The main cost benefit of PCI is the replacement of high cost coking coal. The amount of coke replaced by an injected coal depends on the useable energy released by the PCI coal in the lower zone of the BF. This energy is the partial combustion heat released when coal is gasified to CO and H2 less the sensible heat of the combustion gases and ash.

There is an economic injection rate, namely the injection rate at lowest total coal costs, for every BF. This rate depends on the coke quality and the cost of coking and PCI coals. Most BF’s have PCI systems that can achieve injection rates greater than the economic rate.

The value-in-use benefit of PCI depends on its replacement ratio and its impact on the energy balance of the steelworks. The costs/benefits associated with changes to the energy balance of a steelworks depend on the costs of the alternative fuels.

 

Any coal can be injected with the main requirements, from the viewpoint of chemical analysis being low sulphur, low phosphorus, low ash content and minimum fluctuation in composition.  The primary factor that influences the cost benefit of PCI is the amount of coke that can be replaced by the injected coal. See attached summary for details of various methods for calculating the replacement ratio of an injected coal base on its chemical analysis.

In Figure 1 different methods for calculating replacement ratio have been plotted against the dry ash free carbon content, which is a measure of the rank of a coal.  In this figure, all coal properties have been related back to the carbon content assuming a fixed ash content and using CoalTech's relationships between coal properties.  The figure does show that all methods used to determine replacement ratio give a maximum at around a carbon content of 90%. This is the main reason for the choice of LV coals for PCI.

 

Mr T. Fukishma of F-TeCon Pty Ltd carried out the modeling (Bennett & Fukushima, 2003) to investigate the impact of PCI coal quality on the operation of a blast furnace. The model was used for a quantitative study on the influence of ash content of the PCI coal on blast furnace operation, particularly focusing on the impact on operating costs.  For this study the blast furnace process parameters shaft efficiency, reserve zone temperature, heat losses of upper shaft and lower shaft and theoretical flame temperature at raceway were constant for all calculations. One coal was used in all calculations with different amounts of ash – 8, 9, and 10 % ad.  These calculations indicated that the change in coke requirements (kg/tHM) per 1 percent ash change in the PCI coal varies with injection rate as:

For an injection rate of 150 kg/tHM, an increase of 1% in the ash of the injected coal would result in a 1.2 kg/tHM increase in the coke rate.  In addition to this increase in coke rate there is also a need to adjust BF operating parameters (e.g. blast volumetric rate, temperature and oxygen, and slag chemistry), which results in a net increase in BF energy requirements and other raw materials. Therefore, the cost impact of an increase in ash of the injected coal is dependent on the coke costs and the cost of auxiliary fuel used within the plant.

The increase in coke rate per 1% increase in ash determined by this study is significantly less than that suggested by other authors. Poveromo [1] suggests that for furnaces with total fuel rates of 500 kg/tHM every reduction of coke ash  (or injected coal ash ) of 1 % would reduce coke rate by 6 kg/tHM.  Xi Ping and Suen [2] state that for each one percent of ash will increase the blast furnace fuel rate by two percents.  Therefore, the ash content of the PCI coal must be at least 1.5 percent lower than that of met coke.  

Brouwer and Toxopeus [3] in summarising the PCI operating results at Hoogovens IJmuiden blast furnace derived a relationship between replacement ratio and the properties of the coal injected (given in Appendix A). This relationship shows that ash increases replacement ratio and therefore would lower coke rate.  

There is a value-in-use penalty for increased ash, but for the majority of coals traded as PCI coals the ash differences are small and the composition of the ash may be more of a concern in the future.

[1] Poveromo, J., 2004, Blast furnace fuel injection trends, Met Coke World Summit, Chicago, October 2004.

[2] Xi Ping, P., Suen H., 2003, PCI Application in blast furnace operation and market trend in China, McCloskey’s 8th Annual Australian Coal Forecast 2004 Conference, 24 – 25 November 2003.

[3] Brouwer, R.C., Toxopeus, H.L., 1991, Massive coal injection at Hoogovens IJmuiden BFs, Revue de Metallurgie. Cahiers d'Informations Techniques, V88, N4, Apr 1991

Two market forces control the price of PCI coal, the price of hard coking coal and the price of thermal coal.  The upper price for PCI coal can be determined by the value of coke replaced and the lower price is set by the value of HV PCI coal produced from washing a typical thermal coal to an ash of less than 9%.  Figure 2 show how the coal costs of a typical coke oven blend and the thermal coal price influences the price range for LV PCI coals.

 

As shown in Figure 3, the incremental change in the amount of coke replaced by coal injection decreases as the injection rate increases.  In addition, as the injection rate increases, over about 140 kg/tHM, the quality of the coke must increase to ensure BF productivity is maintained.   Using the curve for all the data in Figure 1and a simple relationship between PCI rate and required coke quality then the most economic injection rate can be determined for known PCI and coking coal prices, as shown in Figure  4.  If the 2005 coal prices into Japan are used then the economic injection rate is determined to be about 115 kg/tHM, just below the average Japanese injection rate for that year.