Newcastle University. EPSRC: Thermal Management of Industrial Processes. Paper test case study report. (June 2010) Report prepared by

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1 Newcastle University EPSRC: Thermal Management of Industrial Processes Paper test case study report (June 2010) Report prepared by Dr Yasmine Ammar, Sir Joseph Swan Centre for Energy Research 1

2 List of figures Figure 1: papermaking process schematic representation (website: fpmp.html)... 4 Figure 2: papermaking process schematic representation... 5 List of tables Table 1: Raw material supply for paper industry... 4 Table 2: Characterization and classification of potentially recoverable low grade heat gas streams in the paper industry Contents 1. Introduction Overall process overview Pulpmill process Board machine process Water and effluent treatment process Low grade heat source characterisation Conclusion

3 1. Introduction The case study presented in this report is an integrated Pulp and board mill producing over 150,000 tonnes of product from virgin timber. It is equipped with a CHP plant owned by EON which supplies the plant for all its steam and most of its electricity requirements. The latter comprises 2 turbines: The Gas Turbine (GT) generator power has a maximum output of 42 MW but it varies depending on the value of the ambient air temperature. It is usually operated at its maximum output and any excess electricity which is not required by Iggesund is exported to the grid. This is fired on gas or distillate, the exhaust gas going to the heat recovery boiler (HRB) to generate steam. The Back pressure steam turbine has a rated capacity of 8 MW. It is used to reduce the steam pressure from 32 bars down to 3.8 bars for the process. The output varies between 4.6MW and 5.5 MW, depending on the steam mass required to meet process requirements. The exhaust gas from the GT passes through the heat recovery boiler and the recovered heat is capable of generating 58 tonnes/yr of steam at 32 bars and 335 C. 42 tonnes of steam is added to the process by further boiler firing, either by gas or distillate up to a load of 105 tonnes / hr. This is the steam load required for drying. The boiler maximum steam capacity is 105 tonnes /yr without the use of exhaust gases from the GT. The boiler has indeed to ensure the total Steam requirements in case of GT shut-down due to maintenance. In case of CHP boiler failure or maintenance, 2 boilers 1 and 3, which are also part of the CHP plant and owned by EON, act as a backup system and can be brought into service in order to meet the steam loads. Steam pressure then needs to be reduced to 10.3 bars at 200 C and 3.8 bars at 165 C (for the dryers). The construction of a bio-fuelled CHP plant would be the best solution for energy savings and CO 2 emission reduction. Furthermore, biofuels which are bark, fibre rich effluent cake, sawdust are produced on site. However, one of the downside aspects is that the biofuels are already sold and could be worth financially to the plant. This has already been a problem in trying to get a payback for a bio-fuelled plant. Also, the wood chips required would then amount to 400 kilotonnes which means doubling the wood supplies. This is a significant price increase implication. However, more wood could be made available as biofuels if the tree biomass currently left in the forest in the form of branches and tree top sections could be exploited on site too. Its Spanish pulping operation also produces large quantities of Eucalyptus waste which could be shipped into Workington Docks. Besides, there is locally a company which processed domestic waste into a dry fuel with a significant percentage of biomass. This is being investigated as a possible source of material but the quality will have to be consistent so that the project is not classified as an incinerator with all the associated restrictions. If the capital proposal is approved, the plant would be commissioned in The total electrical load is 43MW in average, peaking at 60 MW. The maximum grid import power is limited to 44.4MVA which corresponds to about 37 MW. Currently, under normal conditions, about 6 MW of electricity is imported from the grid, the rest being produced on site by the CHP plant. In case of CHP Gas Turbine shut-down, limit production operations procedure is implemented so that the grid import limit is not exceeded. In the past, the plant was able to take loads from the grid of up to 55 MVA at short notice but in 2004, a wind farm has been installed within 15 miles with 9 wind turbines ranging from 10 MW to 180 MW and was connected onto the same section of the grid. As a consequence, Iggesund can no longer operate its plant at full capacity from the grid. More precisely, 3

4 its connection formerly 55 MVA is restricted to 44.4 MVA to allow space on the grid from these wind farms to export their power. This has now forced Iggesund into a major investment to get a new line to the mill. Furthermore, the Natural gas requirements are 6000 Therms / Hr (~176 MW). Below is the summary of the site raw material supply. Table 1: Raw material supply for paper industry Material Quantity (tonnes/year) Type Wood Spruce Chemical pulps bale form of dry sheeted material for re-pulping on site Chemicals, pigments and fillers wet or powder form 2. Overall process overview Figure 1: papermaking process schematic representation (website: fpmp.html) Figure 1 gives a schematic representation of the papermaking process. The timber arriving at the mill has to go through the stock preparation stage. After de-barking, the wood successively goes through the chipping machine and the mechanical pulping process. The mechanical pulp is then separated into a suspension of individual fibres in water in the hydrapulper. Chemical pulp is also added for repulping. In this case study, the chemical pulp is not processed on site but imported. The way mechanical pulp and chemical pulp are mixed together is called the furnish. In many paper making plants, part of the pulp also comes from waste paper but that was not the choice of this test case study. Once the papermaking stock has been prepared, it is often held in a machine chest, from 4

5 which the stock is fed to the paper making machine called board machine, as needed after screening and cleaning. Figure 2 gives the general diagram of the paper making process. The first part of the paper machine, where the sheet of paper is formed, is called the Wet End. The part of the board machine where the paper is successively dried, sized and calendered is called the Dry End. The next sections aim at identifying low grade sources throughout the paper mill. Each process is described by a flow diagram on which the low-grade heat sources are identified in orange. Intermediate heat streams are given in red while incoming products are given in green. Wood Wood yard Bark Chip Hot effluent Pulp mill Chemi cal Wate Bleaching Chemicals Coating mixes Effluent Water & Effluent treatment broke Furni Wate Board machine Chemic Coating & Chemicals Effluent Steam Paper board Finishing Reel Figure 2: papermaking process schematic representation 5

6 2.1. Pulpmill process The pulpmill contains one large pulper of 18 MW, 4 pulpers of 4 MW and 3 pulpers of 1.5 MW. Pulpers are highly energy intensive processes. The total pulping load at full production is around 36 MW. Most of this is released as heat in the form of low grade steam. The largest pulper is pressurised at 5 bars. The mixture of steam and pulp is passed through a cyclone. A tonne of steam is then generated by Megawatts of pulper power. The steam obtained contains wood fibre which prevents from directly using it in the process. A reboiler which produces clean steam will be included in the next investment for a large refiner to replace the 4 MW units and to provide 38 tonnes of steam. The steam has not been identified as a potential for heat recovery since it is currently used to heat up the water in a surface condenser for the washing of the wood chips. Steam from the 4 MW units is at atmospheric pressure and needs a large fan to collect it. This produces hot water using a scrubbing tower. Hot water Wat Chip Scrubbing tower Hot water Chip washing Stea Surface condenser 18 MW Pulper 3 x 1.5 MW Pulpers 4 x 4 MW Pulpers Fan Stea Cyclone Screening and pulp beaching Hot effluent Pulp storage wet & dry Chemi cal Mechani cal pulp Re-pulping Broke processing & storage Brok Mixing Conditioning BOARD MACHINE Postprocessed 2.2. Board machine process The board machine has a length of 200 metres. It is six metre wide at the wet end and produces board with a width of 5.5 metres. Its production speed is 315 m/min and it produces 28.5 tonnes per hour. The machine is faster at the wet end and slows down as the board dries out. 6

7 The paperboard consists of five layers. The first layer, the liner, is made of chemical pulp, comprising softwood (pine and spruce mainly from Sweden) and hardwood (birch from Northern Europe). The three middle layers contain mechanical pulp, made from spruce (all sourced from UK FSC certified forests). The last layer, the back, is chemical pulp mainly made of eucalyptus from Spain or Portugal. Water drains from paper layers and is vacuumed from pulp layers. It is then put through rollers. The power requirements mainly lie in the production of vacuum and the main energy usage is for the steam drying the products. Steam drying systems are now very efficient at getting most of the heat out of the steam without resorting to condensers. The machine consists of 5 sections: - The wire section where the board is formed by the water carrying only 0.4 % fibre draining through a moving wire mesh. The board is formed in 5 layers. - The press section where the wet board has the water removed by passing it between vacuum rolls. A 1 % reduction in the moisture content of the board leaving this section results in a 4 % reduction in drying steam requirement. Moisture content of the board leaving this section is 58%. - A drying section where the board is carried with around 77 steam heated rolls evaporating the moisture. - A coater section where the board is coated back and front by a printable liquid medium. This is dried using gas fired infrared radiators. - Reel-up where the finished board is wound onto 300 mm diameter metal drums to form reels 2.7 metres in diameter. The purpose of the board machine is to produce board with a moisture content of 6 % from a material containing 99.6 % water. Therefore most machine sessions comprise steam heated drying cylinders. The temperature of the condensate reduces from 165 C to 98 C. The condensate is collected in flash vessels where the flash steam is collected and is reused in the lower pressure cylinders. Passing the condensate into lower and lower pressure parts of the system allows maximising the heat recovery from the steam. The whole machine drying section is covered by an enclosed hood fitted with extraction fans in order to pull the moisture from the drying cylinders out of the roof. Fresh air is introduced through floor duct into the hood and is heated with steam which is a crude form of heat recovery consisting of transferring the heat from the exhaust air to re-heat the fresh air. The fact that the board is very wet at the start of the process is ideal for achieving this pressure gradient through the machine, condensing the steam very rapidly in the earlier cylinders creating a vacuum. 7

8 Air balance has been affected by recent plant changes. In particular, the installation of new machinery replaced air heater at ground level and air being ejected from the hood was not replaced. The air in winter is well below zero and drawn into a very moisture machine hall, this causes all sorts of condensation problems. A programme has had to be introduced to try to restore the air balance, these corrections due to the mistaken belief that people were making the process more efficient are likely to cost more than 1 million pound. The low grade heat sources identified from this process are streams with a maximum temperature of 109 C from the after-drying section. No recoverable steam low grade heat was identified within the board machine process. Furnish Mist Wet-end Production of Paper web Liner Fourdrinier table 3 middle stations bar Backs Fourdrinier table High speed roof fans Vacuum system exhaust Wate Paper web Wa Pressing (series of drilled hollow rolls) Stea Exhaust air 1 Exhaust air 2 Hood Extracted fans Pre-drying 41 steam drying cylinders Basement Exhaust air 3 Exhaust Exhaust air 4 Yankee Hood After-drying Machine glazing Heated fresh air Hood drying cylinders 6 m - diameter steam heated roll Hot Gas fired hood Size exha ust Sizing Calendering Gas firing radiant heat coating drying Pape r Exhaust air 4 Exhaust air 5 Combustion Vapou Exhaust air 6 8

9 2.3. Water and effluent treatment process Pulpers and screw presses used to thicken the pulp produces very hot effluent (95 C) tonnes/hr (or 22 million litres/day) effluent comes through the effluent treatment plant. Effluent consists of water laden with short fibres, china clay and pigment. It is kept in settlement before being discharged. 22 million litres/day of water at temperature in excess of 35 C disposed into the sea represent a potential for low grade heat recovery. Solid wastes are sold to animal hygiene companies (potential biomass source). Wate Water abstraction from River Derwent Filtering and chemical treatment Effluent Effluent pumping Water storage Settling ponds Wa Effluent thickening Effluent Discharge to the sea Wate 3. Low grade heat source characterisation 22 million litres/day of water at temperature in excess of 35 C is disposed into the sea. There also exist large quantities of exhaust air leaving the roof at high temperature without heat recovery. Most of the low grade heat available is given by the Board Machine (BM). The low grade air heat streams come from the hoods of the BM drying section and the water is transferred in the water treatment unit from the BM wet-end section. Air and water streams identified in previous sections are classified in terms of their exergy (calculated values) and characterised with their main thermodynamic properties. 9

10 Table 2: Characterization and classification of potentially recoverable low grade heat gas streams in the paper industry Description size exhaust Water discharge to sea Water Water hot water vapour Combustion gas hot water vapour Location size exhaust BM Pre-drying hood- BM After-drying hood -BM Pre-drying hood -BM After-drying hood -BM After-drying hood -BM Pre-drying hood -BM Pre-drying hood - BM MG hood BM effluent and water treatment department Heat exchanger- Pulpmill Vacuum system exhaust- BM Vacuum system exhaust- BM Gas firing radiant heat coating drying- BM Gas firing radiant heat coating drying- BM Temperature ( C) Mass flow rate (kg/s) Moisture content enthalpy (kj/kg) Energy content (MW) Exergy (MW) N/A N/A ?? N/A????????????????????? 10

11 The mill is isolated from population centres, Workington being about 2 km away. This heat has been offered to plan new housing development but there is not a willingness to take this up. This is outside the experience of most housing developers to incorporate district heating into their plans. The process is continuous but once a month, the mill shuts down for maintenance, typically from 12 pm to 9/10 am. There exists also an annual shutdown which lasts 72 hours for replacing machine/ pressure vessels to be inspected every 2 years by insurance inspectors. Unplanned plant shutdown represents ~29 days per year. The payback period for small recovery projects is 18 months while it can be up to 15 years for larger projects. 4. Conclusion The low grade heat sources have been identified as effluent water (~20% of the input energy flow) and air (27% of the input energy flow) from the board machine. The combustion gas and the vapour exhaust from the heat gas firing coating drying system as well as the stream from the vacuum exhaust system could potentially contribute to low grade heat amount but further investigations is needed with regards the usefulness of these streams. The company strategy is often a barrier for heat recovery project. In this case, the company mind set is that production improvements have first call on capital and this limits heat recovery projects. Furthermore, the plant is not divided into profit centres and therefore energy cost reductions do not benefit the department making them. 11