Training Session on Energy Equipment Furnaces and Refractories Presentation from the “Energy Efficiency Guide for Industry in Asia” www.energyefficiencyasia.org 1 © UNEP 2006 Training Agenda: Steam Introduction Type of furnaces and refractory materials Assessment of furnaces Energy efficiency opportunities 2 © UNEP 2006 Introduction What is a Furnace? • 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 3 © UNEP 2006 Introduction Chimney: remove combustion gases Burners: raise or maintain chamber temperature Furnace Components Furnace chamber: constructed of insulating materials Hearth: support or carry the steel. Consists of refractory materials Charging & discharging doors for loading & unloading stock (The Carbon Trust) 4 © UNEP 2006 Introduction What are Refractories: 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 5 © UNEP 2006 Introduction Refractories Refractory lining of a furnace arc Refractory walls of a furnace interior with burner blocks (BEE India, 2005) 6 © UNEP 2006 Introduction Properties of Refractories • 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 7 © UNEP 2006 Introduction Properties of Refractories • 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 8 © UNEP 2006 Introduction Properties of Refractories • Pyrometric cones • Used in ceramic industries to test ‘refractoriness’ of refractory bricks • Each cone is mix of oxides that melt at specific temperatures (BEE India, 2004) • Pyrometric Cone Equivalent (PCE) • Temperature at which the refractory brick and the cone bend • Refractory cannot be used above this temp 9 © UNEP 2006 Introduction Properties of Refractories • Volume stability, expansion & shrinkage • Permanent changes during refractory service life • Occurs at high temperatures • Reversible thermal expansion • Phase transformations during heating and cooling 10 © UNEP 2006 Introduction Properties of Refractories • 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 11 © UNEP 2006 Training Agenda: Steam Introduction Type of furnaces and refractory materials Assessment of furnaces Energy efficiency opportunities 12 © UNEP 2006 Type of Furnaces and Refractories • Type of Furnaces • Forging furnaces • Re-rolling mill furnaces • Continuous reheating furnaces • Type of Refractories • Type of Insulating Materials 13 © UNEP 2006 Type of Furnaces and Refractories Classification Combustion Furnaces Classification method Types and examples 1. Type of fuel used Oil-fired Gas-fired Coal-fired 2. Mode of charging materials Intermittent / Batch Periodical Forging Re-rolling (batch/pusher) Pot Continuous Pusher Walking beam Walking hearth Continuous recirculating bogie furnaces Rotary hearth furnaces 3. Mode of heat transfer Radiation (open fire place) Convection (heated through medium) 4. Mode of waste heat recovery Recuperative Regenerative 14 © UNEP 2006 Type of Furnaces and Refractories Forging Furnace • 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 15 number of reheats © UNEP 2006 Type of Furnaces and Refractories Re-rolling Mill Furnace – Batch type • 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 16 © UNEP 2006 Type of Furnaces and Refractories Re-rolling Mill Furnace – Continuous pusher type • 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 17 © UNEP 2006 Type of Furnaces and Refractories Continuous Reheating Furnaces • 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 18 © UNEP 2006 Type of Furnaces and Refractories Continuous Reheating Furnaces 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) 19 © UNEP 2006 Type of Furnaces and Refractories Continuous Reheating Furnaces 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) 20 © UNEP 2006 Type of Furnaces and Refractories Continuous Reheating Furnaces 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) 21 © UNEP 2006 Type of Furnaces and Refractories Continuous Reheating Furnaces 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) 22 © UNEP 2006 Type of Furnaces and Refractories Continuous Reheating Furnaces 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) 23 © UNEP 2006 Type of Furnaces and Refractories Classification of Refractories Classification method Examples Chemical composition ACID, which readily combines with bases Silica, Semisilica, Aluminosilicate BASIC, which consists mainly of metallic oxides that resist the action of bases Magnesite, Chrome-magnesite, Magnesitechromite, Dolomite NEUTRAL, which does not combine with acids nor bases Fireclay bricks, Chrome, Pure Alumina Special Carbon, Silicon Carbide, Zirconia End use Blast furnace casting pit Method of manufacture Dry press process, fused cast, hand moulded, formed normal, fired or chemically bonded, unformed (monolithics, plastics, ramming mass, gunning castable, spraying) 24 © UNEP 2006 Type of Furnaces and Refractories Fireclay Refractories • Common in industry: materials available and inexpensive • Consist of aluminium silicates • Decreasing melting point (PCE) with increasing impurity and decreasing AL2O3 High Alumina Refractories • 45 - 100% alumina • High alumina % = high refractoriness • Applications: hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks 25 © UNEP 2006 Type of Furnaces and Refractories Silica Brick • >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 Magnesite • Chemically basic: >85% magnesium oxide • Properties depend on silicate bond concentration • High slag resistance, especially lime and iron 26 © UNEP 2006 Type of Furnaces and Refractories Chromite 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 27 © UNEP 2006 Type of Furnaces and Refractories Zirconia 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 Oxide Refractories (Alumina) • Aluminium oxide + alumina impurities • Chemically stable, strong, insoluble, high resistance in oxidizing and reducing atmosphere • Used in heat processing industry, crucible shaping28 © UNEP 2006 Type of Furnaces and Refractories Monolithics • 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 29 © UNEP 2006 Type of Furnaces and Refractories Insulating Materials Classification • 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) 30 © UNEP 2006 Type of Furnaces and Refractories Castables and Concretes • 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 31 © UNEP 2006 Type of Furnaces and Refractories Ceramic Fibers • 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 32 © UNEP 2006 Type of Furnaces and Refractories Ceramic Fibers Remarkable properties and benefits • • • • • • • • • • 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 • Lightweight furnace • Simple steel fabrication work • Low down time • Increased productivity • Additional capacity • Low maintenance costs • Longer service life • High thermal efficiency • Faster response 33 © UNEP 2006 Type of Furnaces and Refractories High Emissivity Coatings • 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% 34 © UNEP 2006 Type of Furnaces and Refractories High Emissivity Coatings (BEE India, 2005) 35 © UNEP 2006 Training Agenda: Steam Introduction Type of furnaces and refractory materials Assessment of furnaces Energy efficiency opportunities 36 © UNEP 2006 Assessment of Furnaces Heat Losses Affecting Furnace Performance Heat input FURNACE Heat in stock Other losses Furnace surface/skin Openings in furnace Hydrogen in fuel Moisture in fuel Flue gas 37 © UNEP 2006 Assessment of Furnaces Instruments to Assess Furnace Performance Parameters to be measured Location of measurement Instrument required Required Value Furnace soaking zone temperature (reheating furnaces) Soaking zone and side wall Pt/Pt-Rh thermocouple with indicator and recorder 1200-1300oC Flue gas temperature In duct near the discharge end, and entry to recuperator Chromel Alummel Thermocouple with indicator 700oC max. Flue gas temperature After recuperator Hg in steel thermometer 300oC (max) Furnace hearth pressure in the heating zone Near charging end and side wall over the hearth Low pressure ring gauge +0.1 mm of Wc Oxygen in flue gas In duct near the discharge end Fuel efficiency monitor for oxygen and temperature 5% O2 Billet temperature Portable Infrared pyrometer or optical pyrometer - 38 © UNEP 2006 Assessment of Furnaces Calculating Furnace Performance 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) 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 39 t2 = Initial temperature of the stock before it enters the furnace in©oC UNEP 2006 Assessment of Furnaces Calculating Furnace Performance Direct Method - example m = Weight of the stock = 6000 kg Cp= Mean specific heat of m x Cp (t1 – t2) stock = 0.12 kCal/kg oC 6000 kg X 0.12 X (1340 – 40) t1 = Final temperature of 936000 kCal stock = 1340 oC t2 = Initial temperature of the stock = 40 oC (heat input / heat output) x 100 Calorific value of oil = 10000 [936000 / (368 x 10000) x 100 = 25.43% kCal/kg Fuel consumption = 368 kg/hr • Heat in the stock Q = • • • • Efficiency = • • • Heat loss = 100% - 25% = 75% 40 © UNEP 2006 Assessment of Furnaces Calculating Furnace Performance Indirect Method Heat losses a) Flue gas loss = 57.29 % b) Loss due to moisture in fuel = 1.36 % c) Loss due to H2 in fuel = 9.13 % d) Loss due to openings in furnace = 5.56 % e) Loss through furnace skin = 2.64 % Total losses = 75.98 % Furnace efficiency = • Heat supply minus total heat loss • 100% – 76% = 24% 41 © UNEP 2006 Assessment of Furnaces Calculating Furnace Performance Typical efficiencies for industrial furnaces Furnace type Thermal efficiencies (%) 1) Low Temperature furnaces a. 540 – 980 oC (Batch type) 20-30 b. 540 – 980 oC (Continous type) 15-25 c. Coil Anneal (Bell) radiant type 5-7 d. Strip Anneal Muffle 7-12 2) High temperature furnaces a. Pusher, Rotary 7-15 b. Batch forge 5-10 3) Continuous Kiln a. Hoffman 25-90 b. Tunnel 20-80 4) Ovens a. Indirect fired ovens (20 oC –370 oC) 35-40 b. Direct fired ovens (20 oC –370 oC) 35-40 42 © UNEP 2006 Training Agenda: Steam Introduction Type of furnaces and refractory materials Assessment of furnaces Energy efficiency opportunities 43 © UNEP 2006 Energy Efficiency Opportunities 1. Complete combustion with minimum excess air 2. Proper heat distribution 3. Operation at the optimum furnace temperature 4. Reducing heat losses from furnace openings 5. Maintaining correct amount of furnace draft 6. Optimum capacity utilization 7. Waste heat recovery from the flue gases 8. Minimize furnace skin losses 9. Use of ceramic coatings 10.Selecting the right refractories 44 © UNEP 2006 Energy Efficiency Opportunities 1. Complete Combustion with Minimum Excess Air • 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 45 © UNEP 2006 Energy Efficiency Opportunities 2. Proper Heat Distribution 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 46 © UNEP 2006 Energy Efficiency Opportunities 3. Operate at Optimum Furnace Temperature • Operating at too high temperature: heat loss, oxidation, decarbonization, refractory stress • Automatic controls eliminate human error Slab Reheating furnaces 1200oC Rolling Mill furnaces 1200oC Bar furnace for Sheet Mill 800oC Bogie type annealing furnaces 650oC –750oC 47 © UNEP 2006 Energy Efficiency Opportunities 4. Reduce Heat Loss from Furnace Openings • 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 48 © UNEP 2006 Energy Efficiency Opportunities 5. Correct Amount of Furnace Draft • 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! 49 © UNEP 2006 Energy Efficiency Opportunities 6. Optimum Capacity Utilization • 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 50 stage © UNEP 2006 Energy Efficiency Opportunities 7. Waste Heat Recovery from Flue Gases • 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, selfrecuperative burner • Up to 30% energy savings • Heat source for other processes • Install waste heat boiler to produce steam • Heating in other equipment (with care!) 51 © UNEP 2006 Energy Efficiency Opportunities 8. Minimum Furnace Skin Loss • 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 52 © UNEP 2006 Energy Efficiency Opportunities 9. Use of Ceramic Coatings • 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% 53 © UNEP 2006 Energy Efficiency Opportunities 10. Selecting the Right Refractory Selection criteria • Type of furnace • Type of metal charge • Structural load of furnace • Presence of slag • Stress due to temp gradient & fluctuations • Area of application • Chemical compatibility • Working temperatures • Heat transfer & fuel conservation • Extent of abrasion and impact • Costs 54 © UNEP 2006 Training Session on Energy Equipment Furnaces and Refractories THANK YOU FOR YOUR ATTENTION 55 © UNEP 2006 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 56 © UNEP 2006
