need occurs for intensification of the burning process and optimization of the shaft kiln operating conditions. This requires knowledge of calcination physicochemical and heat transfer process mechanisms. Thus, the work deals with the issues related to determination of the optimal specific fuel consumption for burning of limestone from a particular deposit. It may be done only basing on thermal calculations for an operating shaft kiln, what, in its turn, causes the need for determination of the whole set of limestone and lime heat transfer properties. The obtained work results may be used to optimize the operating conditions of not only shaft but also rotary kilns intended for limestone heat treatment.

Sustainable circular value chains: From rural waste to feasible urban construction materials solutions

Urban regions in sub-Saharan Africa are growing significantly more rapid than their also growing rural counterparts. However, the employment perspectives in rural areas are decreasing, and thus the urban growth can become a driver for enhanced livelihoods in the rural areas. Since urbanisation in Africa requires high amount of concrete, the conversion of agricultural waste materials to sustainable concrete constituents does not only create new rural and urban income possibilities, it can also help Africa's construction industry to spearhead green concrete technologies, and thus to reduce carbon emission worldwide.From vegetable wastes chemical admixtures can be derived as well as reactive mineral cement substitutes, which help to use concrete more efficiently and reduce the ordinary Portland cement clinker in concrete, respectively. The authors of this paper discuss the potentials and shortcomings of using agricultural waste materials for construction materials and propose a synergistic three-step process chain to obtain polysaccharides, biochar, and a reactive ash. The process requires boiling, hydrolysis and vertical shaft kiln technology, respectively, to obtain maximum useable products with minimum generation of climate gases. Eventually, the process chain is discussed in conjunction with possible rural to urban circular economic potentials in the concrete construction business.

Experimental Study of Packed Bed Heat Transfer in a Shaft Kiln to Pre-Heat Manganese Ore with Hot Air

Research work to demonstrate the feasibility of using hot air to preheat manganese ore to 600˚C before feeding into a submerged arc furnace is underway. A shaft kiln will be used as the transfer unit where energy transfer takes place by passing hot air through a packed bed of manganese ore. This paper report on the results obtained from experimental measurements of the transfer process at the air-manganese ore interface. Hot air at 650˚C was produced with an electrical heater and fed into a packed shaft type column. As a requirement for continuous unit operation, the transient analysis provides details of the required residence time before materials flow to the furnace can meet temperature requirements. Understanding of the transfer processes provides optimum column sizing and process control information to be used as the basis for scaling the shaft kiln design for preheating of manganese ores. This will enable the use of concentrated solar thermal power to preheat manganese resources with air used as the thermal transfer fluid.

Pretreatment of Manganese Ore for Improved Energy Efficiency and Smelting Furnace Stability

Preheating of feed for smelting in submerged arc furnace (SAF), has shown to have numerous advantages in the ferroalloy industry. In ferrochrome smelting industry for example, preheating the feed before smelting in SAF has shown excellent results in stabilizing SAF smelting operation by removing moisture and volatiles. More importantly, preheating has shown to reduce smelting energy consumption and to increase the smelting furnace capacity. Furthermore, reducing SAF energy consumption has significant benefits in reducing CO2 emissions. However, preheating technology is currently not widely used in ferromanganese production. In ferromanganese production, smelting is mostly done using a mixture of lump and sintered ore as feed to SAF. The behaviour of feed during preheating in separate shaft kiln is still poorly known in smelting. This research is aimed at understanding optimal operating conditions/steps that could be suitable for using preheating technology when lump ore is used in the feed mix. This is also one of the goals of the EU- funded PreMa project.