Process Description
Overview of Rotary Kiln
Simtronics’ Rotary Kiln simulator consists of the following equipment:
- Feed Bin
- Process Water Tank & Pumps
- Rotary Kiln
- Primary Air Fan
- Coal Bin – Burner
- Cooling Air Fans (3)
- Product Cooler
- Excess Air Cyclone & Fan
- Product Conveyor
- Flue Gas Spray Cooler
- Electrostatic Precipitator
- Induced Draft Fan
- Stack
The Feed Bin, V-101, transfers feed into the feed end of the Rotary Kiln, F-101, by gravity using a rotary valve.
The Rotary Kiln, F-101, is a very long, rotating, large-diameter tube, which is outfitted with Burner, B-101, opposite the feed end. The tube is lined with brick and refractory lining to keep heat within the kiln and to protect the frame of the kiln. Dry or wet feed from the Feed Bin, V-101, is heated by the burner’s hot flue gas flowing within the tube countercurrent to the feed material. The kiln tube is angled slightly downward from the feed end to the burner end so that the rotation of the tube will cause the feed material to tumble and slowly move to the burner end. The kiln is rotated by a drive system which uses a variable speed electric motor. Any water in the feed is evaporated in the first section of the kiln. The contacting of feed and flue gas is enhanced by an internal set of chains which pull the feed material through the flue gas. In the next section, heating to the calcining reaction temperature (1,800 DEG F) takes place in the next section. The calcining reaction takes place in the center section and in the sections near the burner end. The calcined product exits the kiln and drops into the Product Cooler, E-101.
Hot product from the Rotary Kiln is collected and moved along a moving grate system within the Product Cooler, E-101. Cooling air from three Cooling Fans, G-101, G-102 and G-103, flows into separate distribution chambers under the moving grates. The grates are specially designed to allow air to infiltrate through the hot product moving along the top of the grates without the product material entering into the air distribution chambers below. Cooled product exits the far end of the Product Cooler and falls onto the Product Conveyor, X-102, which transports the product material to storage. The design product flow rate is 13.62 T/H.
Most of the hot air from the Product Cooler combines and flows into the Kiln Hood, H-101. A portion of the air from the cooler end of E-101 is taken off as excess air through the Excess Air Cyclone, V-105, and through the Excess Air Fan, G-105, which sends the warm excess air to other units outside the Rotary Kiln unit. Most of the hot air from the Kiln Hood flows into the Rotary Kiln and is equally distributed around the Burner, B-101 as secondary air. A portion of the hot air in the Kiln Hood is taken off to the Primary Air Fan, G-104, where it is routed to the Burner, B-101.
A sealing system on the Kiln Hood minimizes any unmetered ambient air infiltration through the gap between the Kiln Hood and the Rotary Kiln. The Kiln Hood is operated at a slight vacuum to ensure hot flue gas from the Burner does not flow out through the sealing system. This direct air infiltration is commonly called false air and excessive amounts result in lower thermal efficiency of the Rotary Kiln because this air bypasses the Product Cooler which recovers a sizeable portion of the heat from the hot product. The slight vacuum conditions in the Kiln Hood keep the false air infiltration flow to a minimum, while maintaining safe conditions around the Rotary Kiln.
The hot air from the Primary Air Fan, G-104, will entrain any coal being fed to the Burner, B-101, from the Coal Bin, V-102. In addition to entraining any coal fuel, the Burner is specially designed to use the primary air to shape the burner flame so that the flame does not impinge on the walls of the Rotary Kiln. This shaping, combined with secondary air directly entering the Rotary Kiln from the Kiln Hood, results in a longer, narrower flame that extends into the Rotary Kiln. This shaping also helps avoid overheating the hot material exiting the Rotary Kiln, which could result in unwanted reactions in and physical property degradation of the product. Natural gas is also routed to the Burner through specially designed ports. An ignition system inserted into the Burner maintains essentially instant re-ignition of the burner flame in the event of a flameout.
Warm flue gas containing dust from the processing of the feed in the kiln leaves the feed end of the Rotary Kiln at around 590 DEG F and enters the Feed End Hood, H-102, which is ducted to the Flue Gas Spray Cooler, E-102. The flue gas must be cooled to around 350 DEG F in order not to exceed design temperature limits in the downstream Electrostatic Precipitator, EP-101, which removes most of the dust in the flue gas. Process water is injected into an array of specially designed spray orifices in the Flue Gas Spray Cooler so that the injected water is finely dispersed into the warm flue gas so the water evaporates and quickly cools the flue gas without forming a mixture of wet dust, which would then build up on the equipment in the flue gas treating section.
Cooled flue gas from the Flue Gas Spray Cooler enters the Electrostatic Precipitator, EP-101, which removes nearly all the dust in the flue gas using a set of electrically charged grids to attract the dust. The dust accumulates on the grids and the accumulated dust is shaken off the grids by vibrating the grids and falls into an array of collection hoppers connected to the base. The collected dust is transported to storage for reuse or product blending depending on its composition and properties.
Flue gas from the Electrostatic Precipitator flows into the Induced Draft Fan, G-106, which pulls the flue gas from a vacuum condition into the Stack, S-101, for discharge to the atmosphere. Vacuum conditions must be maintained throughout the Rotary Kiln Unit from the Product Cooler, E-101, through the Electrostatic Precipitator, EP-101, to avoid blowing hot gases and dust out of the unit through the seals on the two hoods attached to the Rotary Kiln, F-101.
Feed Bin, V-101
The Feed Bin, V-101, is conically shaped and normally stores about ten minutes worth of feed. The inventory of the Feed Bin is assumed to be regulated at a reasonable level on the simulator. Also, the Feed Bin and its outlet rotary valve, WV-104, is assumed to be capable of handling both wet and dry feeds. The rotary valve, WV-104, pushes the feed material into the feed end of the Rotary Kiln. The design feed rate to the unit is 30.0 T/H.
In the design operating condition on the simulator, the feed is characterized as consisting of:
- 70 weight % Calcium carbonate (CaCO3, limestone)
- 25 weight % water
- 5 weight % inert material
The normal feed temperature is 80 DEG F.
Water Tank, T-101
Water from makeup treating at battery limits flows Water Tank, T-101, through control valve LV-112 at ambient temperature. The Water Tank normally holds about one hour’s worth of inventory at design water usage flow of 15.2 GPM.
Water Pumps, P-101A/B
The Water Pumps are electric motor driven centrifugal pumps that move water from the Water Tank to the Rotary Kiln process. Normally, water is only used in Flue Gas Spray Cooler, E-102, at design operation through control valve FV-114. However, water can also be pumped directly into the Rotary Kiln, F-101, as needed to control the temperatures in the kiln nearer the feed end. This is done during startup prior to starting feed into the kiln and at shutdown after removing feed from the kiln.
Feed End Hood, H-102
The Feed End Hood connects to the feed end of the Rotary Kiln, F-101. A sealing system between the rotating kiln and the hood minimizes air infiltration into the kiln while allowing the kiln to rotate. This sealing system also allows for thermal expansion of the Rotary Kiln as it heats up and cools down. The Feed End Hood routes warm flue gas from the Rotary Kiln into the Flue Gas Spray Cooler, E-102. The feed pipe is inserted through one end of the hood and routed so it discharges directly into the Rotary Kiln. Piping for water going to the Flue Gas Spray Cooler is also routed through the hood. This warms the water prior to injection so it evaporates more easily.
Flue Gas Spray Cooler, E-102
Warm flue gas containing entrained dust from the processing of the feed in the Rotary Kiln leaves the feed end of the Rotary Kiln at around 590 DEG F, passes through the Feed End Hood and enters the Flue Gas Spray Cooler, E-102. The flue gas is cooled to 350 DEG F in order not to exceed design temperature limits in the downstream Electrostatic Precipitator, EP-101. Process water from Water Pumps P-101A/B is warmed while passing through warm flue gas in the Feed End Hood and is injected into an array of specially designed spray orifices in the Flue Gas Spray Cooler so that the injected water is finely dispersed into the warm flue gas. This ensures the water quickly evaporates and cools the flue gas without forming a mixture of wet dust, which would then build up on the equipment in the flue gas treating section.
Rotary Kiln, F-101
The Rotary Kiln is 300 feet long, rotating, large-diameter tube, which is outfitted with Burner, B-101, opposite the feed end. The tube is a steel shell that is lined with brick and refractory lining to keep heat within the kiln and to protect the shell of the kiln. Dry or wet feed from the Feed Bin, V-101, is heated by the burner’s hot flue gas flowing within the tube countercurrent to the feed material. The kiln tube is angled slightly downward from the feed end to the burner end so that the rotation of the tube will cause the feed material to tumble and slowly move to the burner end. The kiln is rotated by a drive system which uses a variable speed electric motor. The kiln normally rotates at 1.0 RPM. The kiln has four large rings on the outside that are supported by rollers that keep the extremely heavy kiln from bending along its length.
Any water in the feed is evaporated in the first section of the kiln, which is outfitted with an array of chains attached to the inside of the kiln tube. As the kiln rotates, these chains will pass through the wet feed coming into the kiln and lift the material. This improves the heat transfer and drying of the feed within the kiln. The wetted chains also help absorb any dust moving in the flue gas coming from the hot section into the wet feed, thereby reducing the dust load on the Electrostatic Precipitator, EP-101. The evaporation of water in the feed helps keep the flue gas outlet temperature below 600 DEG F.
The dried feed leaves the chain section and heats up to reaction temperature as it moves down the kiln by exchanging heat with hot flue gas. In order for the calcining reaction to take place, the temperature of the dried feed must be higher than 1,800 DEG F. The calcining reaction takes place in the center section and in the sections near the burner end. The calcining reaction also absorbs heat (the reaction is endothermic). Therefore, the temperature of the material in the kiln will not increase as much in this part of the kiln when compared to the feed end sections.
The calcined product exits the kiln and drops into the Product Cooler, E-101. The temperature of the product leaving the Rotary Kiln is normally 2,135 DEG F for lime production. To simulate cement production with the simulator, the product temperature can be increased by increasing the fuel gas rate or switching to coal as a fuel.
Burner, B-101, is inserted through the Kiln Hood, H-101, and into the Rotary Kiln. The combustion of the fuel, normally natural gas, takes place within the kiln. Secondary air, which has been heated in the Product Cooler, flows from the Kiln Hood in the annular space between the Burner assembly and the inner surface of the kiln. The secondary air mixes with the combusted fuel and primary air leaving the Burner to provide the oxygen to totally combust the fuel as it travels into the kiln. This concentric flow of secondary air, combined with a constant flow of primary air through the Burner that provides a fairly high gas velocity at the outlet of the burner, shapes the flame to be narrower so it extends far into the kiln. This longer flame better distributes heat to the material traveling through the kiln so that hot spots are minimized. Overheating of the product leaving the kiln can lead to the formation of less reactive forms of the product, depending on what product is being made.
Kiln Hood, H-101
Heated air from the Product Cooler, E-101, directly enters the Kiln Hood, H-101 at 663 DEG F. A majority of this air flows directly into the Rotary Kiln, F-101, through the annular space between the Burner assembly and the inner surface of the kiln. Some of the air is taken off the hood to Primary Air Fan, G-104, to be injected directly into the Burner, B-101. The Kiln Hood normally operates at slight vacuum (-0.2 INH2O) so that hot air or very hot flue gas does not flow out through the sealing system between the Kiln Hood and the Rotary Kiln. As a consequence, a small amount of cool, ambient air flows in through the sealing system.
Primary Air Fan, G-104
The Primary Air Fan, G-104, takes hot air from the Kiln Hood and blows it to the Burner, B-101. A cold air bleed damper, TV-130, is normally closed, but opens automatically to keep the inlet temperature to the Primary Air Fan below 700 DEG F so as not to exceed the mechanical limits of the fan. The flow rate of primary air is kept at a constant value of 135 MSCF/H to help extend the center of the flame from the Burner well into the Rotary Kiln. This flow rate is also high enough to fluidize and transport any pulverized coal being used as fuel to the Burner. The Primary Air Fan has a variable speed electric motor which is used to control the primary air flow.
Burner, B-101
Pulverized coal is fed to the primary air line to Burner, B-101, from Coal Bin, V-102, through rotary valve WV-132. Normally, no coal is being used as fuel in the design condition of the Rotary Kiln simulator. The Burner is a dual-fuel burner which normally uses natural gas that flows from battery limits through control valve FV-134 and directly into the burner. The fuel gas is radially distributed in the burner along with the primary air and any fluidized coal it is carrying. The design flow rate of natural gas to the Burner is 87.0 MSCF/H. The Burner is outfitted with a cross-fire ignition system to ensure quick re-ignition of the burner in case of a flameout. An interlock system stops both of the fuels in case of low primary air flow, a loss of the ignition system, or a stoppage of the motor of the Induced Draft Fan, G-106. The primary air flow is insufficient to provide for complete combustion of the fuels to the Burner. The secondary air entering the Rotary Kiln directly from the Kiln Hood is kept sufficiently high to completely combust the fuels. Refer to the Instrumentation section below for details on how this is accomplished on the Rotary Kiln simulator.
Cooling Fans, G-101, G-102 & G-103
The cooling fans G-101, G-102 and G-103, take ambient air and blow it into the Product Cooler, E-101, to cool the hot product flowing from the Rotary Kiln, F-101. Recovering this heat improves the thermal efficiency of the Rotary Kiln Process. The heated air is then routed to the Kiln Hood, H-101, for use as primary and secondary combustion air. A smaller portion of the warmed air from the Product Cooler is taken off to battery limits for warming and drying purposes. This air is called excess air.
The flow rate of air is the same in each of the fans. The normal flow rate is 353 MSCF/H. The design flow rate of excess (warming) air is 99 MSCF/H. Therefore, the net combustion air routed to the Kiln Hood is normally960 MSCF/H.
Each fan is driven by an electric motor with a variable speed drive for the purpose of flow control.
Product Cooler E-101
Hot product is discharged from the Rotary Kiln at 2,135 DEG F is collected and moved along a moving grate system within the Product Cooler, E-101. The speed of oscillation of the grates is adjustable. Cooling air from the three Cooling Fans, G-101, G-102 and G-103, flows into separate distribution chambers under the moving grates. The grates are specially designed to allow air to infiltrate through the hot product moving along the top of the grates without the product material entering into the air distribution chambers below. Cooled product exits the far end of the Product Cooler at 293 DEG F and falls onto the Product Conveyor, X-102, which transports the product material to storage. The design product flow rate is 13.62 T/H.
A portion of the heated air from Chamber No. 3 is taken off as excess air to the Excess Air Cyclone, V-105. The temperature of this air is 293 DEG F. The balance of the air from Chamber No. 3 is combined with the hot air from Chamber No. 2 and Chamber No. 1 and is routed to the Kiln Hood, H-101. The temperature of this combined air is 663 DEG F.
In cement manufacturing, the Product Cooler is often called a “clinker cooler” because the product from a rotary kiln in a cement is termed “clinker”.
Product Conveyor, X-102
The Product Conveyor, X-102, is a rubber belt conveyor that transports cooled product to storage. The conveyor is electric motor driven and runs at constant speed. A weighing meter is built in to it to monitor the production rate.
Excess Air Cyclone, V-105
The Excess Air Cyclone, V-105, removes any entrained product dust from the excess air leaving the Product Cooler and returns it to the Product Conveyor, X-102 by gravity. The cleaned, warm excess air flows to the Excess Air Fan, G-105.
Excess Air Fan, G-105
The Excess Air Fan, G-105, moves warm excess air from the cool end of the Product Cooler to the users at battery limits. The design flow rate of warm excess air is 99 MSCF/H. The Excess Air Fan is driven by an electric motor with a variable speed drive for the purpose of flow control.
Electrostatic Precipitator, EP-101
Cooled flue gas from the Flue Gas Spray Cooler, E-102, enters the Electrostatic Precipitator, EP-101, at 350 DEG F. EP-101 removes nearly all the dust in the flue gas using a set of electrically charged grids to attract the dust. The dust accumulates on the grids and the accumulated dust is shaken off the grids by vibrating the grids and falls into an array of collection hoppers connected to the base. The collected dust (fines) is transported to storage for reuse or product blending depending on its composition and properties. Normally about 0.70 T/H of fines is recovered. Removing these fines will keep the opacity of the flue gas as low as possible. When coal is used as fuel, additional fines are created because a small portion of the coal is mineral in nature and will not combust. This mineral portion that ends up as a fluidized solid in the flue gas is called fly ash.
Most of the pressure drop in the flue gas treating equipment occurs in the Electrostatic Precipitator. Normally, the flue gas exits the Rotary Kiln at a vacuum pressure of -2.0 INH2O. The pressure drop across the Electrostatic Precipitator is normally 1.80 INH2O and the outlet pressure is at a vacuum pressure of -4.00 INH2O.
The Electrostatic Precipitator has an interlock that will stop power to EP-101 in case of a high outlet temperature. This is done to prevent damage to the electric grids.
Induced Draft Fan, G-106
Flue gas from the Electrostatic Precipitator flows to the Induced Draft Fan, G-106, which pulls the flue gas from a vacuum condition into the Stack, S-101, for discharge to the atmosphere. Vacuum conditions must be maintained throughout the Rotary Kiln Unit from the Product Cooler, E-101, through the Electrostatic Precipitator, EP-101, to avoid blowing hot gases and dust out of the unit through the seals on the two hoods attached to the Rotary Kiln, F-101. The induced Draft Fan is driven by an electric motor with a variable speed drive for the purpose of inlet draft control.
The inlet temperature to the Induced Draft Fan is normally 346 DEG F. In case of a severe temperature increase in the Flue Gas Spray Cooler, E-102, a cold air damper, HV-167, can be opened to mix cool ambient air with hot flue gas so as to keep the Induced Draft Fan from being subjected to very high temperatures. The cold air damper is also used when starting the Induced Draft Fan.
Stack, S-101
Flue gas from the Induced Draft Fan is received near the base of the Stack, S-101. The stack effect of warm flue gas rising in the Stack helps move the air out to the atmosphere at the top of the stack.