BOILER EFFICIENCY EVALUATIONS

Transcription

1 BOILER EFFICIENCY EVALUATIONS Presenter: John F. La Fond, P.E. President Author: Marcel D. Berz, P.E. Manager, Process Technologies 1

2 OUTLINE 1. Purpose of Evaluating Efficiency 2. Definitions 3. Typical Boiler Losses 4. Case Study 1: Two NG Package Boilers 5. Case Study 2: Biomass Boiler 6. Summary 2

3 PURPOSE OF EVALUATING EFFICIENCY Achieve lower operating cost for power and/or process steam generation Reduce carbon footprint Compare cost of different fuels Evaluate impact of operational changes Find opportunities for improvement 3

4 DEFINITION: ENERGY EFFICIENCY Efficiency = Energy Converted / Energy Input Efficiency = 1 Sum of Efficiency Losses Efficiency = 1 Total Energy Lost / Energy Input Energy Lost to Environment without Use Energy Input Energy Converted (i.e., Electricity, Steam, Heat, etc.) 4

5 DEFINITION 2: BOILER THERMAL EFFICIENCY Efficiency = Q net out / Q in Q net out (Heat Converted to Useful Energy) Energy in steam and continuous blow down minus heat entering in feedwater Q in (Heat Input) Fuel: typically on higher heating value basis in North America Air sensible heat (compared to 77 F, 1.3% H 2 O) 5

6 DEFINITION 3: CALCULATION BOUNDARY Define Boundaries for Mass and Heat Balance Typically at locations where air enters air heater and flue gas leaves the last heat transfer area (air heater or economizer) Sweetwater condenser is inside the boundary Steam coil air heater is typically outside the boundary (external heating) Water coil air heater is typically inside the boundary (internal heating) 6

7 Inputs Outputs Steam Continuous Blow Down (CBD) Air Feedwater Flue Gas Fuel Ash Calculation Boundary 7

8 TYPICAL BOILER EFFICIENCY LOSSES FG sensible heat (function of composition and temperature) FG heat of condensation (function of H 2 O and H 2 in fuel) Incomplete combustion CO formation Unburned fuel/char in ash Other losses that are typically smaller Radiation (function of design and load from Ref.) Ash sensible heat (function of quantity and temperature) 8

9 CASE STUDY 1: TWO PACKAGE BOILERS 9

10 CASE STUDY 1: TWO PACKAGE BOILERS Identical O -type units built in 1964 with superheaters and laminar air heaters (LAH) Design: 100,000 lb/hr steam, 850 psig and 830 F Now burning NG Apparent issue: Boiler A shows lower efficiency Approach: JANSEN engineers visit to measure selected variables (check of mill instruments) followed by in-house evaluation 10

11 CASE STUDY 1: MEASURED OPERATING PARAMETERS Units Boiler A Boiler B Steam flow klb/hr Steam temperature F Steam pressure psig Feedwater flow klb/hr Feedwater temperature F Natural gas flow kscfh Wet flue gas oxygen vol.% 4.4* 3.8* Flue gas temperature F 307* 313* CO O 2 ) ppmv 108* 101* Air temperature to LAH F 107* 111* *From hand-held instruments 11

12 CASE STUDY 1: CALCULATION STEPS A) Calculate apparent thermal efficiency (Efficiency = Q net out / Q in ) 1. Combustion calculations using the known fuel composition and FG oxygen» = excess air and air flow (per lb of fuel)» = FG composition and flow (per lb of fuel) 2. Calculate Q net out with steam flow and properties, and feedwater temperature 3. Calculate Q in with fuel HHV, air flow and temperature 12

13 CASE STUDY 1: APPARENT THERMAL EFFICIENCY Units Boiler A Boiler B Measure steam flow klb/hr Measue natural gas flow kscfh Calculation Results: Excess air % Flue gas moisture vol. % Qnet out MMBtu/hr Q in MMBtu/hr Apparent efficiency %

14 CASE STUDY 1: CALCULATION STEPS 2 B) Calculate efficiency losses (Efficiency = 1- Sum of Efficiency Losses) FG sensible heat from composition and temperture FG heat of condensation from % H 2 O in FG Radiation losses No ash sensible heat and unburned char losses CO formation losses are small (about 0.35% per 1,000 ppm dry CO) 14

15 CASE STUDY 1: CALCULATED LOSSES AND THERMAL EFFICIENCY Units Boiler A Boiler B Measure steam/fw flow klb/hr 84.2/ /77.6 Measure natural gas flow kscfh Calculation Results: FG sensible heat loss % FG condensation loss % Other losses % Calculated efficiency % Apparent efficiency % Conclusions: Natural gas and/or steam flow meters are not reading correctly 15

16 CASE STUDY 2: BIOMASS BOILER 16

17 CASE STUDY 2: BIOMASS BOILER Stoker-fired Boiler (1977) with superheater, tubular air heater (TAH), and economizer Design: 150,000 lb/hr steam, 850 psig and 830 F (up to 140,000 lb/hr from biomass) Now burning wood derived fuel (WDF) and NG Apparent issues: High CO and carryover, need for NG firing Approach: JANSEN engineers visit to measure selected variables (check of mill instruments), take fuel and ash samples, followed by in-house evaluation 17

18 CASE STUDY 2: MEASURED OPERATING PARAMETERS Units Site Visit Data Steam flow klb/hr 152 Steam temperature F 841 Steam pressure psig 837 Feedwater temperature F 282 WDF HHV/moisture/dry inert content Btu/lb-% 4,900/42/1.7* Natural gas flow kscfh 11.9 Wet flue gas O 2 at GB/Economizer outlet vol.% 3.0/5.8** Flue gas temperature F 464** Flue gas CO ppmv, dry 1,500** Average air temperature F 123** Average ash LOI mass %, dry 48* *From Lab, **From hand-held instruments 18

19 CASE STUDY 2: CALCULATION STEPS Solid fuel feed rate is difficult to measure and should therefore not be used in calculations Step 1: Calculate Q net out with steam flow and properties, and feedwater temperature Step 2: Combustion calcs (assumed fuel mix) Result: FG and air flow per lb of fuel, excess air Step 3: Calculate efficiency losses Result: Efficiency, WDF flow, fuel mix Repeat Steps 2 and 3 with calculated fuel mix until converge 19

20 CASE STUDY 2: EFFICIENCY LOSSES Calculate efficiency losses FG sensible heat from composition and temperature FG heat of condensation from % H 2 O in FG Radiation losses (larger % at lower load) CO formation losses (about 0.35% per 1,000 ppm dry CO) Ash sensible heat and unburned char losses» Often difficult to determine accurately 20

21 CASE STUDY 2: CHAR LOSSES Option 1: Fly and grate ash stored together Take LOI sample = unburned/(unburned + inert) Obtain inert quantity from combustion calcs Option 2: Fly and grate ash not stored together Take individual LOI samples from each stream Either estimate or measure ash quantities (difficult to do)» Weigh ash streams for a period» Collect uncontrolled flue gas particulate matter 21

22 CASE STUDY 2: CALCULATED LOSSES AND THERMAL EFFICIENCY Units Site Visit Measured steam klb/hr 152 Measured natural gas flow kscfh 11.9 Reported WDF flow t/hr 28.5 Efficiency FG sensible heat loss % 11.0 FG condensation loss % 15.0 Other losses (radiation, CO) % 0.9 Ash sensible heat & unburned losses % 2.2 Calculated efficiency % 71.0 Calculated WDF t/hr

23 CASE STUDY 2: POSSIBLE IMPROVEMENTS Reduce unburned fuel losses Improve combustion system performance Coarser fuel Reduce FG losses Reduce temperature by installing more economizer surface or condensing economizer Reduce in-leakage (i.e., in TAH) Reduce excess air, but CO and unburned may increase without a good air system Reduce FG evaporation/condensation losses Reduce fuel moisture 23

24 SUMMARY Evaluating boiler efficiency is a useful tool Quantify operation and current cost of each fuel Find opportunities for improvement Flue gas losses can be determined accurately using measurements of FG temperature, oxygen, and CO together with fuel composition and HHV Other losses, such as unburned char losses for boilers burning solid fuels, are more difficult to determine without experience Evaluation by an experienced specialist and field measurements are beneficial 24

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