Energy is essential requirement in industry. In industry cement, energy temperature is most important requirement, to operate for combustion in rotary kiln.
1. Technical Description About Fuel Type
Operation of Combustion at rotary kiln is step that is most critical in every industry cement, either evaluated technically and also economically. operation of Combustion in rotary kiln determines operation at other units, and requires usage of energy temperature assessing it can reach 30% from entirety operating expenses. Productivity from industry cement generally determined by the rotary kiln unit productivity. While rotary kiln productivity generally determined by run factor, which generally determined by resilience of the fire proof stone layer.
Main aspect of which very influential to resilience of flame resistant stone layer and efficiency operation of combustion in rotary kiln, be in fuel type used. For both purpose of the is required operation of combustion which can yield stable flame and temperature that is as high as possible.
Usage of fuel with certain coal type in operating combustion in rotary kiln can yield different productivity if compared to usage of other type fuel. For example operation of combustion with coal fuel will require consumption of association temperature of product larger ones, compared to usage of oil fuel or gas fuel. This thing is caused existence of difference of operation pattern of combustion from third of the fuel type that is gas fuel, liquid and solid. operation of Coal firing will require usage of cool air far bigger while on the contrary operating combustion to use oil fuel or natural gas will use air at high temperature larger ones.
Side that, operation of coal firing also will yield lower flatne temperature and unfavorable stability compared to oil or natural gas, both the things this will cut short age from fire proof stone layer. This situation causes operation of combustion by using coal would less productively is compared to operation of combustion with oil or natural gas. Not productive from the angle of technical for example because :
a. Consumption of association temperature of Product
b. Flame resistant stone layer age or equally rotary kiln productivity meaning cement factory productivity as a whole
Economically can expressed that operation by using coal would less economically is compared to uses oil or natural gas, inter Alia :
a. Rising combustion operating expenses
b. Rising flame resistant stone operating expenses
c. Rising cement production cost as result of degradation of produce of cement
Remembers type and quality of coal in Indonesia very uniform, hence in general can be told that usage productivity of coal in operating combustion at rotary kiln will decline counted 10-20% compared to usage of oil or natural gas.
2. Coal As Component Of Burning In Cement Industry
Coal Characters
Like it is known that coal is a a real heterogeneous solid mixture and there is nature with grade which is different, starts from lignite, sub bitumen, bitumine until anthracite. As solid, coal consisted of gathering of maceral (vitrinite, eksinite and enertinite) and mineral (clay, calcite and others).
Seen from element coal former consisted of carbon, oxygen, nitrogen is rather sulfurs, phosphorus and others. While from the angle of molecule structure, distinguishable to aromatic and aliphatic. Therefore in industry cement, coal is applied as component of burning, hence heat combustion, combustion pickings and combustion pickings it is important to know especially if the things can bother quality of cement which will be yielded.
Visible coal characters with analysis as follows :
a. Proximate analysis
Consisted of :
+ Moist (moisture) which in the form of free moist (free moisture), wafting moist (inherent moisture) and wafting moist (total moisture)
+ Ash content (ash)
+ Carbon (fixed carbon)
+ Matter flies (volatile matter)
b. Ultimate Analysis
Consisted of analysis element : C, H,O, Nitrogen also S and phosphor and Cl
c. Caloric Value
There are two kinds of caloric value, that is :
Caloric value net, that is combustion caloric value is calculated in a state of all water (H2O) in the nature of gas. Gross caloric value, that is combustion caloric value is measured in a state of all water (H2O) in the nature of water.
d. Total Sulfur
Sulfur or brimstone can differ in in coal as mineral pyrite, markasite, Calcium sulfate or brimstone organic which at the combustion will turn into SO2.
e. Dusty Analysis
Dusty happened in coal firing will form oxides as follows SiO2, Al2O3, TiO2, Mn3O4, CaO, MgO, Na2O, K2O. this ash especially would in mixed solid with klinker and influences quality of cement. However coal ash content in Indonesia usually only ranges from 5% until 20% only.
f. Hardgrove Grindability Index
Be a number which points can easy to difficult to of coal is grind to to become powder fuel. More and more small number, more and more situation firm of coal.
As according to in character, coal generally is divided to four kinds of that is :
+ Anthracite, contains a few volatiles matter
+ Bitumen, contains volatile medium matter
+ Lignite, contains many volatiles matter
+ Peat
If we burn coal with free grate, hence flame length yielded, depends on level of the matter volatile content. Coal with volatile rate matter high, will yield long flame to grate fire and coal with volatile rate matter low, will yield short flame. For the reason anthracite usually is called as with short flaming coal and bitumen as long flaming coal.
However coal will yield different result if burned in the form of smooth coal in rotary kiln. Long flaming coal if burned in rotary kiln as smooth coal will raveled immediately and volatile matter evaporating will be combustible swiftly. While coke particle which have been segregation will have a real big surface area so that coal powder can be combustible quickly. This thing causing long flaming coal in rotary kiln will be combustible only in short area from kiln or equally will yield short flame. Short flaming coal contains a few volatiles matter, if burned in rotary kiln as smooth coal will raveled slowly, so that will be combustible in longer distance.
Thereby, coal so-called short flaming coal if burned as smooth coal in rotary kiln, will yield long flame. operation of Combustion in rotary kiln requires combustion with a real flatness temperature height, because klinkerization process requires material temperature around 1450 0C. side that is higher level flatness temperature will yield heat transfer of larger ones. Both the things this hardly having an effect on in the case of effective and efficiency operation of combustion in rotary kiln. Although anthracite has high caloric value, its use as component of burning in rotary kiln unable to be taken a fancy to, because anthracite yields longer flame with lower relative temperature.
And so do lignite, beside has volatile content matter which is high and heating low value, unwelcome because will yield lower flatness temperature. Bitumen is preferred coal type of its(the usage as component of burning in rotary kiln, because having volatile content matter which is enough, but its(the caloric value high relative.
Therefore bitumen can yield higher level flatne temperature. However bitumen which bigger ash content (effect existence of usual impurities from clay etcetera) or high water content nor taken a fancy, because the things will reduce flatne temperature is side to requires also excess water larger ones. This thing will result the low of effectiveness and operation efficiency of combustion in rotary kiln.
Actually theoretically is expected bituminous coal which clear of non combustible material will yield higher and short flatne temperature compared to fuel oil and natural gas. But at the practice content non combustible material either in the form of ash or moisture cannot be obviated, causing requires operation with excess higher level water and requires primary water (which temperature is low) larger ones.
This thing will reduce flatne temperature is side to enlarges flow rate burning gas resulting shorter of retention time gas in rotary kiln from pre heater system and will reduce heat transfer of rate, meaning will enlarge castaway it is hot passed pre heater gas.
3. Preparation of Coal And Bait System into Kiln
Among all fuels that is generally is used, coal is fuel required a real unitral investment height good to grinding and also bait. Flow elementary sheet from installation of same hamper coal in all storeys.
a. Storage (Stock Pilling)
After in unloading in a factory, coal is kept in a storage warehouse. Main attention of which must be passed to this phase is lessen risk as of LDF ignition and loss (looses) material during storage. Because one of solid fuel character is not homogenize, hence before milled need to be done pre-homogenization, that for example by the way of arrangement of heap and winnowing from storage warehouse. Order FIFO need to be executed here to prevent abundant coal.
b. Primary Crushing
Primary crushing can be done in open circuit or close circuit. Softness of product from primary this crushing depends on to kinds of grinding mill used.
c. Grinding and Drying
For coal having water content under 20%, the draining done at coal mill. For coal which the water content is more than 20%, usually there are additional dryer before coal mill. Coal mill differentiated in two types, that is:
- Pack mill/Tube mill
- Vertical mill, what operated in open circuit and close circuit
Draining process here is dry raw maximum coal come up with inherent its moisture. In operation of system coal this mill which must become main attention is lessen detonation risk caused :
- Coal bait that is is not lancer
- Liquidity of bait causes harsh material (drought) re- from separator, contact direct would with hot weather
- Change of coal water content that is too big
- too Low product water content, far under inherent moisture
The detonation risks enlarged by volatile content matter which is high from coal. Operation of operation coal mill based on softness design of coal which has been considered according to requirement of combustion in rotary kiln.
d. Dirt Arrest
Arrest of coal dirt generally is done with filter or electrostatic presipitator. To lessen losing of material, equipment of this impinger must be taken care of operates in an optimal fashion. Which must be paid attention here is smooth dirt tended to causes reaction of detonation. Coal mixture or air explosive would in certain concentration area. Some experts mentions that international 40-150 g/Nml3 as critical region for the happening of the detonation, usual happened in when start up or equipments stop.
e. Bait System Smooth Coal Into Rotary kiln
System smooth coal bait into distinguishable rotary kiln as follows :
- Direct system
- Semi of indirect system
- Indirect system
At direct system, all coals yielded in grinding mill direct is baited into rotary kiln with dryer air. At flourishing indirect system, coal from mill for a while kept in silo intermediate before baited into rotary kiln. For this system there are two kinds of version which depend on coal water content. What has low water content, dryer air from mill some of hypodermic to rotary kiln as primary air, and some of circulation to mill. when high water content, some of gas from mill is released through equipment of impinges.
At indirect system, all coals from mill in keeping in silo intermediate before baited, and gas from mill is not baited to rotary kiln as primary air, except if wanted.
4. Operation of Usage of Coal At Rotary kiln
In usage of coal as component of burning in operating rotary kiln, there are some specific things that need to be paid attention that is :
a. Primary air Usage
Primary air stands inter Alia as :
- Supporting facilities for transportation for coal hypodermic into rotary kiln
- An equipment of aflame controller
Thereby primary air which temperature is this low, hence combustion air consisted of by primary water and secondary water, will have low relative mixture Einstein characteristic temperature. Therefore actually economically usage of this primary air unable to profit. In operating coal usage, usage of this primary air can range from 15-20% from requirement of combustion air.
b. Usage of Excess Big Water
Based on the reaction kinetics theory, gas fuel and liquid is more re actively with oxygen, compared to oxygen with coal. This thing easy to be understood by coal firing will pass steps as follows :
- Heat transfer from burning zone to coal particle in convection and radiation
- Heat transfer through ash layer having the character of isolator towards oxidation front in conduction
- Chemical reaction between C, S, H2 with H2, CO, H2o and SO2
- CO2, SO2, CO and H2 diffusion from front oxidizes to coal particle exterior
- Packer ash around coal particle is decomposition in terms and mechanical
Therefore to reach perfection of combustion using coal as component of burning is required excess big relative water. With usage of air larger ones this, hence will be given on to problems :
- Hot loss because permeated by excess of the air
- Transfer of temperature between airs and material in kiln unable to perfect, because low relative hot weather resident time
c. Water Content In Coal
Water which there is in coal, either as inherent moisture and also partly small other moisture, it is of course will harm because lessening temperature yielded.
d. Feed Stability
Because coal is fuel in the form of powder hence very difficult to be obtained condition of bait which really stable into kiln. Feed instability this means hot instability in kiln, will result instability of coating as fire proof stone protector. Thereby will result stone age which relative short.
e. Impurities
If cleaning of coal process is not good, hence will be met impurities (like clay). With existence of this impurities, it is of course will confuse number of hot baits into rotary kiln.
5. Clauses Quality Of Coal In Cement Industry
Basically all usable coal types as component of burning in rotary kiln. Inferential that clauses quality of coal required by industry cement of operation unit with effective that is enough is height that is :
a. Combustion value net enough heights, that is gt; 6000 cal/gr
b. Volatile matter medium, maximum 36-42%
c. Total moisture, maximum 12%
d. Maximum ash content 6%
e. Rate sulfur maximum 0,8%
f. Alkalinity in ash, maximum 2%
g. Coal measure (raw coal)
For filter 100 mm = 0%
100 mm - 50 mm = 70%
50 - 25 mm = 25%
25 - 15 mm = 15%
Gets away 15 mm = 0%
h. Various quality is upper at the most 10%
Coal with quality that is is not fulfills clauses is upper will yield lower productivity, clauses upper to be explainable as follows :
- Minimum net combustion value 6000 cal/gr, Volatile matter medium, maximum 36-42%, Maximum ash content 8%, meant that usage of the coal in rotary kiln, can yield targets expected at operation of combustion.
- Maximum moisture total of 12% and maximum ash content of 6% and coal measure according to measure, meant in order not to complicate at operation handling.
- Maximum sulfur rate of 0,8% and alkalinity at maximum ash of 2% is meant to ins order not to invite trouble at operation of rotary kiln and not happened degradation of quality of cement.
- Coal measure and volatile matter also is meant ins order not to invite burning during bait, more and more many containing is prilled smoothness, hence coal heap would flammable.
- Various quality of 10% from values mentioned meant that clauses to reach operation of stable combustion can fulfilled
6. Contamination of Area
To reach perfection of coal firing, required excess water which relative many, unhappily that with excess higher level water results Einstein characteristic temperature in kiln would lower. Therefore in reality of practice of often is found that combustion reaction process has not taken place perfection, though gas has gone out from suspension pre heater. This thing is pointed with existence of content CO from the gas. Even not happened, especially at the time of heating up, or existence of fluctuation of coal bait that is big enough, flue exit gas also still having colour black. This thing is pointed not merely just CO which implied in the gas, but combustible has not coal.
If gas content CO from gas towards electro precipator as a means of impinger bigger than 0,6%, hence to avoid detonation, equipment of this impinger off would so that thereby there is no dirt arrest, is meaning around 7% from bait raw meal will fly together secretory gas of flue, it is of course generates inter Alia problems :
- Air pollution, either dirt and also gas CO
- Loss because loss of material
combustion reaction process of This coal would having continuation so is all temperature gas ring, results the gas Einstein characteristic temperature is very diffraction height. In condition of like this is not rarely results damage of impeller from fan passed by or damage of expansion joint from ducting or to ducting itself.
contamination risks of Area, losing of this deductible material and equipments damage or avoided inter Alia by the way :
- Labors perfection of combustion in burning zone in kiln by comprehending combustion process kinetics
- Planning of system kiln and good coal hypodermic
The thing is upper to be source of environmental contamination passed flue gas, side that is source of other contamination happened during storage and during operation of exploitation and preparation of coal, also happened leakages generating contamination of area.
Sources : Batubara and Gambut, Ir. Sukandarrumidi, MSc. Phd