Mechanical Pulping Sessions. Session 1 The Mechanical Pulping Process Electrical Energy Requirements

Transcription

1 Mechanical Pulping Sessions Session 1 The Mechanical Pulping Process Electrical Energy Requirements

2 Introduction Mechanical pulps used to manufacture printing papers are produced either by: > Stone Groundwood process (SGW, PGW) - <20% > Thermomechanical Pulping process (TMP,CTMP) - >80% A major limitation to mechanical pulping is the high levels of energy required compared to chemical pulping: > SGW: 1000 to 2000 kwh/bdt > TMP: 1600 to 3000 kwh/bdt > Chemical pulping: approx 500 kwh/bdt Energy costs alone represent 25 to 50% of total costs depending on mill location. Focus here is on energy cost reduction in TMP process and on TMP heat recovery

3 Mechanical Pulping (MP) Background Mechanical Pulping in BC Represents > 10 mill sites (9 currently in operation) > Up to 8,000 GWh/yr and 950 MW of Capacity > About 11% of BC Hydro total supply BC Mills are old and inefficient > Most mechanical pulp mills are 15 to 30+ years old > Low CAPEX vs. Life Cycle Costing (LOC) installation Poor Industry Profitability > P&P Mature industry in slow decline Insufficient investment to maintain asset base > Low ROCE > Management focused on survival Estimated potential savings > Energy savings up to 950+ GWh/yr > Demand reduction 120 MW

4 TMP Electrical Energy Overview (kw) 750 BDt/D Plant 2,400 kwh/bdt SRE, P1/S1/R1 Screening, 4 Stage TMP Cleaning

5 TMP Process - Newsgrades

6 Chip Handling - Design Concepts Uniform-high quality chips to process > Low % bark - 0.5% goal > Low % chip fines goal - 0.5% (-5 MM) > Low % over thick chips - 5% (>8 MM) maximum (effects mass flow rate and increases variability) > High moisture chips - 40% BD minimum Short Chip Storage > Softwood 5 days > Hardwood 5 days First in first out concept Accurate blending/metering of component wood species

7 Chip Handling - Design Concepts

8 Impact of Chip Fines (5 MM Round Holes) Increasing Chip fines from 0.5 to 2.0% > More shives 100% increase > Brightness % lower (function of % bark in fines) > Strength impact Tear 2-4% lower Tensile 1-2% lower Burst 2-4% lower

9 Impact of Thick Chips (+8 MM Thickness) The center of thick chips are difficult to heat in atmospheric presteaming (APS) and pre-heater resulting in refiner mechanical pulp (RMP) (ie. cold non-impregnated chips to refiners). Impact of increasing content of thick chips from 5 to 10% (species dependent) > Power consumption increase of kwh/bdmt > Shives content increase of 25% > Strength reduction of 1-3% > Pulp pitch contentof 1-2%

10 Chip Handling Electrical energy consumption ranges from 1% (just chip handling) to 3.5% if wood room and whole log chipping present > Equipment drives > Conveyors > Fans/blowers Process Significance > More important in terms of supplying good quality chips to process > In the current economic climate, very little can be done to improve chip quality Consider > Motor Management > Process changes to Screen out fines and thick chips minimize fines generation

11 Chip Pre-Heating and Chip Washing Design Concepts APS No.1 90 C or higher for 30 min. APS No C or higher for 15 min (initiate wood softening and improved pitch/extractive removal) Automatic flow control to chip washer control specific water consumption > target 15m 3 /BDt (effluent discharge toxicity limited) 20m 3 /BDt wash water flow to chip drainer Chip cyclone > Grit and sand removal Chip drainer pressate cleaning (pins/fines, grit) > Sidehill screens > Sand settling tank (Sunds) > Sand cleaner cyclones 125% grit cleaner capacity Counter current WW purge via chip washing

12 Chip Pre-Heating and Washing BYPASS CHIPS FROM CHIP HANDLING BYPASS CHIP DRAINER CHIP CYCLONE APS BIN NO 1 REFINER STEAM FROM PRESSURIZED SCRUBBER CHIP DRAINER SIDEHILL SCREENS APS BIN NO 2 REFINER STEAM FROM PRESSURIZED SCRUBBER CLEAR TMP WW MAKE-UP TO EFFLUENT TREATMENT CHIP WASHER STEAM GRIT CLEANERS CLEAN DIRTY CHIP SUMP CHIP DRAINER FILTRATE TANK REJECTS CONVEYOR PRESSATE FROM CHIP PRETREATMENT

13 Design Concepts

14 Chip Pre-Heating and Washing Less than 1% of MP electrical energy consumed in this area > Pumps > Equipment drives > Screw Conveyor drives Refiner generated steam used for chip heating Process Significance > More important in terms of preparing chips for mainline refining Adequate pre-heating Efficient Washing and grit/pins/bark removal > White water purge point to effluent treatment Consider > Pumping system analysis > Motor Management > Specific water consumption analysis

15 Chip Conditioning Design Concepts Prepare chips for mainline refining > Supply primary refiner with a material of uniform bulk density, moisture content and temperature High compression PSF for extractives removal > Up to 4:1 compression ratio > Extractives screened to remove solid material (pins, bark, etc,). > Solid waste material discharged to rejects conveyor > PSF pressate discharged to effluent treatment Impregnation device for water and chemical uptake depending on the nature of the raw material Pressurized steaming vessel heats chips to C just before discharge to primary refiner feed to soften chips

16 Chip Conditioning WASHED CHIPS APS BIN NO 2 REFINER STEAM FROM PRESSURIZED SCRUBBER CHIP PRETREATMENT REFINER STEAM FROM PRESSURIZED SCRUBBER TO REJECTS CONVEYOR TO HEAT RECOVERY PSF SIDEHILL SCREEN PSF PRESSATE TANK PRIMARY REFINER FROM GRIT CLEANERS TO EFFLUENT TREATMENT

17 Chip Pre-Heating and Washing About than 2.5-3% of MP electrical energy consumed here > PSF drives up to 40kWh/BDt > Equipment /screw conveyor drives > Pumps Refiner generated steam used for chip heating Process Significance > More important in terms of preparing chips for mainline refining Chip de-structuring and heating Pitch/extractives removal, Water/chemical impregnation bark/pins removal > White water/chip pressate purge point to effluent treatment Consider > Motor Management > Pumping system analysis

18 Mainline Refining In BC mainline refining is typically carried out in two stages Low consistency refining may be present to reduce refining energy or increase production rate Design objectives > Even load split between primary and secondary refiners > Efficient steam handling and effective separation of pulp and refiner steam to minimize fibre loss and carry-over into refiner generated steam system > Minimize refiner load swings and pulp quality variability

19 Mainline refining TO HEAT RECOVERY TO HEAT RECOVERY TO HEAT RECOVERY PRESSURIZED PRIMARY CYCLONE PRESSURIZED SECONDARY CYCLONE CHIP PRETREATMENT PSF PSF TRANSFER CONVEYOR LC REFINERS SIDE FEED SCREW TRANSFER CHEST LATENCY CHEST PRIMARY REFINER SECONDARY REFINER

20 Mainline Refining Up to 60% of total MP energy is consumed in the primary and secondary refining stages > Mainline refiner drives > PSF and conveyor drives > Pumps (refiner services and stock dilution/transfer) > LC refiner motors (if present) Process Significance > Mainline refining is the most important unit process in the MP plant > The development of all important strength and surface properties is directly proportional to specific refining energy. > The average fiber length is determined by the primary refiner and the total specific refining energy. > Optical pulp properties are developed in mainline refining

21 Mainline Refining Lowering the specific refining energy requirements of the MP process to a given pulp quality profile is the single most important focus of the MP Initiative > Process measurement and advanced control Power metering Process measurement and control Real time /continuous pulp quality monitoring Fibre development models and multi-variable process control > Standard measurement and verification (M&V) reporting for projects impacting pulp quality and specific refining efficiency > Low energy refiner plate development > Heat recovery for steam and power generation

22 Mainline Refining MP in BC consumes about 5,600 GWh/yr On average mainline refining represents 60% of this total or about 3,360 GWh/yr Energy savings possible in this area alone > 5% reduction in SRE 168 GWh/yr > 10% reduction in SRE 336 GWh/yr > 25% reduction in SRE 840 GWh/yr Overall energy savings target for the MP Program is 941 GWh/yr

23 Mainline Screening/Cleaning Screening Various screening configurations employed with and without cleaners > Send fully developed shive-free pulp to the disc filter > Direct debris and underdeveloped long fibre fraction (rejects) to the rejects refining system for further development Centrifugal Cleaning Energy intensive- requires low feed consistency ( %BD) plus high pressure drop across cleaner to function properly (2 bar) Separation of under-developed fibre, ray cells plus further removal of sand and grit from process

24 Mainline Screening and Cleaning WW DISC FILTER PRIMARY SCREENS SECONDARY SCREENS REJECTS SCREENS WW WW STOCK TO BRIGHTENING AND HD STORAGE REJECTS LATENCY STORAGE TANK REJECTS CHEST LATENCY CHEST THICKENED UNTHICKENED WHITE WATER CHEST CLEAR CLOUDY

25 Screening and Cleaning Screening uses about 1-1.5% of total MP plant electrical energy consumption > Pumps > Screens > Agitators Process significance > Screening is the second most important operation in the MP plant > Effective screening is essential for optimizing mainline and rejects specific refining energy loads. The MP Initiative will focus on improving screening operation > Optimizing screening system configuration > Continuous pulp quality monitoring of the mainline and rejects screen accepts and rejects streams > Rotor and screen basket design > Other pulp screening/cleaning technologies that enhance system fractionation efficiency

26 Cleaners Centrifugal Cleaning Whole plant 4 stage cleaner systems use about 2-2.5% of total MP plant electrical energy consumption > Pumps > Agitation > Dewatering > White water management Process significance > Considered essential in plants producing higher value SC and LWC grades Increasingly considered unnecessary given recent improvements in screening technology new rotors and high capacity slotted screen baskets

27 Rejects System Typically carried out in a single stage of HC refining Purpose is > Shive and debris reduction, and > Continued treatment of underdeveloped long fibre fraction Rejects typically 30-40% of mainline production Screens/cleaner rejects thickened to 30% prior to refining Refined rejects diluted and re-screened > Screen accepts sent forward to disc filter > Rejects returned to rejects system for further treatment

28 Rejects System FROM LATENCY CHEST WW PRIMARY SCREENS SECONDARY SCREENS REJECTS SCREENS DISC FILTER WW WW REJECTS PRESS COLLECTING CONVEYOR REJECTS SIDEHILL SCREENS TO HEAT TO HEAT RECOVERY RECOVERY PSF REJECTS CYCLONE REJECTS CHEST REJECTS CHEST THICKENED UNTHICKENED REJECTS CONVEYOR REJECTS REFINER PSF REFINED REJECTS TRANSFER CHEST REJECTS LATENCY STORAGE TANK WHITE WATER CHEST CLEAR CLOUDY TO CHIP WASHING TO PROCESS TO PROCESS TO PROCESS

29 Rejects System Approximately 20% of MP electrical energy consumed here > Refining > Equipment drives > Screw Conveyor drives > Pumps > Agitators Process Significance > Completes development of the long fibre fractions in the MP process The MP initiative will focus on > Optimizing screening operation to improve refining efficiency and balance mainline and rejects specific refining energy > Process measurement and advanced refiner control > Low energy plate development > Heat recovery

30 Annual Ring Cross Section LLW ELW LEW EEW Relative Calender Pressure Relative Spring back Factor Transition-wood

31 White Water Management Ultimately concerned with managing the level of dissolved solids in the TMP process water released during pulping Accepted practice is to introduce fresh water to services where water quality is most critical and cascade it counter-currently as far back in the process as possible before purging it to effluent treatment For TMP, paper machine white water is pumped to the cloudy side of the white water chest for use in TMP process and to the high density storage tank for thick stock dilution White water is purged from the TMP to effluent treatment either directly from the clear side of the white water chest or indirectly through down stream processes such as chip washing and chip pretreatment

32 White Water Systems TMP STOCK CHIP DRAINER DISC FILTER TO REJECTS CONVEYOR CHIP DRAINER SIDEHILL SCREENS CHIP PRETREATMENT TO REJECTS CONVEYOR PSF SIDEHILL SCREEN TO CHIP WASHER STEAM CLEAN DIRTY GRIT CLEANERS PSF PRESSATE TANK BLEACH TUBE TMP HD STORAGE TANK TO REJECTS CONVEYOR CHIP DRAINER FILTRATE TANK WHITE WATER CHEST TO HEAT RECOVERY AND EFFLUENT TREATMENT CLEAR CLOUDY TO PAPER MACHINE TO PROCESS COLLECTION BASIN MEDIUM CONSISTENCY PUMP SODIUM HYDROSULPHITE PAPER MACHINE WW MAKE-UP FROM REJECTS SIDEHILL SCREENS TO PROCESS

33 White Water Systems Approximately 1.5-2% of MP electrical energy consumed here > Pumps > Equipment drives > Agitators Process Significance > Other than high consistency refining, pulp passes through each unit process as a suspension in water Consider > Pumping systems analysis > White water usage analysis > Operational set points analysis to reduce the volume of re-circulating water in the process i.e. screen feed consistencies > Heat recovery on waste water streams leaving MP plant

34 Energy Efficiency in Mechanical Pulping Heat Recovery

35 MP Plant Energy Balance 15 C PROCESS WATER 18 m 3 55 C REFINING ENERGY 2400 kw-h/t AUXILLARY DRIVES TMP PLANT SOLIDS TO 75 C PROCESS VENTS PULP C EFFLUENT 15 m 3 60 C Heat Recovery Potential

36 MP Heat Recovery Features of a modern MP plant heat recovery system > Primary all, secondary and rejects refiners all pressurized and contribute to heat recovery system > Clean steam generated using a reboiler for process heating or power generation > Process and make-up water heating from effluent stream heat extraction Design goals > Minimize MP water usage > Minimize fibre carry-over in dirty steam system > All MP process heating supplied internally, external steam is used for start-up only

37 TMP for Standard News

38 Overview Heat Recovery in Modern TMP Plant

39 Collection of and Distribution of Refiner Generated Steam Steam to Mainline Latency Chest Steam to Rejects Latency Chest

40 Chip Conditioning and Pretreatment

41 Clean Steam Generation and Process Water Heating

42 Mainline Refining Mass/Energy Balance

43 Mainline Refining Mass/Energy Balance

44 TMP Heat Recovery - Case Study

45 TMP Energy Flows without Heat Recovery

46 TMP Energy Flows with Heat Recovery

47 Energy Balance Comparison

48 Heat Recovery Potential per 750 BDt/d Line/mill

49 Total BC Heat Recovery Potential

50 MP Heat Recovery Economics

51 Summary Energy savings opportunity possible from mechanical pulping is enormous Initial focus is on improving efficiency of existing processes MPI focus is on process efficiency i.e. kwh/bdt > Efficient Grade Appropriate Quality The only PSP program that makes the connection between product quality and energy > We recognize that an energy saving technologies that negatively impact product quality will not go forward