Technical Market Support » Metallurgical Coal
The aim of this project was to develop a technique by which the handleability of Australian coals in pulverised coal injection systems for blast furnace applications can be more reliably predicted and compared with other international coals on an equal footing.
Injection of pulverised coal into the blast furnace to replace part of the coke burden is now common practice and has the potential of becoming a coal conversion process of equal importance to the use of coal for the production of metallurgical coke. Many different coals have been used for injection into blast furnaces and these coals are selected largely on the criteria of grindability, specific energy, and ash chemical composition. At present the upper limit of coal injection is around 200 kg/tonne of hot metal. Increasing injection rates will increase the sensitivity of the injection systems to coals that have poor handleability and flow characteristics.
Laboratory scale tests exist to predict the behaviour of bulk materials in silos and hoppers, assessing material properties such as permeability, wall friction, compressibility and cohesion. The apparatus include the Johanson Indicizer system (JIS), Jenike shear cell, Edinburgh Cohesion Tester (ECT). Research has been undertaken in the field of powder mechanics using these testers to understand and improve the flowability of bulk materials from silos and hoppers. These tests have not been adopted for assessing pulverised coal and only limited attention has been devoted to explain the pneumatic conveying behaviour of crushed materials. It is well established that coal crushing efficiency is affected by Hardgrove Grindability Index (HGI), but neither the size distribution of the product nor the actual throughput of the plant can be fully predicted. It is apparent that the performance of coals in the crushing and handling process routes is susceptible to coal type and properties.
The aim of this work has been to develop a series of tests and use them to characterise more precisely the performance of coals in the crushing, grinding and transport systems. A detailed examination of the coals before, during and after these processes was made to relate physical properties to performance in both pilot and plant scale systems wherever possible.
Testing showed that the coals varied in Hardgrove Grindability Index, chemical analysis, size, mineralogy and handleability. At the particle size required for Pulverised Coal Injection (PCI), both the JIS and ECT show different results compared to the “as received” coals. All the coals show a lack of clear Arching or Rathole Indices at all free moistures. The ECT analyses show considerably higher free unconfined column strengths for the PCI coals.
Coal size at Granular Coal Injection (GCI) decreased during handling and conveying with an increase in the amount of fines. Problems with blockages were encountered with two high volatile coals when conveying GCI at low conveying air rates and high solids loadings. For PCI conveying no blockages occurred. Operation of the pilot plant using the PCI sized coals resulted in lower solids mass flow rates, lower phase densities and higher system pressure drops compared to GCI conveying for all the coals tested.