Furnaces and refractories

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Information about Furnaces and refractories
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Published on February 13, 2008

Author: Stella

Source: authorstream.com

Slide1:  Training Session on Energy Equipment Furnaces and Refractories Presentation from the “Energy Efficiency Guide for Industry in Asia” www.energyefficiencyasia.org Training Agenda: Steam:  Training Agenda: Steam Introduction Type of furnaces and refractory materials Assessment of furnaces Energy efficiency opportunities Introduction:  Introduction Equipment to melt metals Casting Change shape Change properties Type of fuel important Mostly liquid/gaseous fuel or electricity Low efficiencies due to High operating temperature Emission of hot exhaust gases What is a Furnace? Introduction:  Introduction Furnace Components (The Carbon Trust) Furnace chamber: constructed of insulating materials Hearth: support or carry the steel. Consists of refractory materials Burners: raise or maintain chamber temperature Chimney: remove combustion gases Charging & discharging doors for loading & unloading stock Charging & discharging doors for loading & unloading stock Introduction:  Introduction Materials that Withstand high temperatures and sudden changes Withstand action of molten slag, glass, hot gases etc Withstand load at service conditions Withstand abrasive forces Conserve heat Have low coefficient of thermal expansion Will not contaminate the load What are Refractories: Introduction:  Introduction Refractories Refractory lining of a furnace arc Refractory walls of a furnace interior with burner blocks (BEE India, 2005) Introduction:  Introduction Melting point Temperature at which a ‘test pyramid’ (cone) fails to support its own weight Size Affects stability of furnace structure Bulk density Amount of refractory material within a volume (kg/m3) High bulk density = high volume stability, heat capacity and resistance Properties of Refractories Introduction:  Introduction Porosity Volume of open pores as % of total refractory volume Low porosity = less penetration of molten material Cold crushing strength Resistance of refractory to crushing Creep at high temperature Deformation of refractory material under stress at given time and temperature Properties of Refractories Introduction:  Introduction Pyrometric cones Used in ceramic industries to test ‘refractoriness’ of refractory bricks Each cone is mix of oxides that melt at specific temperatures Properties of Refractories Pyrometric Cone Equivalent (PCE) Temperature at which the refractory brick and the cone bend Refractory cannot be used above this temp (BEE India, 2004) Introduction:  Introduction Volume stability, expansion & shrinkage Permanent changes during refractory service life Occurs at high temperatures Reversible thermal expansion Phase transformations during heating and cooling Properties of Refractories Introduction:  Introduction Thermal conductivity Depends on composition and silica content Increases with rising temperature High thermal conductivity: Heat transfer through brickwork required E.g. recuperators, regenerators Low thermal conductivity: Heat conservation required (insulating refractories) E.g. heat treatment furnaces Properties of Refractories Training Agenda: Steam:  Training Agenda: Steam Introduction Type of furnaces and refractory materials Assessment of furnaces Energy efficiency opportunities Type of Furnaces and Refractories:  Type of Furnaces and Refractories Type of Furnaces Forging furnaces Re-rolling mill furnaces Continuous reheating furnaces Type of Refractories Type of Insulating Materials Type of Furnaces and Refractories:  Type of Furnaces and Refractories Classification Combustion Furnaces Type of Furnaces and Refractories:  Type of Furnaces and Refractories Used to preheat billets/ingots Use open fireplace system with radiation heat transmission Temp 1200-1250 oC Operating cycle Heat-up time Soaking time Forging time Fuel use: depends on material and number of reheats Forging Furnace Type of Furnaces and Refractories:  Type of Furnaces and Refractories Box type furnace Used for heating up scrap/ingots/billets Manual charge / discharge of batches Temp 1200 oC Operating cycle: heat-up, re-rolling Output 10 - 15 tons/day Fuel use: 180-280 kg coal/ton material Re-rolling Mill Furnace – Batch type Type of Furnaces and Refractories:  Type of Furnaces and Refractories Not batch, but continuous charge and discharge Temp 1250 oC Operating cycle: heat-up, re-rolling Output 20-25 tons/day Heat absorption by material is slow, steady, uniform Re-rolling Mill Furnace – Continuous pusher type Type of Furnaces and Refractories:  Type of Furnaces and Refractories Continuous material flow Material temp 900 – 1250 oC Door size minimal to avoid air infiltration Stock kept together and pushed Pusher type furnaces Stock on moving hearth or structure Walking beam, walking hearth, continuous recirculating bogie, rotary hearth furnaces Continuous Reheating Furnaces Type of Furnaces and Refractories:  Type of Furnaces and Refractories 1. Pusher Furnace Pushers on ‘skids’ (rails) with water-cooled support push the stock Hearth sloping towards discharge end Burners at discharge end or top and/or bottom Chimney with recuperator for waste heat recovery (The Carbon Trust, 1993) Continuous Reheating Furnaces Type of Furnaces and Refractories:  Type of Furnaces and Refractories 2. Walking Beam Furnace Stock placed on stationary ridges Walking beams raise the stock and move forwards Walking beams lower stock onto stationary ridges at exit Stock is removed Walking beams return to furnace entrance (The Carbon Trust, 1993) Continuous Reheating Furnaces Type of Furnaces and Refractories:  Type of Furnaces and Refractories 3. Walking Hearth Furnace Refractory blocks extend through hearth openings Stock rests on fixed refractory blocks Stock transported in small steps ‘walking the hearth’ Stock removed at discharge end (The Carbon Trust, 1993) Continuous Reheating Furnaces Type of Furnaces and Refractories:  Type of Furnaces and Refractories 4. Continuous Recirculating Bogie Furnace Shape of long and narrow tunnel Stock placed on bogie (cart with wheels) with refractory hearth Several bogies move like train Stock removed at discharge end Bogie returned to entrance (The Carbon Trust, 1993) Continuous Reheating Furnaces Type of Furnaces and Refractories:  Type of Furnaces and Refractories 5. Rotary Hearth Furnace Walls and roof remain stationary Hearth moves in circle on rollers Stock placed on hearth Heat moves in opposite direction of hearth Temp 1300oC (The Carbon Trust, 1993) Continuous Reheating Furnaces Type of Furnaces and Refractories:  Type of Furnaces and Refractories Classification of Refractories Type of Furnaces and Refractories:  Type of Furnaces and Refractories Common in industry: materials available and inexpensive Consist of aluminium silicates Decreasing melting point (PCE) with increasing impurity and decreasing AL2O3 Fireclay Refractories 45 - 100% alumina High alumina % = high refractoriness Applications: hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks High Alumina Refractories Type of Furnaces and Refractories:  Type of Furnaces and Refractories >93% SiO2 made from quality rocks Iron & steel, glass industry Advantages: no softening until fusion point is reached; high refractoriness; high resistance to spalling, flux and slag, volume stability Silica Brick Chemically basic: >85% magnesium oxide Properties depend on silicate bond concentration High slag resistance, especially lime and iron Magnesite Type of Furnaces and Refractories:  Type of Furnaces and Refractories Chrome-magnesite 15-35% Cr2O3 and 42-50% MgO Used for critical parts of high temp furnaces Withstand corrosive slags High refractories Magnesite-chromite >60% MgO and 8-18% Cr2O3 High temp resistance Basic slags in steel melting Better spalling resistance Chromite Refractories Type of Furnaces and Refractories:  Type of Furnaces and Refractories Zirconium dioxide ZrO2 Stabilized with calcium, magnesium, etc. High strength, low thermal conductivity, not reactive, low thermal loss Used in glass furnaces, insulating refractory Zirconia Refractories Aluminium oxide + alumina impurities Chemically stable, strong, insoluble, high resistance in oxidizing and reducing atmosphere Used in heat processing industry, crucible shaping Oxide Refractories (Alumina) Type of Furnaces and Refractories:  Type of Furnaces and Refractories Single piece casts in equipment shape Replacing conventional refractories Advantages Elimination of joints Faster application Heat savings Better spalling resistance Volume stability Easy to transport, handle, install Reduced downtime for repairs Monolithics Type of Furnaces and Refractories:  Type of Furnaces and Refractories Material with low heat conductivity: keeps furnace surface temperature low Classification into five groups Insulating bricks Insulating castables and concrete Ceramic fiber Calcium silicate Ceramic coatings (high emissivity coatings) Insulating Materials Classification Type of Furnaces and Refractories:  Type of Furnaces and Refractories Consist of Insulation materials used for making piece refractories Concretes contain Portland or high-alumina cement Application Monolithic linings of furnace sections Bases of tunnel kiln cars in ceramics industry Castables and Concretes Type of Furnaces and Refractories:  Type of Furnaces and Refractories Thermal mass insulation materials Manufactured by blending alumina and silica Bulk wool to make insulation products Blankets, strips, paper, ropes, wet felt etc Produced in two temperature grades Ceramic Fibers Type of Furnaces and Refractories:  Type of Furnaces and Refractories Low thermal conductivity Light weight Lower heat storage Thermal shock resistant Chemical resistance Mechanical resilience Low installation costs Ease of maintenance Ease of handling Thermal efficiency Ceramic Fibers Remarkable properties and benefits Lightweight furnace Simple steel fabrication work Low down time Increased productivity Additional capacity Low maintenance costs Longer service life High thermal efficiency Faster response Type of Furnaces and Refractories:  Type of Furnaces and Refractories Emissivity: ability to absorb and radiate heat Coatings applied to interior furnace surface: emissivity stays constant Increase emissivity from 0.3 to 0.8 Uniform heating and extended refractory life Fuel reduction by up to 25-45% High Emissivity Coatings Type of Furnaces and Refractories:  Type of Furnaces and Refractories High Emissivity Coatings (BEE India, 2005) Training Agenda: Steam:  Training Agenda: Steam Introduction Type of furnaces and refractory materials Assessment of furnaces Energy efficiency opportunities Assessment of Furnaces:  Assessment of Furnaces Heat Losses Affecting Furnace Performance Assessment of Furnaces:  Assessment of Furnaces Instruments to Assess Furnace Performance Assessment of Furnaces:  Assessment of Furnaces Direct Method Thermal efficiency of furnace = Heat in the stock / Heat in fuel consumed for heating the stock Heat in the stock Q: Q = m x Cp (t1 – t2) Calculating Furnace Performance Q = Quantity of heat of stock in kCal m = Weight of the stock in kg Cp= Mean specific heat of stock in kCal/kg oC t1 = Final temperature of stock in oC t2 = Initial temperature of the stock before it enters the furnace in oC Assessment of Furnaces:  Assessment of Furnaces Direct Method - example Heat in the stock Q = m x Cp (t1 – t2) 6000 kg X 0.12 X (1340 – 40) 936000 kCal Efficiency = (heat input / heat output) x 100 [936000 / (368 x 10000) x 100 = 25.43% Heat loss = 100% - 25% = 75% Calculating Furnace Performance m = Weight of the stock = 6000 kg Cp= Mean specific heat of stock = 0.12 kCal/kg oC t1 = Final temperature of stock = 1340 oC t2 = Initial temperature of the stock = 40 oC Calorific value of oil = 10000 kCal/kg Fuel consumption = 368 kg/hr Assessment of Furnaces:  Assessment of Furnaces Indirect Method Heat losses Flue gas loss = 57.29 % Loss due to moisture in fuel = 1.36 % Loss due to H2 in fuel = 9.13 % Loss due to openings in furnace = 5.56 % Loss through furnace skin = 2.64 % Total losses = 75.98 % Furnace efficiency = Heat supply minus total heat loss 100% – 76% = 24% Calculating Furnace Performance Assessment of Furnaces:  Assessment of Furnaces Typical efficiencies for industrial furnaces Calculating Furnace Performance Training Agenda: Steam:  Training Agenda: Steam Introduction Type of furnaces and refractory materials Assessment of furnaces Energy efficiency opportunities Energy Efficiency Opportunities:  Energy Efficiency Opportunities Complete combustion with minimum excess air Proper heat distribution Operation at the optimum furnace temperature Reducing heat losses from furnace openings Maintaining correct amount of furnace draft Optimum capacity utilization Waste heat recovery from the flue gases Minimize furnace skin losses Use of ceramic coatings Selecting the right refractories Energy Efficiency Opportunities:  Energy Efficiency Opportunities Importance of excess air Too much: reduced flame temp, furnace temp, heating rate Too little: unburnt in flue gases, scale losses Indication of excess air: actual air / theoretical combustion air Optimizing excess air Control air infiltration Maintain pressure of combustion air Ensure high fuel quality Monitor excess air 1. Complete Combustion with Minimum Excess Air Energy Efficiency Opportunities:  Energy Efficiency Opportunities When using burners Flame should not touch or be obstructed No intersecting flames from different burners Burner in small furnace should face upwards but not hit roof More burners with less capacity (not one big burner) in large furnaces Burner with long flame to improve uniform heating in small furnace 2. Proper Heat Distribution Energy Efficiency Opportunities:  Energy Efficiency Opportunities Operating at too high temperature: heat loss, oxidation, decarbonization, refractory stress Automatic controls eliminate human error 3. Operate at Optimum Furnace Temperature Energy Efficiency Opportunities:  Energy Efficiency Opportunities Heat loss through openings Direct radiation through openings Combustion gases leaking through the openings Biggest loss: air infiltration into the furnace Energy saving measures Keep opening small Seal openings Open furnace doors less frequent and shorter 4. Reduce Heat Loss from Furnace Openings Energy Efficiency Opportunities:  Energy Efficiency Opportunities Negative pressure in furnace: air infiltration Maintain slight positive pressure Not too high pressure difference: air ex-filtration Heat loss only about 1% if furnace pressure is controlled properly! 5. Correct Amount of Furnace Draft Energy Efficiency Opportunities:  Energy Efficiency Opportunities Optimum load Underloading: lower efficiency Overloading: load not heated to right temp Optimum load arrangement Load receives maximum radiation Hot gases are efficiently circulated Stock not placed in burner path, blocking flue system, close to openings Optimum residence time Coordination between personnel Planning at design and installation stage 6. Optimum Capacity Utilization Energy Efficiency Opportunities:  Energy Efficiency Opportunities Charge/Load pre-heating Reduced fuel needed to heat them in furnace Pre-heating of combustion air Applied to compact industrial furnaces Equipment used: recuperator, self-recuperative burner Up to 30% energy savings Heat source for other processes Install waste heat boiler to produce steam Heating in other equipment (with care!) 7. Waste Heat Recovery from Flue Gases Energy Efficiency Opportunities:  Energy Efficiency Opportunities Choosing appropriate refractories Increasing wall thickness Installing insulation bricks (= lower conductivity) Planning furnace operating times 24 hrs in 3 days: 100% heat in refractories lost 8 hrs/day for 3 days: 55% heat lost 8. Minimum Furnace Skin Loss Energy Efficiency Opportunities:  Energy Efficiency Opportunities High emissivity coatings Long life at temp up to 1350 oC Most important benefits Rapid efficient heat transfer Uniform heating and extended refractory life Emissivity stays constant Energy savings: 8 – 20% 9. Use of Ceramic Coatings Energy Efficiency Opportunities:  Energy Efficiency Opportunities Selection criteria Type of furnace Type of metal charge Presence of slag Area of application Working temperatures Extent of abrasion and impact 10. Selecting the Right Refractory Structural load of furnace Stress due to temp gradient & fluctuations Chemical compatibility Heat transfer & fuel conservation Costs Training Session on Energy Equipment:  Training Session on Energy Equipment Furnaces and Refractories THANK YOU FOR YOUR ATTENTION Disclaimers and References:  Disclaimers and References This PowerPoint training session was prepared as part of the project “Greenhouse Gas Emission Reduction from Industry in Asia and the Pacific” (GERIAP). While reasonable efforts have been made to ensure that the contents of this publication are factually correct and properly referenced, UNEP does not accept responsibility for the accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this publication. © UNEP, 2006. The GERIAP project was funded by the Swedish International Development Cooperation Agency (Sida) Full references are included in the textbook chapter that is available on www.energyefficiencyasia.org

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