Proceeding of the international scientific conference “Refractories, furnaces and Thermal insulations” April 24-26, 2006 Strrbske Pleso-High Tatras, Slovakia, 2006. – P.84-89.

Theoretical and experimental research of pulverized coal burning process under conditions of raceway has been carried out. Method of enriching blast with oxygen and tuyere design for intensifying have been developed. Industrial tests of this method have shown the possibility of considerable increase of PC combustion degree under high PC consumption.

1. Introduction

Pulverised coal injection into the blast furnace is an effective technique as a means of stable operation, cost reduction, productivity increase and environmental protection. The first in Europe commercial complex for PC preparation and injection was built in 1980 at Donetsk Steel Plant (DSP), Ukraine /1/. At present more than 100 complexes operate in countries of European Community, China, Japan, Korea, U.S.A. and other countries /2,3/.

The effective use of expensive PCI complexes can be attained when consumption rate of coal gets to maximum. The main factors which hinder the increase of PC quantity being injected into the hearth of furnaces is the maintenance of its complete gasification within the raceway and smooth operation of a furnace due to change of heat, slag-formation and gas distribution conditions.

The results of research of PC burning process in the raceway, investigations of different type of coals and changes in reduction and temperature processes are given in this paper.

2. PC Combustion in the Raceway

The character of burning process of PC flow is expressed by non-dimension expression /4/:

K₁d₁=NuD (1)

where, K₁ : constant of burning velocity (m/s),

d₁: maximum size of particle (m),

Nu: diffusional criterion of Nusselt (-),

D: coefficient of oxygen diffusion in gas into the zone of burning (m²/s).

The results of calculation showed that under conditions of BF operation the value of complex (1) is more than 1, which testify that process of PC burning exists in the diffusional regime.

Experimental examination of theoretical results was carried out on the laboratory installation, which allows imitating PC burning in the raceway /5,6/. Method for investigation of PC combustion was following. Air blast was blown by blow fan through silica tube, situated in the furnaces, where it was heated up to 900-1000 ⁰C. Blast flow rate was controlled by rotameter. Pulverised coal from lean coal which used for injection into BF at DSP fed in PC bin, and the transported by air, argon or oxygen to combustion chamber. Incompletely burnt coal particles were caught in water bowl, and after filtration and drying technical analyse of particles (ash content) was determined. Series of tests have been carried out to study influence of blast temperature and concentration of oxygen on PC combustion degree.

Analysis of results is as follows: increasing consumption rate of PC from 5 to 20 g/min (50-200 g/m³ of blast) decreases PC combustion degree by 40%. Increasing temperature of blast from 600 to 1000^oC increases the degree of PC combustion by 10-15%. Concentration of oxygen in the blast considerably influences the level of PC combustion. Increase of the content of oxygen by 10% (from 21 to 31%) has led to increase of degree of coal powder combustion by 25%.

Results of tests confirm the diffusional character of burning regime.

Oxygen enrichment of blast under PC injection was proved by industrial experiments /7-9/. However theses methods do not ensure maximum oxygen utilization efficiency because design of tuyere doesn’t give even mixture of coal particles with blast flow.

Due to complication of complete PC mixing with blast is proposed to increase the local oxygen concentration in the place of moving PC jet. These proposals are known and put into practice at Thyssen Stahl and british Steel BFs. However they do not provide rational use of oxygen because of decreasing due to diffusion oxygen concentration by the beginning of burning coke residue of coal. Therefore it is necessary to determine place and time for adding oxygen to PC flow with purpose to provide maximum oxygen concentration in the moment of coke residue burning.

This is due to the fact that combustion of coal particles occurs in four stages /10/

where t ₁, t ₂, t ₃, t ₄- times of heating of particle before ignition of volatile matter, volatile matter burning, heating of coke residue and coke residue burning, respectively, (s),
k₁, k₂, k₃, k₄ : coefficients which depend on the coal grade ( - ),
T - the temperature of the surrounding atmosphere (K),
d - the average initial particle size (m),
A - ash content in fuel (%),
O₂ - oxygen concentration in the blast (m³/m³),
p - the apparent density of coke residue (kg/ m3).

An increase in oxygen concentration affects only the time over which the last stage occurs. Therefore preliminary mixing of fuel with oxygen is inexpedient: when it enters the tuyere a considerable portion of the PC particles burn before they emerge into the oxidizing zone because of the high local oxygen concentration around the coal particles. However, combustion PC in the tuyere assembly is undesirable because of the increase in gas temperature and melting of ash, and on account of deviation of the stream of combustion products and unconsumed particles upwards along the furnace periphery. Furthermore, as usual, the fine particles burn first, and consequently their combustion occurs under more favourable conditions. Burning of coarse particles, which occurs later, already proceeds in a region of lower oxygen concentration.

Consequently it is most effective to increase the oxidising agent concentration after heating and emergence of volatile matter and heating of the coke residue, i.e. after time t₁ + t₂ + t₃. Calculations show that this time is 0.005 – 0.020 s, depending on PC characteristics and the temperature and blast conditions of the operation.

The zone of active oxygen diffusion to PC particles was taken to be 25% of the tuyere cross-sectional area. With PC delivery into tuyere, this quantity can vary over the range 15-35%, depending on the diameters of the dust line and tuyere, and the design of the dust introduction unit.

Complete combustion of 100 kg of PC requires 35-50 m³/t HM of oxygen spread among the tuyeres if oxygen enrichment of blast is excluded.

To ensure timely arrival of oxygen in the stream of preheating pulverised coal, the depth of its penetration and flow trajectory in the tuyere cavity must be known. For determination of these parameters there are analytical procedures in jet hydraulics and aerodynamics theory for calculating the trajectory of a round jet in a carrier stream /11/.

An experimental industrial installation was created to carry out investigations on a blast furnace at DSP /12,13/. Oxygen delivery into the tuyere is effected from a manifold through an oxygen line provided with the necessary fitting and instruments. As for the natural gas delivery system, a return valve, which operates if the oxygen pressure falls, is provided. There is provision for instantaneous remote disconnection of the system if gas tightness is lost. There is a “blow off pipe” for flushing the route.

Hydraulic tests were carried out on the installation before it was brought into service, and the inner surface of the oxygen line and fittings was degreased and flushed with stream. During the investigations, the 1033 m³ blast furnace produced steelmaking hot metal on an iron ore burden consisting of 94% sinter from the Southern Mining and Benefication Plant and pellets from the Central Mining and Benefication Plant in a ratio of 1.5 : 1 with the necessary additions of iron and manganese ores. The average consumptions were as follows: coke 487 kg/tHM, pulverised coal 76 kg/tHM, natural gas 84 m³/tHM, oxygen 75 m³/tHM. The furnace was provided with a system for monitoring the distribution of blast and PC among the tuyere. The oxygen pressure in the oxygen line was 570-1000 kPa, the oxygen flow rate per tuyere was 320-350 m³/h.

The effectiveness of the proposed method of delivering oxygen was assessed with the aid of a quantitative method for determining the completeness of PC combustion in the tuyere zones, which consists in probing of the hearth with sampling of materials from different points along the tuyere assembly and the oxidising zones with a water cooled tube.

Investigations were carried out on blast oxygen enriched to 25% and atmospheric blast (oxygen delivery to the blower inlet was stopped). The amount of oxygen delivered through the tuyere corresponded to the oxygen consumption for blast enrichment of the furnace. Each trial included two experiments, carried out in sequence with an interval of several minutes: probing of the hearth while oxygen was delivered into the tuyere and with the oxygen turned off.

The results of mineralogical analysis of samples of materials taken from the hearth showed that, with and without oxygen delivery to the tuyere, fine, acute angle coal dust particles of 0.004-0.008 mm size, and more rarely up to 0.02 mm in size predominate at the oxygen delivery point. At the tuyere end there are residues of burnt PC particles in the form of thin rings, as well as unchanged PC particles. At a distance of 0.25 m from the tuyere nose, PC particles no longer exist. Only residues of dust particles in the form of 0.004-0.012 mm rings remain. At a distance of 0.25 mm and 0.5 m, rounded globules of acid and iron containing slag appear in samples. The particles are generally irregular in shape, up to 0.4 mm in size /21/. In all experiments, the PC consumption was 8 t/h and the consumption of blast enrichment oxygen 6000 m³/h, the flow rate of oxygen delivered to the tuyere being 320-340 m³/h. The natural gas flow rate was 4200-6200 m³/h.

Comparison of corresponding pairs of experiments shows that the content of PC particles at the end of the tuyere with individual delivery of oxygen into it is on average 15-20% lower than with the conventional method. At a distance of 0.5 m from the tuyere nose the amount of dust particle residue is also lower in experiments with local oxygen delivery. The results obtained point to acceleration of pulverised coal combustion with individual oxygen delivery.

The design of an industrial system for individual oxygen delivery to the tuyeres of a blast furnace has been proposed for Donetsk Steel Plant. The system provides for delivery of process oxygen to the blast furnace; the laying of oxygen lines with the necessary shutoff and control fittings to individual tuyeres from a distributing point situated in the casthouse; automatic control of the total oxygen flow rate and the flow rate to each tuyere; emergency disconnection with the aid of high speed cutoff valves. It is also intended to create an automated system for proportioning the flow rates of blast, fuel additions, and oxygen around the furnace.

3. Optimisation of Coal Grinding and Complex Compensating for the Negative Changes in the BF Operation with PCI

Increase in the level of coal grinding leads to intensification of its combustion, but certainly increases the cost of PC complex.

Overgrinding of coal dust does not only increase energy-waste on grind and reduce mill productivity, but at the same time worsens transportation of PC, since the smallest particles have the tendency to stick together. Therefore in order to increase productivity of mills it is profitable to produce PC of coarse grind.

However with coarseness of grind combustion occurs with difficulties in the raceway that causes a whole complex of negative phenomena: reducing gaseous passage capacity through burden column, increasing viscosity of slag, reducing the coke/coal replacement ratio and others.

Economy of PC injection is the highest when total consumption of energy for preparation and combustion of dust get to minimum.

To substantiate optimum fractional content of PC, research were carried in different ways with dust, prepared from concentrate of lean coal and used for injection into BFs in Ukraine.

The desire to reduce size of particles is explained with the hope of increasing the reaction surface area of PC, which at the determining level indicates its combustion degree in the furnace. At the same time unit surface of particles increases also in consequence of high temperature thermal hit at their arrival in the tuyere cavity.

Therefore optimum coal grinding for injection into blast furnaces exists: main mass (60-80%), obviously, must consist of fraction 50-100 ěm. Grinding of PC 100 ěm below 50 ěm is not proved technologically.

Attaining maximum effectiveness of PC use is possible with the help of principle of complete and complex compensation for the negative changes of heat fluctuation, slag-formation and gas distribution processes determined by the combustion of PC and simultaneously reducing of coke rate in burden.

To evaluate the effect of PC on heat regime of hearth, the value of flame temperature was determined from equation of heat balance for lower zone of heat exchange. Viewing two technological regimes – with (index 1) and without using PC (index 0), equation is got for flame temperature, needed to save initial heat and composition of pig iron under new technological conditions:

T₁=T_n+(1-A(r_d0-r_d1)/r_d0)(Q_k0V_g0)/(Q_k1V_g1)(T₀-T_n) (6)

where, T- flame temperature (^oC),
Tn - temperature of burden and gases in the slowed down heat exchange zone (^oC),
r_d- direct reduction rate ( - ),
Q_k - coke consumption (kg/tHM),
V_g - output of hearth gases (m³/t of coke),
A - constant, determined by initial value of water equivalent of burden ( - ).

From Eq. (7) it follows, that technological conditions of BF operation functionally determine level of necessary flame temperature. E.g., reducing of coke consumption or raising the level of direct reduction by 1% leads to necessity of increase in temperature T1 correspondingly by 20-30^oC or 15-25^oC.

Injection of PC into the hearth is accompanied by reducing in theoretical temperature of burning by 100-200?C for every 100 kg of fuel/t HM. Lowering the percentage of direct reduction by 2-4% for every 100 kg fuel per 1 t HM makes it possible to reduce necessary flame temperature by 30-100^oC, which cuts down by the indicated value the temperature compensating calculated on the basis of maintaining initial temperature level of products of melting /14,15/.

Compensation with increase in the blast temperature (by 250 ^oC for every 100 kg of Pc/t HM) maintain the best result: total coke/coal replacement ratio 1.45-1.55, saving of 40-45 kg/t HM (8-10%) of additional fuel, increasing the productivity by 1.4-7.0%, improving of tuyere gas efficiency (2-6%), and reducing direct reduction rate by 3.6%. Using this method coke/coal replacement ratio does not practically reduce when increasing PC injection from 0 to 300 kg.t HM.

4. Conclusions

Theoretical and laboratory research of PC burning showed that under conditions of BF operation process of PC burning exists in the deffusional regime.

Complete gasification of high PC quantities can be provided by special measures on intensifying its combustion in the raceway: enriching blast with oxygen, complete mixing of PC and oxidizer.

To accelerate PC combustion it is proposed to increase the local oxygen concentration in the region where the jet of suspension is flowing in the tuyere assembly, rather than the total oxygen content of the blast. This should be done 0.005-0.020 s after the coal dust particles have emerged into the inner cavity of the tuyere assembly.

Experimental industrial trials with probing of the hearth have confirmed the faster disappearance of PC particles on leaving the tuyere compared with the traditional method of blast oxygen enrichment.

Using the principle of complex compensating enables to get the value of total coke/coal replacement ratio of more than 1.0 kg/kg. As a result the output of blast furnace is increased and the utilization rate of gases is improved.

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