LIME KILN PROCESS (NUNG ĐÁ VÔI)

Lime kiln producing process The calciner – ver t ical shafts PFR lime kiln types of PFR lime kiln The layout of circular PFR kiln bag filter stack reversible belt- conveyor kiln stack weighing hopper (top) bur n er lances crossover channel optical ther m ometer rotar y hopper fuel piping bag filter combustion air piping inducing fan suspended cylinder control room air shock blower discharging table MCC room blower house hydraulic unit Lime cooling air skip skip bucket weighing hopper (bottom) What is PFR PFR is the abbreviation of Parallel Flow Regeneration ● limestone Parallel Flow ： Limestone is fed from the top of the kiln and lime discharg es from the bottom Combustion direction is from the top of the burn ing zone to the bottom So the limestone flow and the combustion flow are parallel ● Regeneration ： It means heat-reuse Hot off-gases from the bur n ing shaft will flow through the crossover channel then exhaust from the top of the non-bur n ing shaft Wh ile ● lime Characteristics of PFR lime kiln ● ● ● ● ● The standard PFR-Kiln is a kiln with twoshafts which are connected by a crossover channel and defined by alternating burning and non-burning shaft operation The regenerative process requires two connected shafts One shaft operates in the burning mode and Simultaneously , the other shaft operates in the nonburning or exhaust mode In burning shaft, combustion gases flow and raw stone flow are parallel, whereas in non-burning shaft, off-gases flow and raw stone flow are counter-current All the combustion air will be preheated in the regenerative process Each shaft spends an equal amount of time in both the burning and non-burning modes of operation The advantages of the PFR process ● ● ● ● In the parallel flow regenerative kiln the combustion air is preheated in an ideal manner The regenerative preheating of the combustion air makes the thermal efficiency of the kiln practically independent from the excess combustion air factor This considerably simplifies the setting of the correct length of the flame to produce the desired quality of soft-burnt lime This kind of regenerator is completely insensitive to dust-laden or corroding gases and, at the same time, shows excellent heat transfer characteristics Because of the conditions created by the parallel flow of the stone and the combustion gases in the burning shaft, the kiln is ideally suited to produce soft-burnt, high reactive lime The temperature profile of the PFR kiln ● ● ● In parallel flow kilns the fuel is introduced at the upper end of the burning zone and the combustion gases travel parallel to the material The left side picture shows a typical temperature profile Green line means material, blue line in the preheating zone means the combustion air, the blue line in the cooling zone means cooling air, the red line means combustion gas and kiln off-gas As the fuel is injected at the upper end of the burning zone where the material can absorb most of the heat released by the fuel the temperature in the burning zone is typically 950°C in average Because of this, parallel flow heating is the best solution for the production of soft-burnt, reactive lime The operating principle of the PFR kiln We would like to introduce the operating principle of the PFR kiln by parts : Charging process Preheating process Calcining process Cooling and discharging process Charging process handling system charging system Limestone charging equipments are divided into two systems: ● handling system ● charging system Air canon max pressure : 10 bar Operation temperature : from - 10 to 80 °C Canon surface : hot-dip galvanized, in- and outside Capacity of tank : 100 liter Tank diameter : 500 mm Piston diameter : 125 mm Voltage : 24 V DC, Voltage tolerance : +/- 10% Temperature profile of c rossover channel An important objective for kiln operation is to control the temperature in the calcining zone in a consistent, uniform manner ● ● gases temperature in the crossover channel area lime temperature in the crossover channel area Temperature within calcining zone can vary due the stone grain size, the chemical analysis of the stone, variations in the amount and distribution of air flow, and variations of the heat value of the fuel Animation of gases flow in crossover channel Suspended cylinder old design ( 300 TPD PFR kiln ) new design ( exceed 500 TPD PFR kiln ) In order to : 1.increase the working volume of the kiln; 2.decrease the use of refractory bricks ( refractories are expensive ); 3.improve the flowability of crossover channel; 4.avoid lime ash depositing and agglomerating to block the crossover channel; we use suspended cylinder to replace the old design ● Gas flow without obstacle Advantages of suspended cylinder ● Best lime quality of all PFR lime ● kilns cooling air duct (metal structure) a nchorin g Smaller surface area in the gas ● channels for dust to be deposited prefabricated refractory bricks Unproblematic channel cleaning ● ● High availability of the kiln ● Free ver t ical expansion of suspended cylinder, no rigid restraints Insensitive to high horizontal loads ● from material bed anchoring ● Smaller amount of refractor y material required suspended cylinder cooling air Th e refractor y material rests on an air cooled steel cylinder ● Cooling air maintains the steel cylinder within a ● safe temperature range and leaves it at a temperature of about 200°C To recover this heat, : cooling air (inlet) the cooling air is : cooling air (outlet) used as combustion ● Original / Updated design of suspended cylinder Screwing and welding many steel plates to ● make brick molds Then pouring refractor y material into molds to make suspended cylinder bottom bricks Th e constr uction quality affect the lifetime of bricks directly ● Th is kind of refractor y is easy to be broken original by falling limestone design ● Without refractor y protecting, the steel Suspended cylinder bottom bricks are structure precastable of suspended cylinder could not bear the ● ● ●The quality, hardness and tensile strength of high precastable temperature bricks of bur nare ingbetter zone than original design Reduced open porosity – to reduce the effects of chemical infiltration ● updated ● Updated Design has advantages of rapid design change, small segments need to be replaced, repair is more easier and can be done by a local company Using slide rail to assemble the bottom refractory brick to the suspended cylinder, this rapid way is more easier for bricks changing and could reduce the maintenance down time ● slide rail steel plate Th e anchorings not only could make a good combination between steel plate and refractor y material but also improve the tensile strength of the brick ● ● ● anchorin g Special steel grades for the anchorings, to deal with the temperature, stresses and the chemical attack Refractory material optimized for the atmosphere of the lime kiln (sulfur, alkaline,…) Cooling and discharging process ● Th e range of cooling zone: From the lower end of the suspended cylinder to the top end of the distribution cone Wh en limestone leave the burn ing zone, it means calcination has already completed and most of limestone has been tur n ed to lime product ● cooling zone distribution cone Although leaving bur n ing zone, lime has the temperature of 1000 °C approximately around the top of the cooling zone In ord er to prevent discharging and conveying devices being damaged from high temperature, an important ● Lime cooling air is introduced from the discharg e hopper then flow upward to discharg e table, finally enter the cooling zone ● ● ● ● The calcined lime is continuously discharged from both shafts A roof-like distribution cone is constructed above the discharge table to maintain lime flow and discharge This kind of PFR lime kilns use two crosswise hydraulically operating tables - discharge tables to help lime discharging : lime discharging direction : lime cooling air direction ● ● ● Discharging hopper are situated underneath each discharge table to collect the lime discharged from the tables during the 720 second burning period Discharging hoppers are sealed by airtight, hydraulically operated traps During each reversal period the traps open, lime drops into the pressurefree receiving hopper and is then discharged by vibrating feeders The rate of discharge is automatically regulated by the stone level control system located in the preheating zone ● : lime discharging direction : lime cooling air direction Vertical view of distribution cone distribution cone Side view of discharging table distribution cone rolling cylinder rolling cylinder : lime discharging direction discharging table Working principle of discharging table ● ● ● There are two sets of discharging table, like the right side picture, the blue area is one set and the orange is the other hydraulic cylinder Each set of discharging table operated by two hydraulic cylinders Each table has rolling cylinders to help moving and the directions are showed as picture rolling cylinder [...]...Limestone handling system 4 4 ● ● 3 3 The Limestone handling system is used to transport the limestone from ground level to the top of the lime kiln The limestone handling system comprises of: 1 Weighing hopper (kiln bottom ) 2 Skip Bucket 3 Skip Bridge 2 4 Skip Winch (It could be set on the ground level or on the 1 2 top of the kiln ) load cells 4m3 hydraulically operated trap ● This limestone... 710 mm ● Rope diameter : 2 x 28 mm ● Limestone charging system weighing hopper (kiln top,10M3) level indicator cone closure of rotary hopper distribution cone load cells vibrating feeder reversible belt- conveyor rotary hopper (4M3) shaft trap limestone char g ing process for each shaft : ● 1 1 Limestone is charged to the weighing hopper 2 2 Limestone weighing 3 Limestone is charged to the reversible... elongated to open and let limestone fall into the shaft 8 Distribution cone will help to ensure a uniformly distributed stone size across the kiln cross section In conventional types of PFR kiln, owing to keep the kiln pressure, charging process only occurs during the reversal time , but modern type PFR kilns are improved to be able to charge during burning cycle ● 8 Preheating process ● preheating zone... Preheating process of combustion air : 1 Combustion air is introduced to the burning shaft from the top of the kiln preheating zone of burning shaft This limestone bed has been preheated during last burn ing cycle 2 Th e limestone in the preheating zone has been charged and preheated during last burn ing cycle 3 Heat will transfer to combustion air from the stone when it pass through the preheating zone limestone... warm combustion air : limestone bed in burning shaft preheating zone preheating zone of non-burning shaft limestone bed is under preheating ● This figure shows a non-burning shaft ● Preheating process of limestone : 1.Hot off-gas from the bur n ing shaft flows to the non-burning one via crossover channel and finally emits to the air from the top 2.When hot off-gas pass through the limestone bed of the... the 1 2 top of the kiln ) load cells 4m3 hydraulically operated trap ● This limestone weighing hopper (kiln bottom ) is located on load cells and equipped with a hydraulically operated trap to dump the lime stone to skip bucket ● ● The skip bucket is transporting the limestone from the ground to kiln top and is cladded with rubber Skip bucket net volume is 4m3 approximately Skip slope : 80 ° (degree)... n ing zone limestone bed where fuel and air meet ● Burning cycle of conventional type PFR kiln A B During 660s burning time, fuel and combustion air are fed into A shaft (the burning shaft) while B shaft serves as the preheating and offgas shaft - the non-burning shaft A B During 60s kiln reversal time : 1.The fuel flow is stopped 2.combustion and cooling air are vented to the open 3.The kiln is de-pressurised... lance cooling air blowers ( 1 in use and 1 is spare) During kiln reversal, the COG will be returned to the suction side of the blowers without differential pressure COG safety valve COG orifice for flow measurement COG safety valve start-up burner The start-up burner will be used for the first heating-up of the lime kiln COG boosters for one kiln ( 2 in use and 1 is spare) COG flow data and heat value... Ring mains and ducts of a kiln : combustion air duct : ring main of lance cooling air : ring main of COG : ring main of suspended cylinder cooling air (outlet) : ring main of suspended cylinder cooling air (inlet) : off-gas duct (for kiln) : off-gas duct (for bag filter) : lime cooling air Burner lances ● ● This diagram shows the distribution of burner lances from top view of the kiln Each shaft has 33... the kiln shafts The PFR -Kiln can be fired with virtually all kinds of gaseous, liquid and pulverised solid fuels such as : Natural Gas , LPG (Propane, Butane) , Coke Oven Gas , Low Calorific Value Gas ,Fuel Oil ,Waste Oil , Coal , Wood Fuel input has to be controlled as a function of the calcining temperature The length of the flame is one of the key factors to control the reactivity of burnt lime