aloke mookherjea paper sponge iron project

8/2/2019 Aloke Mookherjea Paper Sponge Iron Project

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PRELIMINARY POSITION PAPER

ON

PROJECT PROPOSAL

FOR

REPLACEMENT OF INJECTION COAL BY

PARTIALLY STEAM REFORMED

COAL - BED METHANE (CBM)

IN

CONVENTIONAL COAL BASEDSPONGE IRON PLANT IN A

DIRECT REDUCTION PROCESS

Aloke MookherjeaChairman

Flakt (India) Limited[Email : [email protected]]


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Introduction

Originally, when direct reduction of iron or DRI plants were envisaged, the idea was to replace

coke whose availability and cost became highly uncertain and prohibitive. The original process

of DRI replacing coke and using coal was hailed as a precursor of a new regime of making iron

and steel.

Then started the problem always associated with the use of coal as fuel. Coal, while being a

comparatively cheaper fuel and (which is a common knowledge today) has a no. of reasons for

being used as a fuel, particularly in developing countries like China and India, also has inherent

problems of creating environmental pollution. This happens because of air/gas pollution, solid

waste, causing water pollution and very high emission of greenhouse gases like carbon dioxide.

While use of any hydrocarbon like methane etc. will not remove the generation of carbon

dioxide totally, it certainly reduces the emission and takes away the problem of other kind of

environmental degradation. This degradation is very prominent now all over the country

wherever there is a concentration of sponge iron plant, like Durgapur and Birbhum area of West

Bengal.

It has been proposed to establish theoretically, as well as practically that Coal Bed Methane(CBM), abundantly available in these areas or near about in West Bengal, could replace the use

of injection coal.

The basic improvements expected from such a process modification would be a much higher

reducing efficiency for the iron ore because of higher hydrogen density of methane compared to

that of coal and of course much less generation of all kinds of waste which is normal with the

use of coal.

Metallurgy

Direct reduction of iron ore, an alternative to blast furnace technology of producing iron from

ore, has become very popular because of reasons stated above. In this process, the ore isconverted into metallized pellets, the so called sponge iron, by removing the oxygen from the

iron to leave metallic iron. The amount of metallic iron produced from a given quantity of ore is

termed as the degree of metallization and is defined by the ratio of the amount of the metallic

iron produced to that of total iron in the ore.

The iron left after the removal of oxygen has a honeycomb like construction and hence the name

sponge iron. A considerably high degree of purity of ore is required for sponge iron because the

gangue is not removed at the iron making stage but later in the steel making process. Both

pelletised concentrate and screened natural lump ore of similar purity can be utilized. A typical

layout of the sponge iron is given in Diagram 1 where the heart of the process is the Rotary

Kiln. Ore (Fe2O3) with coal and a small amount of dolomite (flux) is fed to the Rotary kiln

which also has an arrangement of submerged air injection. Obviously there is a lot of material

handling involved in different stages because of use of the ore and coal. The chemistry of the

iron formation is given in Diagram 2. Haematite or iron ore and the non-coking coal are the

major raw materials. The significance of this process is that coal plays a dual role by acting as a

reducing agent as well as the fuel for raising the requisite temperature inside the kiln to more

than 1000C. Another point in this process is that the reduction occurs in solid state. Controlled

combustion as well as the conversion of carbon monoxide to remove oxygen from the iron ore

are the crucial factors in this process.


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Diagram 1

Diagram 2


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Reactions

Using haematite or Fe2O3 as the ore, the carbon monoxide and hydrogen which are both

reducing gases act on the ore and reduces it to metallic iron at a temperature above 800 deg. C.

In the pre-heat zones, the temp is around 850-1000 deg C while the reduction zone temp is

around 1000/1050 degC. As normal in all kiln type of operation, the moisture is driven off first

and after that thermal decomposition of coal and subsequent generation of reducing gases like

HCs and H2 take place. Also, as normal in a rotating kiln, the major heat transfer to the chargetakes place through the mechanism of radiation. Gases are burnt above the bedassisted by

controlled air injection, also helping in better fuel consumptionand more heat is transferred

through mixing and constant change in the charge layer.

The stages are :

Fe2O3+ CO= 2FeO + CO2 (Ferric to Ferrous Oxide)

FeO+ CO= Fe+CO2 (reduction in metallizing zone)

CO2+C (presence of excess solid fuel) = 2CO

The beds are not excessively heated up as the 3rd

reaction above is endothermic, thus no melting

or stickiness.

Methane & Steam Reforming of Methane

Thermodynamically methane has a great reducing capability and can be activated to produce

synthetic gases over heated metal oxide as an oxygen donor. Metal oxide reduction and methane

activation, two concurrent thermo-chemical process, can be combined as an efficient and energy

saving process. This new reduction process combines energy efficiency and much lower degree

of GHG emission compared to the conventional process using coal.

Partial steam reforming of methane with sub-stoichiometric amount of H2O ensures that there is

hardly any water gas shift reaction taking place and very little or no CO is lost as carbon dioxide

before the main reaction zone of direct reduction.

The steam reforming of methane consists of 3 reversible reactions as given below where 1 and 3

are strongly endothermic reactions while the water gas shift reaction in equation 2 is moderately

exothermic.

CH4 + H2O CO + 3H2 deltaH deg 298 = +206 kJ/mol (1)

CO + H2O CO2 + H2 deltaH deg 298 = - 41 kJ/mol (2)

CH4 + 2H2O CO2 + 4H2 deltaH deg 298 = +165 kJ/mol (3)

One must note that CO2 is not only produced via the shift reaction as in 2 above but also directly

via the steam reforming reaction in equation 3 above. It only emphasizes the fact that 1 and 2

are not the only reactions occurring during the reforming processes.

Because of its endothermic character, reforming is favoured by high temperature. Also because

of volume expansion it is favoured by low pressure. On the other hand, exothermic shift reaction

is favoured by low temp but unaffected by pressure change. Increasing the amount of steam will

no doubt enhance the methane conversion but would also require additional amount of energy to

produce the steam. In practice, steam to carbon ratio (S/C) of around 3 are applied.

The steam reforming process is divided into 2 sections, a section of high temperature and

pressure, of around 800 to 1000 deg. C and 30 to 40 bar pressure, in which the reforming and the

shift reaction occur. This is followed by additional 2 step shift section at lower temperature


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(typically of the order of 200-400 deg. C) in order to maximize the CO conversion via reaction

no. 2 shown above. In such a process, CO concentration can be as low as 0.1%.

The steam reforming further decreases the demand of elemental oxygen for converted carbon to

CO, thus requiring less volume of air. This also helps in reducing the amount of diluents in the

reaction zone. It is clear that a very important feature of steam reforming is the generation of

elemental hydrogen which is extracted from both methane as well as water vapour. This

generates hydrogen which is theoretically 3 moles per mole of methane reformed which in turn

will act as an even more efficient reductant as compared to CO. In this process the carbondemand also decreases.

OBJECTIVE

It has been proposed that coal based methane as a fuel and a reductant can be very helpful for

sponge iron plants in Eastern India. CBM has the same benefits as normal natural gas and can

be substituted for natural gas as long as it has a low sulphur content and high methane content of

around 94% plus. This should normally pose no problem based on the analysis of the CBM

available. This process also is expected to comply with very stringent environmental

compliance.

A techno-economic feasibility study is being planned for making the above possible. It isproposed that initially certain laboratory studies would be needed to find out the actual high

reducing ability of steam reformed methane. It has been given to understand that NML at

Jamshedpur can carry out this study and come out with the answer. After this study the pilot

plant will be required to be set up at one of the sites. Based on the current operational

parameters of variable load vis--vis amount of coal injected as well as its properties such as

chemical analysis, calorific value etc. trial and error method has to be adopted for gradual

replacement of coal by steam reformed CBM to arrive at the optimum condition. Theoretical

calculations beforehand would no doubt help but actual experimentation must be carried out at

site in a pilot plant to achieve the optimum conditions. Also, the normal current practice of coal

injection at the discharge end has to be looked into. Today, this is done purportedly for

preventing carbon starvation as also for maintaining the temperature profile in the kiln.

Even if the pilot study is done for part load and/or a small sized kiln, low pressure steam would

be required at site as well as a small reformer using heated nickel plate or other material.

Extensive trials and tabulations of data would be necessary.

The next step would be analysis and interpretation of data for subsequent use in a scaled up plant

with continuous supply of steam and CBM. It is also understood that supply of methane will not

be a problem since even for todays production of CBM, a lot of gas is being flared up. The

pilot plant study could be made with piped supply if possible but even otherwise it can be

supplied from a tanker. For final implementation of course, continuous piped supply would be

necessary.

The first step in the project will be the initial formulation. Next will be the laboratory studies

after which calculations and design of the experiment could be taken up. The pilot plant

including the additional equipment required could be set up after this. Actual experiments

including collection of site data etc. would follow. The last step would be the interpretation of

the data and analysis of the same after which the work to scale up model can be started.

The time period for all these steps will depend upon the resource provided and time spent. A

rough estimate could be around 32 weeks after work starts in right earnest up to the pilot plant

stage but this can vary widely depending upon the working conditions.

aloke mookherjea paper sponge iron project

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