Post on 16-May-2015
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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