Fundamentals of Heat Recovery Recover heat that would otherwise be wasted up the stack.. This energy has already been paid for $$ BENEFITS of HEAT RECOVERY Higher efficiency Lower Pollution (Thermal and emissions) $$ saved
Opportunities for Heat Recovery Packaged Boilers Utility boilers Furnaces Ovens Kilns Fired Heaters Diesel Generators Process industries Chemical & petrochemical Food & beverage Paper Pharmaceuticals Industrial applications Steel production Equipment manufacturers Textile industries Other Hospitals Schools Laundries Offshore Platforms
Example: Pulp and Paper Average total steam energy use, is 1,947 trillion Btu (end use). The sources of the steam in pulp and paper manufacturing include recovery boilers (at chemical pulping facilities), power boilers, and waste heat recovery boilers.
Example: Pulp and Paper There is approximately 370,000 million Btu per hour (MMBtu/hr) of boiler capacity in the pulp and paper industry. Approximately half of this boiler capacity is fired by waste fuels.
EXAMPLE : Heat Recovery in a 700 HP Boiler 300 ºF 460 ºF 271 ºF 210 ºF 24,150 lb/hr 30,202 SCFH Fuel: Natural Gas Economizer Heat Recovery : 1,076,721 BTU/hr Operation Hours per Year : 7,000 Boiler Efficiency: 80% Cost of Economizer: $13,500 Cost of Installation: $10,000.00 Cost of Natural gas: $ 8.00 /MMbtu FUEL SAVINGS : ( 8.00$/MMbtu x 1.076 MMbtu/hr x 7000 hrs ) / 0.8 = $75,370 per year ( 8.00 $/MMbtu x 1.076 MMbtu/hr x 7000 hrs ) / 0.8 = $75,370 per year Installed Payback : ($13,500) + ($10,000) / ($13,500) + ($10,000) / ($75,370) = 0.31 3.7 mon
Partial Experience List – Pulp & Paper Economics of heat recovery are dictated by capital and operating costs of the heat recovery equipment… Babcock & Wilcox- Economizer International Paper- Economizer Georgia Pacific- Economizer Boise Cascade- Economizer Smurfit Stone- Economizer Temple Inland- Steam Coil Air heaters Tolko Industries- High temperature air pre-heater Irving Paper- Economizer Wausau Paper- Economizer Pepperell Paper - Economizer Tembec LVL- Air pre-heater West Fraser- Economizer Higher Fuel price… Faster the ROI
Implications on Economizer Design Waste fuels Bare tube - Larger Units Modular units Fouling considerations High Operating Pressure High design pressure : heavier tube wall – SA-178 C / seamless High gas temperature : Casing and structure metallurgies Larger Boilers Larger economizers Structural design to support large weights
Rectangular Cylindrical Basic Economizer Designs Built to Section I of the ASME code Vertical or horizontal gas flow From 100 hp to 100,000 pph designs Solid or serrated finned tubes Built-in soot-blowers Minimal transitions Vertical or horizontal gas flow Size ranges to 1,000,000+ pph Operating pressures from 50 psig to supercritical Horizontal or vertical tube designs Can support large loads ASME Section I or VIII, Division I
Dirty Fuel Usage o# 6 oil oCoal Inline Soot-blowing Normally Solid Fin with low fin density Best Heat Transfer Characteristics Clean Fuels oNatural Gas o#2 Oil Mass soot-blowing if needed, lance parallel AeroSeg finned tubes Tube Layout in Economizers Square tube pitch Triangular tube pitch
Large Economizers for Recovery Boilers Modular Construction
Enhanced heat transfer surfaces significantly decrease the cost of heat recovery by making the unit very compact Enhanced Heat Transfer Surfaces Serrated (1970) Solid (1959) Studded Externally finned tubes Internally ribbed tubes X-ID AeroSeg (2004 )
Finning Configurations Fuel type Square tube pitch with Solid Finning (fpi) Triangular (Staggered) tube Serrated Finning (fpi) Sootblowers Recommended Natural Gas OnlyN/A Natural Gas with #2 oil backup (<2 wks/yr) Natural Gas with #2 or #4 backup N/A 6 5 4 - 5 No Natural Gas with #5 or #6 Oil backup #5 or #6 Oil #2 or #4 Oil Heavy Residual (Bunker C or Coal Tar) N/A 4 - 5 3 N/A Yes Wood 3 2.5 2 N/A Yes
FLAT COILS FOR LARGE FIELD ASSEMBLED ECONOMIZERS
AeroSeg finned tubes are the new generation of finned tubes AeroSeg ® finned tubes can significantly decrease the number of finned tubes in an HRSG, thus decreasing cost. Cost models show that with tube-layouts ideal for AeroSeg, HRSG module costs can be reduced by up to 8%. Pursuit of higher efficiencies have driven rapid increases in gas turbine exhaust temperature and mass flow rates. AeroSeg finned tubes are ideally suited to handle higher heat duties since they can transfer more heat without a proportional increase in pressure drop. AeroSeg ® finned tubes in HRSGs
Tubular Air pre-heaters Plain tube or X-ID tube. Gas temperature up to 1500 F. Down-stream of gas, wood, coal, biomass fired units. Modular units for minimum field assembly Characteristic of high efficiency, high pressure and temp. reheat steam cycles for utility boilers is high feed-water temperature High feed-water temperature eventually leads to higher gas temperatures and thermal loads in the air heater. Larger units already have higher gas loads.
What is X-ID ® ? X-ID is an internally enhanced tube. Tube augmentation (passive or active) techniques are used to increase the heat transfer rate. The augmentation in an X-ID tube is achieved by helical ribs on the inside of the tube. Ribs modify the behavior of the fluid flow leading to heat transfer augmentation.
What is X-ID ® ? Boundary Layer Heat Transfer Rate Flow
Increases overall boiler efficiency by transferring more heat than a bare tube. – Tube-side heat transfer rate 85% greater than bare tube. – Increased boiler efficiency – Number of tubes in boiler can be reduced. – X-ID boiler requires less space (smaller design size). – Cost savings on larger boilers up to 20%. – Foul less than plain tubes – Self-Cleaning. – Same tube expanders as for plain tubes – Can be rolled, beaded, seal-welded. X-ID® Tubes in Firetube Boilers
Need high quality heat. – High enough temperature of gas stream. Amount of heat recovery is limited by corrosion considerations. This dictates feed- water temperature and gas exit temperatures. –Oxygen corrosion : ( minimum feed-water temp = 180 F to de-aerate) Use stainless steel tubes if water temperature is lower than 180 F. –Acid dew-point ( ~ 210 F - ~ 250 F) for sulphur bearing fuels Use feed-water pre-heat systems to raise tube-wall temperature. As a practical matter, most samples of gases from combustion processes where coal or oil is the fuel, contain about 10%-12% water….Gases from combustion processes where natural gas is the fuel contain about 22% water because the fuel has more hydrogen to form water. PROPER SIZING OF HEAT RECOVERY EQUIPMENT IS VERY IMPORTANT TO AVOID PROBLEMS IN THE ECONOMIZER AS WELL AS THE STACK. Considerations in Heat Recovery Equipment
Acid dew-point depends on sulfur content of fuel Stack temperature after economizer is dictated by acid-dew point. Wall temperature is controlled by feed-water temperature FuelTypical Stack temperature (F) Recommended minimum feed-water temp (F) Natural gas260 – 280 F180 F # 2 Oil ( < 0.5% S)300 – 325 F210 F #6 Oil ( ~ 2% S )350 F240 F Acid dew-point Corrosion
Corrosion Considerations Pitting Corrosion Feedwater quality and temperature Oxygen removed adequately? Chemical / Mechanical /Thermal de-aeration…If not, use stainless steel tubes Undeaerated Feedwater <9 Months
Performance of Products has been extensively tested Fintube R & D encompasses four key areas : Performance of Externally finned tubing Performance of Internally enhanced X-ID tubes Computational Fluid Dynamics (CFD) Fundamental Research Fresh air inlet Inlet duct Gas burner Finned tube bundle Exhaust duct 65 Overall length of test facility
ESCOA Engineering Manual Available size ranges and fin parameter combinations New rating calculations General information on all divisions Fintube Test Facility Data Updated Library Fintube History Download online at www.fintubetech.com
SUMMARY Recovering heat from flue gas can result in significant energy savings Enhanced heat transfer surfaces like finned tubes make economizers, air pre-heaters, waste heat boilers very economical…Typical payback is less than one (1) year Is Time for Heat Recovery!!