Heat Recovery Systems and Heat Exchangers
in LNG Applications
Landon Tessmer
LNG Technical Workshop 2014
Vancouver
Presentation Overview
• LNG plant arrangement with heat recovery
(OSMR Process by LNG Limited)
• HRSG/OTSG Introduction
• Design considerations in waste heat
recovery behind compressor drives
• Duct Burners
• LNG Cold Box Heat Exchangers
OSMR LNG PLANT ARRANGEMENT
4 Major LNG Technologies
• APCI (SMR, C3MR, AP-X, DMR).
• SHELL (DMR, PMR).
• LINDE (MFC).
• AXENS (LIQUEFIN).
SIMPLIFIED C3MR CYCLE
Pillarella, M. 2010. PAPER PS2-5: The C3MR LIQUEFACTION CYCLE: VERSATILITY FOR A FAST GROWING, EVER CHANGING LNG INDUSTRY
Propane Pre-Cooled Mixed Refrigerant Process
Possible LNG Plant Schematic – Optimized Single Mixed Refrigerant Process by LNG Limited (Perth, WA)
OSMR – Optimized Single Mixed Refrigerant Process
OSMR Explained
• The process is based on a simple single mixed refrigerant cycle but the performance is significantly enhanced by the addition of conventional combined heat and power technology and conventional industrial ammonia refrigeration.
• The heart of the process is a very simple single mixed refrigerant cycle which consists
of a suction scrubber, compressor, after-cooler and cold box. It uses a standard single stage centrifugal compressor which does not require a gear box, helper motor or inter-stage components as do most other LNG plants.
Major differences from Typical LNG Cycles: • GT waste heat recovery to produce power. • GT inlet air cooling using ammonia. • Pre-cooling of Mixed Refrigerant (MR) using ammonia. This is successfully used in a
small LNG plant in Western Australia. Since the cold box is a very simple design with minimal streams, the addition of ammonia to cool the MR from ambient temperature down to around 0C only, is not technically challenging.
HRSG/OTSG INTRODUCTION
AND
DESIGN CONSIDERATIONS BEHIND
COMPRESSOR DRIVES
Heat Recovery OTSG
for Power Generation
Direct Fired OTSG for
Enhanced Oil Recovery
Purpose of the heat recovery
OTSG
Direct drive to compressor for refrigerant loop
OR
OR
Direct drive to compressor for refrigerant loop
LNG plant usually needs steam as heating media for acid gas removal unit and reboiler duties for fractionation, therefore cogeneration cycle application will contribute the plant efficiency.
OTSG vs Drum-Style HRSG
LM6000 Installation – overall size comparison
OTSG
HRSG
Courtesy of
HRSG vs OTSG Drum-Type HRSG
Fixed Sections
OTSG Type HRSG
Non Fixed Section
Design Considerations - Metallurgy
• Incoloy 800/825 tubing designed to mitigate the following
failure modes:
– Dew point corrosion (water/acid)
• Allows cold feedwater to 60°F (17°C)
– Flow assisted corrosion
– Thermal shock
– Creep/fatigue failures
– Cycling/daily start – stop
– 409SS & 316SS Liners
– CS, 409SS, & 316SS brazed fins
– Allows dry running capability up to 1100°F (593°C)
Thin wall tubes & mechanical design
Bundle Growth – Thermal Cycling
Blue – hot/expanded condition Black hidden – Cold condition Note the tubesheet movement, tube growth, and flex tubes Makes the OTSG ideally suited for cycling application as stress and start up time are minimized compared to a traditional drum-type boiler.
Typical OTSG P&ID
OTSG and Plant Feedwater
Treatment
• No blowdown so water quality is critical (~50 ppb TDS)
• Requires demineralized and polished feedwater.
– Cation Conductivity Limit: 0.25 μS/cm
• IST recommends stainless FW piping from polisher to OTSG (particularly for cycling plants)
• Eliminates:
– Tube scaling
– Deposition and carry over
– Active chemical treatment
OTSG Feedwater Specification Parameter Target
Value
Water Cation Conductivity (μS/cm)
<0.25
pH (stainless piping)
(CS piping)
8 to 8.5
9.3 to 9.6
Dissolved Oxygen (ppb) (stainless piping)
(CS piping)
<300
<7
Sodium (ppb) <6
Chloride (ppb) <6
Sulfate (ppb) <6
Silica (ppb) <20
Parameter Target Value
Iron (ppb) <10
Copper (ppb) <2
Total Organic Carbon (ppb)
<100
Hardness (ppb) <1
Note: Typically, the water quality required in gas turbine injection applications is more stringent than the OTSG FW spec.
Typical Condensate Handling Diagram
SCR IMPLICATIONS
Distribution Grid (if req’d)
SCR Module
Typical Layout of OTSG w/ SCR & CO
CO Catalyst
Catalyst Loading Platform
Note split tube bundle
• SCR catalyst is located in the appropriate gas temperature zone for maximum efficiency
• Dual range catalysts have peak
efficiency at ~750 to 775F. Maximum continuous temperature is 950F (with reduced efficiency)
• OTSG bundle is designed to balance temperature exposure of catalyst in all operating scenarios (ie. unfired, fired, turndown, etc.)
Location of SCR Catalyst
SCR CATALYST REACTIVITY
• Cross-section enlarged to offset increased gas-side pressure drop (SCRs can add ~4”WC)
• Reduced fin pitch below dew points – particularly if liquid fuels burned in gas turbine (ammonium bisulphate concerns)
• In traditional low temp catalysts, start up times are prolonged in an effort to maintain low temperatures at the catalyst face.
• Cost impact is roughly +$2.5M for a ~50MW gas turbine OTSG install and 80%-90% NOx conversion on the catalyst.
OTSG Design Implications with SCR Catalysts
DUCT BURNERS
Supplementary Firing
• In LNG Plants duct burners may be used to consume: pipeline gas, lean gas, treated regen gas, or vaporized HC condensate. All to add to the available energy for heat recovery
• Common in cogen applications where the value of the steam exceeds the cost of additional fuel burned
• Natural gas is piped through “runners” and distributed by nozzles across the width of the duct.
• Scope consists of runners, gas distribution manifold, fuel handling skid (may need separate skids depending on range of fuel
compositions), and auxiliary blower skid
Supplementary Firing – Velocity Distribution
Supplementary Firing – Velocity Distribution
• Distribution Grid + Flow Straightener
• Flatten velocity profile and remove swirl
• Target 75 ft/s normal operation
• 35 ft/s minimum
• ±10% of average free stream velocity after distribution grid
• Burner duct length provision
• 1.5x flame length
• Burner duct liner material
• 409SS, 304SS, 316SS, Piro Block
Supplementary Firing – Velocity Distribution
• Typical temperature distribution guarantee +/-10% of the average temperature given a particular velocity profile input guarantee
• Typical heat release from a burner runner is 3 MMBtu/hr per liner foot
• Increase total heat release by wider duct or more runners (taller duct)
• Duct size is driven by a balance between space required for runners (heat release) and the 75 ft/s target
Module Material Considerations in Fired Applications
Tubesheets <1050 F – Chromoly 1050 – 1400 F – 347SS 1400 – 1500 F – NO6617
Steam Headers P22 or P91
Fin Material 409SS, 316SS and spacing
Fin Material Considerations
Design Limits CS < 454 C 409SS < 593 C 316SS < 871 C Corrosive duty must be considered as well
MAIN CRYOGENIC HEAT EXCHANGER (MCHE) /
“COLD BOX” HEAT EXCHANGERS
NG LIQUEFACTION TECHNOLOGY IS
BASED ON TWO PRIMARY HE DESIGNS:
MAIN CRYOGENIC HEAT EXCHANGER
SIMPLIFIED C3MR CYCLE
Pillarella, M. 2010. PAPER PS2-5: The C3MR LIQUEFACTION CYCLE: VERSATILITY FOR A FAST GROWING, EVER CHANGING LNG INDUSTRY
Propane Pre-Cooled Mixed Refrigerant Process
Possible LNG Plant Schematic – Optimized Single Mixed Refrigerant Process by LNG Limited (Perth, WA)
PLATE-FIN HEAT EXCHANGER
Courtesy of the Linde Group
PLATE-FIN HEAT EXCHANGER
Courtesy of the Linde Group
PLATE-FIN HEAT EXCHANGER
Courtesy of the Linde Group
• Brazed plate-fin heat exchanger is stack of alternating flat and corrugated plates. The corrugations form the flow channels for the process fluids (up to 10 fluids)
• Typical materials are aluminum alloys 3003 (blocks) and 5083 (attaching components).
• Maximum operating temperature is roughly +65oC.
• The fins/corrugated plates are serrated or solid (more heat transfer area but higher fouling + pressure drop with serrated)
• Fabricated using vacuum brazing (vacuum furnace at 600oC). Plates have filler metal cladding rolled on both sides. Attachments such as half pipes are welded.
• Results in a light-weight compact design
• AVOID: thermal shocks, large delta T (in mediums), dirty fluids, cyclic loads.
PLATE-FIN HEAT EXCHANGER
Courtesy of the Linde Group
COIL-WOUND HEAT EXCHANGER
Courtesy of the Linde Group
COIL-WOUND HEAT EXCHANGER
Courtesy of the Linde Group
COIL-WOUND HEAT EXCHANGER
Courtesy of the Linde Group
Tube bundle before insertion into the pressure vessel shell
COIL-WOUND HEAT EXCHANGER
Courtesy of the Linde Group
• A tubular heat exchanger but the bundle does not consist of straight tubes.
• Long length, small diameter tubes are wound in alternating directions around a centre pipe (the mandrel)
• The complete tube bundle is inserted into a pressure vessel shell. Every tube starts and terminates in tubesheets which are integral in the pressure vessel shell
• The shell-side distributes the 2-phase steam over the whole cross section of the tube bundle.
• Shell material is typically aluminum alloy 5083 and the tubes are a special grade aluminum allow as well. There are also CS and SS variants.
• It is a flexible bundle that can withstand much higher temperature gradients than a plate-fin design.
COIL-WOUND HEAT EXCHANGER
Courtesy of the Linde Group
Courtesy of the Linde Group
COIL-WOUND
HEAT
EXCHANGER
PLATE-FIN HEAT
EXCHANGER
Questions?