IMPORTANCE OF HEAT BALANCE IN CEMENT INDUSTRY BY Ahmed Adel

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A heat balance is a basic tool for manufacturing performance Reasons :

1. Performance Test
2.  Evaluation of a possible modifications and CAPEX
3.  Recording of kiln performance before / after a modification BASIS >>> VERIFICATION
4.  High heat consumption or abnormal kiln operation data
5. Kiln optimization campaign

Basic principles and formulas for calculation

Evaluation of data means to establish a heat balance calculation according to the principle

“input = output“.

Guidelines for Test Duration

1. A long test duration would allow for a good accuracy but the available time is limited by practical considerations.
2. As a rough guideline the minimum test duration should be about ten times the material retention time in the complete kiln system:

Test duration = 10 x retention time

3. If the process is very unsteady longer times should be envisaged.

4. Kiln Operation shall be fixed.

• During the test, the kiln must run at constant and steady conditions.
• Changing of set points should be avoided whenever possible.
• Interruptions have to be logged. If serious problems occur, the test has to be extended or even postponed.
• Therefore it is often worthwhile to plan a certain time reserve.

5. Kiln Data shall be defined

In order to facilitate the final discussion it is usually necessary to collect the main data of the system such as:

• process ∗ type of kiln ∗ nominal capacity
• type of preheater / precalciner data on fans, derives,
• type of cooler ∗ supplier ∗ year of commissioning
• fuel and firing system ∗ type of burner nozzle
• dust reintroduction system
• dimensions of main equipment (sizes,inclinations, etc.)

6. Units / References shall be fixed

• Unit of energy:

1 kJ =1 / 4.187 kcal = 1 kWs

• Reference quantity:

1 kg clinker

• Reference temperature:

20° C

Generally

Calculations

1. Heat Input

Heat from the combustion of coal

Sensible Heat from the coal

Sensible heat of the raw meal feed

Sensible heat from cooler air

Sensible heat from primary air

Sensible heat from false air

Sensible Heat from coal conveying air

Sensible heat from combustion

Sensible Heat from the moisture in the feed

2. Heat Output

Heat of Clinker reaction

Heat loss through the clinker at cooler outlet

Heat loss through the dust in the exhaust gas

Heat loss through evaporation of moisture

Heat loss from exhaust gases

From preheater exit

From Cooler outlet gases

Unaccounted losses

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practical example 

HEAT BALANCE OF UNIT I

Date- 05.07.2019

Time- 12 pm

Basis- 1 kg clinker

Reference temperature- 0⁰ C

Ambient temperature- 40⁰ C

Atmospheric Pressure- 9772 mmWg

HEAT OUTPUTS –

1. Heat of Formation –

Q = 2.2A + 6.48M + 7.646C – 5.1165S – 0.59F

Considering loss free clinker as loss free meal feedstock under equilibrium conditions –

Q = 2.2*5.4 + 6.48*2.52 + 7.646*63.33 – 5.1165*21.3 – 0.59*3.71 = 401.26 kcal/kg clinker

1. Heat in preheater exit dust –

Gross dust loss = 13.2 TPH

Clinker output rate = 132 TPH

Dust in preheater exit dust =

13.2 * 1000

132 * 1000

= 0.1 kg/kg clinker

Mean Specific heat of dust CP_D =

0.206 + 101*427*10^-6 + (-37*427² *10^-9)

= 0.242 kcal/kg/⁰C

Heat Lost in dust =

Qd = md* Cp_d* (Td – Tr)

= 0.1 * 0.242 * (427- 0)

= 10.33 kcal/kg clinker

3) Heat in preheater exit gas –

Pitot tube measurement – 65.47 kg/sec

Clinker production – 3171 TPD

= 36.7 kg/sec

Therefore, mp = 65.47/36.7

= 1.784 kg/kg clinker

Exit gas composition –

CO₂ = 29 %

O₂ = 5.2 %

CO = 0.01 % N₂ = 65.8 %

Converting to weight fraction –

The specific heat of exit gas is the sum of specific heats of individual gas components.

Therefore, Cp_p =

5.05 (0.218 + 30*427*10^-6) +38.75 (0.196 +118*427*10^-6 – 43*427²10^-9 )

100 100

+ 56.18 (0.244 + 22*427*10^-6 )

100

= 0.0116 + 0.238 +0.142

= 0.391 kcal/kg⁰C

Therefore Q_p = 1.784*0.391*(427-0)

= 297.85 kcal/kg clinker.

1. Heat in clinker from cooling discharge –

Q_c = m*Cp_c*( Tc- Tr )

Cp_c = 0.186 + 54*122*10^-6 = 0.192 kcal/kg⁰C

Therefore, Qc = 1*0.193*( 122 – 0) = 23.49 kcal/kg clinker.

1. Heat in cooler exhaust air-

Qc_e = m_ce*Cpe*(Tc_e – Tr)

Pitot tube measurement = 58.17 kg/sec

Clinker output rate = 36.7 kg/sec

Therefore, m_ce= 1.58 kg air/kg clinker

Cp_ce= 0.237 + (23*225*10^-6)

= 0.242 kcal/kg⁰ C

Therefore, Qc_e= 1.58*0.242*(225-0) = 86.01 kcal/kg clinker

1. Heat from cooler to coal mill-

Qc_m = m_cm*Cp_m*(Tc_m – Tr) m_cm= 0.056 kg/kg clinker Cp_cm = 0.247

Therefore, Qc_m = 0.056*0.247*(435-0) = 6 kcal/kg clinker

1. Heat of evaporation of Raw Meal Moisture-

Raw meal feed rate = 220 TPH

Moisture in raw meal = 0.5 %

Therefore, weight of moisture = 1.623*0.5/100 = 0.008 kg water/kg clinker

Heat of evaporation of moisture = 597 kcal/kg

Therefore, heat of evaporation of raw meal moisture = 0.008*597 = 4.776 kcal/kg clinker

1. Heat of evaporation of coal moisture =

Let coal consumption be = x kg/kg clinker

Moisture in coal = 0.8 %

Moisture in coal in terms of kg/kg clinker

= x*0.8/100

= 0.0008x

Heat of evaporation of coal moisture =

.0008x*597

= 4.776x kcal/kg clinker

1. Heat loss due to incomplete combustion –

The heat lost in burning carbon to carbon monoxide instead of CO₂ is 2417 kcal/kg of Carbon Monoxide.

Volume % of CO in waste gases = 0.01

Weight % of CO in waste gases = 0.0085

Amount of waste gases = 65.47 kg/sec

Amount of CO in waste gases = 0.0085*65.47/100

= 0.0056 kg/sec

= 0.0056/36.7 kg/kg clinker

= 0.000152 kg/kg clinker

Therefore, heat loss in incomplete combustion = 0.000152*2417 = 0.367 kcal/kg clinker

10) Radiation and Convection losses –

The surface loss consists of radiation and convection losses.

RAD = 4.87*10^-8*€*( T⁴ – Tø⁴) kcal/hm²

= {4.87*10^-8*0.95*(480⁴ – 313⁴)*99.22} + {4.87*10^-8*0.95*(546⁴ – 313⁴)*533.3}

+ {4.87*10^-8*0.95*(503⁴ – 313⁴)*62}

= 199619.8 + 1955967 + 156087.4

= 2311674.2 kcal/h

CON = {80.33*[(T +T_ø)/2]^-0.724 *(T – T_Ø)^1.333} kcal/hm²

= {80.33*[(480 +313)/2]^-0.724 *(480 – 313)^1.333*99.22} + {80.33*[(546

+313)/2]^-0.724 *(546 – 313)^1.333*533.3} +{80.33*[(503 +313)/2]^-0.724 *(503 – 313)^1.333*62}

= 96212 + 760840 + 701663

= 1558715.2 kcal/h

Total radiation and convection losses from kiln =

2311674.2 + 1558715.2

= 3870389 kcal/h

Kiln Output = 3171*1000/24 = 132125 kg/hr

Therefore specific radiation and Convection losses from kiln shell =

3870389/132125

= 29.29 kcal/kg clinker

On the kiln shell losses, 5% can be added to compensate for heat loss occurring from kiln line rings. So the kiln shell losses =

29.29*1.05

= 30.75 kcal/kg clinker

Radiation Losses from preheater (kcal/hr) –

Convection Losses from preheater (kcal/hr) –

Total radiation and convection losses from preheater = 1372747.35 kcal/h

Specific Radiation and Convection losses from preheater = 1372747.5/132125 = 10.38 kcal/kg clinker

Taking radiation and convection losses from cooler as 5 kcal/kg clinker,

Total radiation and convection losses = 30.75 + 10.38 + 5

= 46.13 kcal/kg clinker

HEAT INPUTS –

1. Heat of coal consumption = 7456x kcal/kg clinker
2. Heat in kiln feed –

• 1. = m_f*Cp_f*(T_f – Tr)

Cp_f = 0.206 + 101*75*10^-6 = 0.213 kcal/kg⁰C m_f = 220 TPH = 220/132.125

= 1.66 kg/kg clinker

Therefore, Q = 1.66*0.213*(75 – 0) = 26.16 kcal/kg clinker

1. Heat in cooling air –
   1. = m_c*Cp_c*(Tc- Tr)

Cp_c = 0.237 kcal/kg⁰C

Therefore, Q = 2.47*0.237*(40 – 0)

= 23.5 kcal/kg clinker

1. Heat in primary air –

• 1. = m_pc_p(Tp – Tr)

m_p = Total primary air (kiln and calciner)

= 0.041 kg/kg clinker

Tp = 40⁰C

Cp = 0.237 + 40*23*10^-6

Therefore, Q = 0.041*0.2379(40 – 0) = 0.39 kcal/kg clinker

1. Sensible heat of fuel –

• 1. = m_fuel*Cp*(T_fuel – Tr)

= x*0.289*(70 – 0)

= 20.23x kcal/kg clinker

HEAT BALANCE –

Total heat inputs –

= 7456x + 26.16 + 23.5 + 0.39 + 20.23x

= 7476.56x + 50.05

Total heat outputs-

= 401.26 + 10.33 + 297.85 + 23.49 + 86.01 + 6 + 4.776 + 4.776x + 0.367

+ 46.13

= 876.21 + 4.776x

Calculation of heat consumption –

Total heat inputs = Total heat outputs

7476.56x + 50.05 = 876.21 + 4.776x

=> 7471.78x = 826.16

=> x = 0.1105

Heat consumption (from coal consumption) =

0.1105*7456

= 824 kcal/kg clinker

FALSE AIR MEASUREMENT IN UNIT- II

Date – 30.06.06

Time – 3.05 pm

O₂ at Preheater outlet –

Pyro String = 4.1 %

Kiln String = 5.8 %

O₂ at kiln inlet – 3.84 %

O₂ at calciner outlet – 2.6 %

Now, false air at the kiln-string = 5.8 -3.84

20.9 – 3.84

= 11.48 %

False air at the pyro-string = 4.1- 2.6

20.9-2.6

= 8.19 %

CHARACTERISTICS OF ENERGY CONSUMPTION IN

CEMENT PRODUCTION

ENERGY CONSUMPTION –

The cement industry is said to be an energy intensive industry together with steel, paper and petrochemical industries. The percentage of energy cost in Portland cement production cost is 20 to 30 %.

Compared percentage of fuel consumption by use –

AREAS OF ENERGY WASTAGE –

The main areas of energy wastage are-

• High exit flue gas temperature for preheater and cooler exhaust.
• High clinker temperature from cooler.
• False air infiltration in kiln, preheater and ESP circuits.
• Improper combustion.
• High Radiation losses from kiln and preheater.

SUGGSTIONS AND RECOMMENDATIOS FOR

IMPROVEMENT OF THERMAL EFFICIENCY

COGENERATION OF POWER THROUGH WASTE HEAT RECOVERY –

The preheater exhaust gas has about 20 % of the heating value brought in by the fuel. In the Unit I of SCL the temperature of the preheater exhaust gas is about 450⁰C and it is about 250⁰C in Unit II. Upto 25-30 % of total power requirement can be met through cogeneration of power utilizing waste heat in a cement plant.

A circuit diagram has been shown below for the use of preheater gas for power cogeneration –

COGENERATION OF POWER BY PREHEATER WASTE GAS

The preheater gas can be used for power cogeneration as shown in the diagram above. A part of the preheater exhaust gas passes to the raw mill and the rest is sent to a separator for separating dust from the gas. The clean gas is used in the High Pressure Boiler for generation of steam which is used in the turbine for electricity generation. The cooled gas is sent to the ESP for precipitating the dust.

There are also some barriers in introduction of cogeneration systems in Indian

Cement Industry. They are-

1. High cost of imported technology and difficult access to funds.
2. High cost of capital without any fiscal incentive. iii) Reluctance of funding agencies to fund an untried technology in India.

iv) Contemporary barriers in some states for putting up captive power generation units.

Nevertheless, the system promises to return the capital investment within a short period.

COPROCESSING OF WASTES –

There is a huge potential for waste coprocessing in the Indian Cement Industry. There is a need for waste management strategy in India.

GENERATION OF WASTES –

Almost all industries- petroleum/automobile/power/steel/chemical/mineral and many more generate wastes which can be classified as –

1. Liquids hazardous/non-hazardous with fuel value
2. Solids hazardous/non-hazardous with fuel value
3. Sludges without fuel value

SOURCES OF WASTES –

• Metallurgical, steelmaking industries.
• Industrial and port/dock cleaning.
• Mechanical and automobile industries.

TYPES OF HAZARDOUS WASTES –

• Waste oil.
• Oil containing residues.
• Oil emulsions.
• Paints, adhesives and varnishes

USE OF WASTE IN CEMENT INDUSTRY AS FUEL–

To satisfy the total and safe destruction of the wastes, the system has to reach the following conditions –

850⁰C at 6% O₂ if the waste contains less than 1 % chloride.

By contrast, coprocessing achieves a more beneficial environmental impact by reclaiming waste materials. The very high retention time, high temperatures and high turbulence in kiln ensure complete combustion of the wastes. Coprocessing facilitates use of 10-50 % of wastes as fuel. The cement kiln has the capacity to burn wastes due to –

• High temperature profile.
• High turbulence.
• Long residence time.
• Sufficient oxygen in the system
• Capacity of the end product (cement) to absorb the metals without affecting its quality.
• Cement production is an energy intensive process. It has potential to accept wastes which are otherwise not accepted by incinerator.

Shree Cement has a huge scope for coprocessing wastes and thus conserve the conventional sources of energy.

USE OF MINERALIZER –

By using mineralizer along with feed the fuel consumption can be brought down as the mineralizer helps in reducing the clinkerisation temperature. The results of an experiment in a kiln where 0.72 % of baryte is used as mineralizer are shown below –

TPD of kiln – 3700 Addition of barite – 0.72 % Kiln feed per day – 5730 MT.

Baryte required per day – 42 MT

Additional cost of baryte – Rs. 62000 per day

COAL AND RAW MATERIAL SAVING POTENTIAL –

Reduction in kiln feed required per day- 42 MT

Reduction in clinkerisation temperature – 55⁰C

Coal consumption without baryte per day – 725 MT

Coal consumption with baryte per day – 678 MT Coal saving per day – 47 MT

SCOPE FOR INCREASE IN PRODUCTION –

Total air required per day without baryte – 5430 kg

Total air required per day with baryte – 5070 kg

Reduction in combustion air – 6.6 %

Scope for increase in production – 6.6 %

If the use of mineralizer is implemented in Shree Cement a lot of raw material and fuel can be saved and production can also be increased.

INSTALLATION OF ONLINE ANALYSER –

To produce a good quality product and to maintain optimal and efficient condition in the kiln, it is crucial that the raw meal is completely homogenized. Quality control starts in the quarry and continues to the blending silo. Online analyzers for raw mix control are an integral part of the quality control system. If online analyzers are installed then the raw mix can be optimized and thus the thermal energy can be used to the optimum.

CSIRO worked in collaboration with Fuel and Combustion Technology (FCT) to develop two key online analyzer systems- an on-conveyor belt bulk elemental analyzer for the raw materials and an on-line analyzer for determining the composition and phases in the cement. These analyzers were designed to improve cement processing in the Australian Cement Industry.

The CSIRO X-belt analyzer (XENA) can be used to control the raw mix composition of cement. It can be positioned directly on the conveyor belt transporting the raw material to the mill. XENA can accurately measure the concentrations of calcium, silicon, aluminium, iron and minor elements independent of both horizontal and vertical segregation and independent of changes in belt loading. XENA is based on a newly-developed fast neutron and gamma ray technique that uses highly penetrating radiation so that measurements can be averaged over a large volume of material on a conveyor belt.

INSTALLATION OF DEFLECTOR PLATES IN PREHEATER CYCLONES –

If the III Stage and IV Stage Cyclones are provided with Deflector Plates the hot gases along with the kiln feed material would be guided by the Deflector Plates to obtain better cyclonic effect (Whirling Motion). The turbulence would be more which would result into better heat transfer as well as reduction in pressure drop in both the cyclones. Thus the radiation losses and preheater exhaust gas temperature would come down ant the load on ID fan would also decrease.

IMPROVEMENT OF BURNING CONDITIONS IN CALCINER –

An analysis of GCT inlet sample of Unit-I has shown the following results –

The analysis above shows that calcined material is getting carried away with the gas.

Also as the calcination is almost negligible in cyclone I, the calcined material is coming from bottom. This puts a question on the efficiency of C-II, C-III and C-IV. Fuel is also coming out from the top. This means fuel is also getting carried away with the gases from the calciner and it is very much possible that this fuel is getting burnt in C-III, C-II or C-I. This will certainly increase the exhaust gases temperature.

As the highly calcined material resides in calciner, C-IV & C-III, this material is getting carried away from these sections. Further more in the course of traveling it is getting carbonized as the gas which carries the calcined material is also having CO₂. The reaction between calcined material and CO₂ is exothermic so it leads to high temperature in the exhaust of preheater. Calcined material from these sections is going upwards with air and this CaO is not getting separated. Even though a part of calcined material is getting carbonized we get nearly 4 % CaO at the exhaust. So separation efficiency of C-III and C-II needs to be improved.

It is found that 1.3 % fuel is there at the exhaust. This suggests calciner burning conditions should be improved. For improving the calciner burning conditions we should use finer fuel. For proper mixing of fuel and air we can have a swirl vane installed in the burner. Even increasing calciner length can serve the purpose to certain extent.

MODIFICATION OF PREHEATER CYCLONE –

The following are the advances in preheater designs which can be implemented in the Unit I of Shree Cement for better heat transfer – – Low pressure drop Cyclones

• Enlarged inlets and outlets
• Sloped shelves
• Favorable inflow geometry
• Gas velocity in preheater cyclone duct in the range of 10-15 m/sec

Here is the diagram of FLS Low Pressure drop Cyclone-

USE OF BOILER ASH IN FEED –

Mr. Bhatty and others in year 2000 & 2001 demonstrated the use of a fly ash that contained approximately 20 % unburned carbon at a preheater kiln. The composition of the fly ash used in the trial run is shown in the table below –

OXIDE COMPOSITION OF THE HIGH CARBON FLY ASH –

The high carbon fly ash was blended with the raw materials (crushed limestone and a small amount of shale) and ground into raw feed. The composition of the raw feed was targeted to the normal raw feed. The fly ash composition and that of other raw materials (limestone and shale) limited the fly ash substitution to 6 %. The total heat content (calorific value) of the ash was estimated to be greater than 740 KJ/kg, which translated to an anticipated energy contribution from the fly ash of 16 kcal/kg of clinker.

During the demonstration, the operation ran in a more efficient, stable and predictable manner. Consequently, the cement plant incurred a fuel savings of nearly 4 %, and the clinker production increased by almost 10 %.

Characteristics of clinker produced during the demonstration as determined by XRF, XRD and free lime tests are shown below. The XRF analysis indicates lower sulphate in the fly ash clinker than in the normal one.

OXIDE COMPOSITION OF CLINKER –

The results and ASTM C150 standard physical requirements for cements presented in table indicate that the fly ash used cement is comparable to the commercially produced cements.

ASTM C150 DATA FOR CEMENTS PRODUCED BEFORE AND DURING THE DEMONSTRATION –

A similar study for Electric Power Research Institute (EPRI) was carried out by

Bhatty and others (1998) on several high-carbon fly ashes (carbon ranging from 6 % to 16 %) that produced similar results. The ashes were used as a component for raw feed collected from different cement plants across the US. Clinkers and cement were produced from these mixes and tested. The data indicated that clinkers had normal formation and distribution of the major clinker phases, and the cement had strength and setting properties comparable to that produced at the actual cement plants.

A cement plant in Canada has also used upto 10 % high carbon fly ash as a raw kiln feed component. The primary motivation for the plant was fuel savings.

USE OF BOILER ASH IN SHREE CEMENT –

Shree Cement produces around 150 MT of boiler ash from its captive power plant. The analyses of the boiler ash are as follows –

Proximate analysis – (in %)

Chemical analysis –

Comparing the composition with the previous fly ash there is not much difference between the two. SO3 content is little higher but it will get absorbed in the clinker and get reduced. The calorific value is 5147 Kcal/kg and its carbon content is 26 % compared to 20 % in the previous fly ash. Therefore if 6 % of boiler ash is used in the feed then fuel savings of about 6 % can be made and clinker production would increase by 10 %.

Thus it would be a huge benefit from the energy efficiency point of view.

Attention!! to Download Most Important Books in Cement industry reviewed from more than two thousands people + Manuals and noted from big Cement Companies like F L  S , Holcim Lafarge , Cemex , + Excel sheets for calculations process Click here Now