用水与用能5

nd im *, T ent, 2004 line 1 ioned ould s po A case study of a Swedish kraft pulp mill showed that 5–6 MW excess heat with a temperature above 90 �C could be released if the system were rebuilt. Two different alternatives for using the excess heat have been investigated. � 2005 Elsevier Ltd. All rights reserved. closed water loops and greenhouse gas abatement poli- cies. This could lead to new opportunities for energy fined by the hot streams that produce process water or are cooled in coolers and the cold streams that represent opportunities for energy saving; see e.g. [3–5]. Combin- ing energy and water pinch to include possible synergis- tic effects has also been studied, e.g. [6]. The definition of usable excess heat in this work corresponds to the one stated by Wising [2]: . * Corresponding author. Tel.: +46 31 772 30 22; fax: +46 31 82 19 28. E-mail address: roger.nordman@chalmers.se (R. Nordman). Applied Thermal Engineering 2 1359-4311/$ - see front matter � 2005 Elsevier Ltd. All rights reserved savings. The fact that the price of energy has increased substantially in recent years makes energy-saving pro- jects ever more attractive [1,2]. In existing mills, warm and hot water is often over- produced and both warm and hot water tanks are commonly overflowed to a cooling tower, or drained away. Instead of producing this surplus, the heat (cool- ing demand) could be used for other purposes [1,2]. Since the main objective for the pulp mill is to produce high-quality pulp, a change in the heat recovery system the process water heat demand. The method shows how much energy at a high temperature it is theoretically and practically possible to release through a new design method for the HWWS. 2. Theory Many process integration (PI) studies in the pulp and paper industry have been done, mostly to investigate the Keywords: Pulp mill; Heat recovery, Pinch analysis; Optimization; Hot and warm water system 1. Background The pulp and paper industry faces new challenges and new operating conditions. Examples are stricter environmental regulations that increase the need for should not affect the process in a negative way. There- fore a new method has been developed to maximize the amount of usable excess heat (hereafter referred to as Qxs) in the hot and warm water system, HWWS (sec- ondary heat system) in a pulp mill. The HWWS is de- Design of kraft pulp mill hot a method that max Roger Nordman Heat and Power Technology, Department of Energy and Environm Received 28 December Available on Abstract Many pulp mill hot and warm water systems are overdimens overproduction could be used for other applications, if heat c method to release as much excess heat of a high temperature a doi:10.1016/j.applthermaleng.2005.06.001 hore Berntsson Chalmers University of Technology, SE 41296 Go¨teborg, Sweden ; accepted 3 June 2005 9 July 2005 and produce more hot and warm water than demanded. This be released at high enough temperatures. In this paper a new ssible in the secondary heat system has been developed. warm water systems—A new izes excess heat www.elsevier.com/locate/apthermeng 6 (2006) 363–373 Nomenclature AHX heat exchanger area (m 2) AEvap total evaporator area (m 2) Cel electricity price (US$/MW h) Cfuel fuel price (US$/MW h) CDH district heat price (US$/MW h) Clost lost electricity production (kW) h00 saturated steam enthalpy (kJ/kg) h 0 saturated liquid enthalpy (kJ/kg) n number of evaporator effects used by excess heat N total number of evaporator effects Qxs usable excess heat (kW) QC cooling demand (kW) QH heat demand (kW) Qtot total evaporation heat demand (kW) Qfuel fuel demand (kW) top time of operation (h/yr) VTank tank volume (m 3) C ratio of saved live steam to excess heat (–) DTmin minimum temperature difference (K) gb boiler efficiency (–) gT turbine efficiency (–) gm+g mechanical and generation efficiency (–) CC composite curve CTMP chemi-thermo mechanical pulp FOB free on board NAS neat annual savings HEN heat exchanger network HWWS hot and warm water system (also referred to as ‘‘secondary heat system’’) TTL tank temperature level 364 R. Nordman, T. Berntsson / Applied Thermal Engineering 26 (2006) 363–373 ‘‘The definition of usable excess heat is heat sources with a high temperature, typically above 80 �C but still below the pinch temperature, which is not needed for warm and hot water heating. The expression �usable excess heat� implies that it can be used in the process to reduce energy demand but by pinch analysis definition there cannot be any �excess heat� below the pinch temperature. However if the evaporation plant is regarded as a flexi- ble unit that is allowed to be redesigned, the heat source below pinch temperature can be used for evaporation, substituting live steam. Heat could also be exported to other processes, where it could be used, hence the expression usable excess heat.’’ Q (kW) Excess heat T (° C ) a Fig. 1. (a) Composite curves show how the temperature of the cooling deman until reaching the HWWS DTmin yield the maximum usable Qxs. 3. Process description The kraft process consists of two main parts: the fibre line where pulp is produced from wood, and the chemi- cal recovery cycle that recovers cooking chemicals and produces process heat. Heat recovery is applied to reduce energy consumption, and in a pulp mill this is achieved mainly in a secondary heat system (HWWS) consisting of heat exchangers and tanks. The HWWS has four main functions: 1. To supply warm and hot water to the process, for example to dilute the pulp. Qxs Q (kW) ∆Tmin Excess heat of low temperature Excess heat of high temperature T (° C ) b ds, which could be used for Qxs, varies. (b) Shifting the tank curve left 2. To heat cold process streams in heat exchangers. 3. To cool hot process streams, thus producing warm and hot water. 4. To act as a heat buffer when the heat demands and supplies vary. The heating and cooling demand in the HWWS in a pulp mill is illustrated in Fig. 1a. Arrows illustrate heat recovered by heat exchange. The remaining heat varies with temperature and is presently cooled away. When the hot streams are cooled, more warm and hot water than needed is produced; this overproduction can be seen as cold utility. It is, however, not necessary to use the HWWS as a cold utility. Instead the cooling could be separated from process water heating. Shifting the cold CC leftwards would increase Qxs at high temperatures, but also decrease the DTmin and increase the area required for heat exchange, Fig. 1b. below. 3. Identify the process demands and the cooling demands from the cold streams. 4. Construct the cold CC from the cold streams for the process demands. 5. Calculate the theoretical target for the maximum excess heat at a given DTmin. 6. Replace the cold CC with the ‘‘tank curve’’. The tank curve is defined below. 7. Vary the number of tanks and their temperature levels to maximize the amount of excess heat. Step 1. Construct the hot CC from all hot streams in the HWWS. Steps 2–4. In our method the cold CC represents only net process demands of hot or warm fresh water. All cold streams� start temperature is set to the inlet temperature of the fresh water. The target temperature is set to the process demand temperature. Defining the Q (kW rom th rs. l cold R. Nordman, T. Berntsson / Applied Thermal Engineering 26 (2006) 363–373 365 1. Identify the HWWS hot streams and construct the hot CC. 2. Identify the HWWS cold streams. T (° C ) 0 10 20 30 40 50 60 70 80 90 0 5000 10000 15000 The tank curve for one tank level (at 85 ºC) The original cold curve f demands of the final use Fig. 2. Tank curves are constructed based on knowledge of the origina Note that the DTmin decreases, although QC is the same. The purpose of this shift would be to make the excess heat at high temperature usable. 4. New design method The method developed is based on composite curves [7]. In addition to the hot and the cold CC�s, a third curve is introduced, called the ‘‘tank curve’’. The steps in the method are listed and explained in more detail and their temperature-wise placement. ) 20000 25000 30000 35000 TTL 1, curve 1 Two tank curve with two tank levels e composite curve, and look different depending on the number of tanks cold CC in this way the slope will contin- uously increase, since the slope equals 1/F * Cp (Fig. 1a). Step 5. Shift the cold CC leftwards until a user set DTmin is reached. The part of the hot CC that overshoots on the right of the cold CC is the theoretical maximum of Qxs (Fig. 1b). Step 6. Heating of fresh water to the different TTL�s in the system composes the tank curve. All fresh water is heated from the fresh water inlet temperature to the temperature levels where tanks are placed in the system. The number of stream intervals in the tank curve is therefore equal to the number of tanks in the HWWS. The tank curve has the same T = 85 ºC TTL 1, curve 2 TTL 2, curve 1 and 2 trarily on the cold CC. The amount of excess heat that can be made available differs depending on the combina- 6. Case study 6.1. General description To demonstrate the new method a case study has been carried out in the Storaenso Skoghall mill. It is an integrated board mill with both CTMP and kraft pulp, and 550,000 tonnes board/yr capacity. The pro- duction of CTMP is 200,000 tonnes/yr and the kraft pulp production is 290,000 tonnes/yr. This study has only covered the kraft pulp plant. The Skoghall mill is considered a low-water-usage mill. The opportunities for a grass-root and/or retrofit 366 R. Nordman, T. Berntsson / Applied Ther tion of TTL�s (Fig. 2). For each combination of temper- atures, a tank curve will replace the original cold curve. This curve is then shifted leftwards in the diagram until DTmin is reached. By systematically checking the value of Qxs for all TTL combinations in the HWWS, the maximum Qxs is reached for a specified number of tanks. A theoretical optimum of Qxs is not necessarily an economic optimum; thus an economic evaluation must be done for different DTmin�s. A small DTmin will give more excess heat but the heat exchanger area needed to release this heat will increase. Hence there is a trade-off between investment costs for new heat exchanger area, and expected savings from the released excess heat. It could also be practical to place a tank at a temperature level where there is a process demand even though it is not optimal to maximize Qxs. In this way, mixing of the tank water with cold water to reach the demanded temperature is avoided. More details on the economic evaluation are presented in the case study. The method has been implemented in a Visual Basic code. Computation effort for a four-tank problem involves approximately 3.2 · 107 function evaluations, requiring about 780 CPU seconds on a 2.4 MHz Pentium 4 PC. 5. Applications using excess heat Two ways to use the released energy were investigated: to replace live steam in a non-conventional evaporation plant [1], or to provide district heating. A third, but not studied, way to use the Qxs would be heat pumping. Raising steam from the excess heat could replace live steam in the evaporation plant. Because of the low tem- perature of the steam raised, it will not be possible to use it in all evaporator effects. The total heat needed for evaporation, Qtot, can thus be expressed as (see Fig. 3 for designations): heat demand as the original cold CC. The two curves coincide at the TTL�s, as well as at the start point. Step 7. Maximize the amount of excess heat by varying the number of tanks and their temperature levels, as well as the DTmin. One tank level always has to be placed at the highest temperature to supply the warmest demands. If only one tank were installed in the system, all water would have to be heated to the highest temperature. With two TTL�s, one tank still has to be placed at the highest temperature, while the second TTL can be placed arbi- Qtot ¼ QI � N I ð1Þ or Qtot ¼ QII � N II þ Qxs � n ð2Þ The reduction in live steam demand, DQ, by using Qxs is then DQ ¼ QI � QII ¼ Qtot N I � Qtot � Qxs � n N II ð3Þ For the case when NI = NII = N: DQ ¼ Qxs � n N ð4Þ Here we introduce the factor C as the ratio of saved live steam (DQ) to the amount of Qxs when live steam is re- placed in non-conventional evaporation. The factor C typically takes values between 0.6 and 0.9 but values greater than unity can occur, e.g. when the surface con- denser operating temperature is lowered. Excess heat is then allowed to cascade through more evaporator units. The reduction of live steam results in lowered fuel con- sumption and lowered electricity production, since less steam runs through the steam extraction back-pressure steam turbine. Using the Qxs for district heating requires that the plant is located close to a community that already has a DH network; otherwise the investment cost would be unrealistically large for almost any value of Qxs. QXS QIIQI NI n NII Fig. 3. Scheme over traditional (left) and excess-heat-driven (right) (also known as non-conventional evaporation) evaporation. mal Engineering 26 (2006) 363–373 design of the HWWS were investigated, using the new T 1 1 1 1 1 7 75 1500 Thermal Engineering 26 (2006) 363–373 367 method, with the aim of releasing Qxs at temperatures as high as possible. Economic consequences for different use of the released Qxs were studied. The existing HWWS contains four tank levels with water at 45, 65, 75 and 85 �C. These tanks supply the warm and hot water demanded in the process. The system also handles the cooling demands in the process, about 41 MW. The overproduction of hot water is drained away. Stream data for the HWWS are taken from previous work by Table 1 Stream data representing the HWWS streams in the case study Stream name Type Clean condensate from condenser Hot Whitewater cooler after ClO2 reactor Hot Stripper condensate cooler Hot Wash liquor cooler, wash filter Hot Scrubber Hot HX D007 Hot Evaporator 1 Hot Evaporator 2 Hot Evaporator 3 Hot Pre-evaporator Hot Condensing of relief vapors Hot After HX D004, steam Hot After HX D004, supercooling condensate Hot Wash press after ClO2 Cold Wash press after Q bleaching Cold Oxygen bleaching Cold Mixing plant Cold Internal and external heating Cold Tank B-900 Cold WW 65 unbleached pulp distribution Cold WW 45 unbleached pulp distribution Cold R. Nordman, T. Berntsson / Applied Bengtsson [8] where the streams were defined. Table 1 lists these streams and the corresponding cold and hot CC�s are shown in Fig. 1a. Because only process streams that have a heating demand are included, the CC�s are not balanced. Hot streams that are not fully utilized in internal heat exchange to reach its target are cooled to the target temperature. 6.2. Excess heat potential A total of 16 combinations of DTmin and tanks (DTmin of 1, 3, 5 and 10 K and 1, 2, 3 or 4 tanks) were optimised to maximize Qxs. The results showed that al- ready with two tanks 100% of the theoretical maximum Qxs was reached. More tanks would therefore not bene- fit excess heat. The minimum temperature of Qxs is di- rectly related to the amount of excess heat available; when Qxs increases, the lowest temperature of the excess heat also decreases. 6.3. Sensitivity analysis The sensitivity of choosing the tank temperatures in an HWWS with two tanks at different temperatures has been analysed. One TTL is chosen at the highest tem- perature demanded in the system, which for the case study is 85 �C. The second TTL can be chosen arbi- trarily on the cold CC. Fig. 4 shows the amount of Qxs as a function of the second tank temperature level for different DTmin�s. When DTmin is large, no excess heat can be released, which is indicated by zero value in the diagram (unpinched problem). In a system with a small DTmin the choice of temperature level need 7 65 8570 7 85 5000 7 55 4210 7 65 2150 7 45 1410 start (�C) T target (�C) Q (kW) 70 20 3060 66 57 810 10 48 5390 02 89 2860 67 60 5830 10 100 1600 76 42 6340 68 67 24,530 67 48 830 77 55 18,960 80 50 3270 23 122 980 22 43 1960 7 85 6860 7 85 6860 not be very exact, because of the flat optimum. With larger DTmin it becomes increasingly important to choose the right temperature level. The optimal TTL for achieving a high Qxs decreases as the DTmin in- creases, and the optimal TTL is found where the system pinches. 6.4. Economic evaluation Good industrial practice means that DTmin values lower than 5 K would not be used [9]. Therefore, in the economic evaluation, only DTmin�s of 5 K and 10 K were evaluated, although even larger amounts of Qxs could theoretically be released by smaller DTmin. The economic evaluation for the different design alterna- tives is based on the net annual savings method: Net annual savings (NAS) = Income from sold excess heat � Income losses because of lost electricity produc- tion � Annualized investment costs. Positive values of NAS mean that the investment is recommended. The annualised investment costs are expressed as I � a ð5Þ 45 mper perat Elect Thermal Engineering 26 (2006) 363–373 0 1000 5 15 25 35 Te Fig. 4. Usable Qxs for different DTmin as a function of the tem Table 2 Cost parameters for the scenarios used in the economic evaluation Scenario Oil price (US$/MWh) Bark price (US$/MW h) 2000 3000 4000 5000 6000 7000 8000 Q x s (kW ) ∆Tmin = 15K ∆Tmin = 10K ∆Tmin = 5K ∆Tmin = 3K ∆Tmin = 1K 368 R. Nordman, T. Berntsson / Applied where I is the capital investment, a is the annuity factor. The investment is annualised using an annuity factor based on interest rate, i, and assumed economic lifetime, n. The annuity is expressed as [14]: a ¼ i 1� ð1þ iÞ�n ð6Þ Annuity factors of 0.1 and 0.25 were used in this case study. Four different energy market scenarios [10] were used to evaluate the economic consequences. The eco- nomic parameters for each scenario are listed in Table 2. 6.5. Investment costs A ‘‘grass-root’’ design would be an alternative only if one has to invest in a new HWWS. In the economic eval- uation of the grass-root design, only the extra cost for building a system that releases the excess heat has been included. In the retrofit cases the total investment cost is included. The calculations include cost for new heat exchangers and new tanks, and the extra cost of building non-conventional evaporation compared to traditional evaporator design. Piping cost is calculated as 40% of FOB price of new heat exchangers [11]. The cost for the steam reformer is in each case calculated as a heat exchanger, Table 3. The volume of the tanks is chosen 1 26.9 13.4 38.7 2 22.0 13.9 40.3 3 29.3 18.2 52.5 4 41.1 26.4 76.2 55 65 75 85 95 ature (˚C) ure of the second tank level in a system with two tank levels. ricity retail price (US$/MW h) District heat price (US$/MW h) to carry 24 h of hot water demand. A new evaporation plant is so costly that investment is only justified if new evaporators have to be built anyhow. Instead the marginal cost of building a non-conventional evapora- tion is calculated. As reference, a traditional evapora- tion plant with six effects and a surface condenser at 60 �C has been used. Other ‘‘traditional’’ configurations exist, e.g. seven effects with surface condenser at 40 �C, but the one chosen represents the most common design. The non-conventional evaporator plant is suggested to have seven effects with excess heat used in the last four or five effects, depending on the temperature of the excess heat. Cost equations used are: Heat exchanger costs [12]: Costð$Þ ¼ 30800þ 1644 � A0.81HX ð7Þ Tank costs [13]: Costð£Þ ¼ 2.5 � 1200 � V 0.55Tank ð8Þ Evaporator costs [1]: Costð$Þ ¼ 8.5� 105 � nþ 333 � AEvap ð9Þ Cost of lost electricity production: Costð$=yrÞ ¼ Elost � top � Cel ð10Þ 20.2 20.8 26.0 35.8 R. Nordman, T. Berntsson / Applied Therm Table 3 Investment cos