Introduction to Solid Fuels

Date:01.09.09

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Fuel is something consumed to produce energy, especially:

a. A material such as wood, coal, gas, or oil burned to produce heat or power.

b. Fissionable material used in a nuclear reactor

Anything that is capable of producing energy in a form that is useable for producing power is called fuel.

Definition

General Grouping Primary/Natural Fuels

Secondary Fuels

Solid Fuel Anthracite Coal, Bituminous Coal, Lignite, Peat, Wood

Coke, Charcoal, Coke Breeze

Liquid Fuel Petroleum Kerosene, Diesel, Tar, gasoline

Gaseous Fuel Natural Gas Producer Gas, Coal Gas, Furnace Gas

Types of fuels

Advantages Disadvantages

Easy Transportable High ash content

Storage convenience Low thermal efficiency

Production cost is low Combustion operation not controllable

Moderate ignition temperatures Burns with clinker formation

Comparatively low S content i.e. low atmospheric pollution

Handling cost is high

A large excess of air needed

Calorific values lower than liquid fuels

Solid Fuels

Coal is useful as a fuel because it is 1. Abundant2. Has a relatively high heating value.3. Many manufacturers changed to natural gas

in making their products but are now switching back to coal because it is less expensive.

4. Coal is used for many things in addition to fuel for heating homes or running engines.  Products left after heating coal are coke and coal tar

Why Coal?

Iron smelting-where the carbonaceous fuel is used for reducing iron ore as well as melting it.

Copper smelting-initially carbonaceous fuel was used for reducing Copper oxides.

Lead & Zinc Smelting- here also carbonaceous fuel is added for reducing the ores & melting them.

Solid fuels in Metallurgy

Chemical composition (as determined in terms of fixed C, ash, S, P)

Reactivity as determined by its physical structure (porosity)

Size range Thermal stability at high temperature Strength & abrasion resistance.

Requirements of fuel for blast furnace

The poor productivity of a blast furnace (1 t/m3/day) as compared to Japanese furnace which can produce 2-2.25 t/m3/day is mainly due to the low quality fuel used.

An increase in ash content in coke by 1% results in decrease in production by 3-6% & increases coke consumption by 4-5%.

Open, well-developed structure influences Rate of burning.

More than chemical composition, permeability of the burden is important. This is decided by the strength of the coke.

Requirements of fuel for blast furnace

The more porous the coke, the better. 7% porosity saves 100kg of coke per tonne of pig iron produced.

The size range of the coke affects the distribution of materials inside the furnace & consequently gas flow, which affects productivity.

Smaller coke size is better for better thermal stability.

Requirements of fuel for blast furnace

Although porosity is required, strength is more important.

Fuel must stand the rigors of handling & charging It must avoid the abrading action of the

surrounding particles. It must also stand the high temp & the nearly 20-25m of tallburden lying over. Fuel must not break down in tofines. In fact the height of the modern furnace is controlled by the strength of the fuel

Requirements of fuel for blast furnace

Pb-Zn smelting also takes place in a blast furnace under less reducing conditions.

Reduction of PbO & ZnO is exothermic. This helps maintain temp of the Zn vapors formed.

PbO+CO Pb+CO2 (10000C) ZnO+CO Zn+CO2 (11000C)

Lead-Zinc Smelting

Reverberatory Furnaces are used. Gases produced from combustion of coal &

petroleum are used. Thermally inefficient.

Cu & Sn smelting

Ash content of coal is the non-combustible residue left after coal is burnt.

Fixed carbon The fixed carbon content of the coal is the carbon found in the material which is left after volatile materials are driven off.

Volatile matter in coal refers to the components of coal, except for moisture, which are liberated at high temperature in the absence of air.

Properties of coal

Clinker the incombustible residue, fused into an irregular lump, that remains after the combustion of coal.

In terms of coke strength, the coke stability and Coke Strength After Reaction with CO2 (CSR) are the most important parameters.

The stability of coke is also important.

Properties of coal

Fixed C atleast 85%. Ash not more than 10% Volatile matter not more than 2% P content approx. 0.018%-0.04% S content approx. 0.6%-1.5% A reduction in S by 0.1% can reduce coke

rate by 2% & consequently improve productivity.

Requirements of fuel for blast furnace

The most important properties of coal to the combustion engineer are as follows:

Proximate analysis - to determine the moisture, ash, volatiles matter and fixed carbon

Ultimate or elementary analysis - to determine the elemental composition of the coal

Calorific value Caking properties - for bituminous coals only Grindability - where the coal is to be pulverised

Important properties of coal

Proximate analysis is the simpler of the tests and is used to determine the moisture, ash, volatile and fixed carbon content.

Ultimate analysis is used to determine the elemental composition in terms of Carbon, Hydrogen, Sulphur, Nitrogen and Oxygen by difference.

Important properties of Coal

The enthalpy of formation of iron oxide is -821.3kJ at 250C.

1 mol of oxide contains 111.6g of Fe. Hence the energy required to dissociate Fe: (821300/111.6)x106=7.4GJ/t

The actual energy used in a blast furnace is 11.45GJ/t due to heat losses to lining & as gases.

Coke has a gross calorific value ~23865 kJ/kg which is higher than all other forms of coals.

Energy Requirement in Blast Furnace

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