Concept of Heat Recovery from Exhaust Gases

Transcription

1 IOP Conference Serie: Material Science and Engineering PAPER OPEN ACCESS Concept of Heat Recovery from Exhaut Gae To cite thi article: Maria Bukowka et al 2017 IOP Conf. Ser.: Mater. Sci. Eng View the article online for update and enhancement. Related content - Experimental and numerical analye on a plate heat exchanger with phae change for wate heat recovery at off-deign condition Roberto Cipollone, Giueppe Bianchi, Davide Di Battita et al. - Analyi of Deep Heat Recovery From Flue Gae Y V Shatkikh, A I Sharapov and I G Byankin - CO2 trancritical refrigeration cycle: potential for exploiting wate heat recovery with variable operating condition M Pieve, G Boccardi, L Saraceno et al. Thi content wa downloaded from IP addre on 14/02/2018 at 02:26

2 IOP Publihing IOP Conf. Serie: Material Science and Engineering (2017) doi: / x/245/5/ Concept of Heat Recovery from Exhaut Gae Maria Bukowka 1, Krzyztof Nowak 1, Danuta Prozak-Miąik 1, Sławomir Rabczak 1 1 Rzezow Univerity of Technology, al. Powtańców Warzawy 12, Rzezów, Poland mbuk@prz.edu.pl Abtract. The theme of the article i to determine the poibility of wate heat recovery and ue it to prepare hot water. The cope include a decription of the exiting ample of coal-fired boiler plant, the analyi of working condition and heat recovery propoal. For thi purpoe, a erie of calculation neceary to identify the energy effect of exhaut temperature decreaing and tranferring recovery heat to hot water proceing. Heat recover olution from the exhaut gae channel between boiler and chimney ection were propoed. Etimation for the coteffectivene of uch a olution wa made. All calculation and analyi were performed for typical Polih condition, for coal-fired boiler plant. Typicality of thi olution i manifeted by the volatility of the load during the year, due to ditribution of heat for heating and hot water, determining the load variation during the day. Analyed ytem of three boiler in cae of load variation allow to operational flexibility and adaptation of the boiler load to the current heat demand. Thi adaptation require change in the operating condition of boiler and in particular aurance of properly condition for the combution of fuel. Thee condition have an impact on the exiting thermal lo and the overall efficiency of the boiler plant. On the boiler plant efficiency affect particularly exhaut ga temperature and the exce air factor. Increaing the efficiency of boiler plant i poible to reach by following action: limiting the exce air factor in coal combution proce in boiler and uing an additional heat exchanger in the exhaut ga channel outide of boiler (economizer) intended to preheat the hot water. 1. Introduction The aim of the analyi performed wa a concept of heat recovery from exhaut gae from the boiler room, determination of the poibility to recover wate heat and uing it for preparing hot tap water in neighbouring reidential building. Thi article decribe a typical Polih boiler room in which bituminou coal i burnt, and the poibility and uefulne of heat recovery with that boiler room a an example. 2. Decription of current condition of the analyed boiler room 2.1. Decription of the boiler room The houing etate boiler room of a Houing Cooperative i located on one of houing etate in the town of Radymno. It i intended for upplying heat for central heating and hot tap water purpoe. The boiler room i equipped with SWC Rumia boiler with the power rating of 900 kw each. The boiler are fired with fine coal. Two boiler operate with the purpoe of central heating, and the third one to the need of hot tap water. In the ummer eaon only one boiler operate, upplying building with hot water. The boiler room ha a cloed cycle. Hot tap water i prepared in a JAD-D2-6/50 heat exchanger and gathered in two 2000/6 heat accumulator. Cold water i upplied from the water Content from thi work may be ued under the term of the Creative Common Attribution.0 licence. Any further ditribution of thi work mut maintain attribution to the author() and the title of the work, journal citation and DOI. Publihed under licence by IOP Publihing Ltd 1

3 IOP Publihing IOP Conf. Serie: Material Science and Engineering (2017) doi: / x/245/5/ upply ytem, heated in JAD exchanger, pumped to hot water tank and then to conumer uing pump Decription of exhaut ga dut extraction ytem All boiler have individual fan carrying exhaut gae away to the flue. Three WPW40/1.8 A+K centrifugal ingle-tream fan have been intalled and they have the efficiency Qv=2.2 m/. Exhaut gae from boiler are conveyed to the flue through CE-4x400/0.5 multi-cell cyclone cooperating with the fan. Exhaut duct are connected with the flue. A ection area of the flue i 580x850 mm and it length i approx mm. 2.. Decription of chimney The exhaut gae with the catalogue temperature of 45 K are conveyed from each of the three Rumia SWC 900 boiler though the flue to a joint chimney. It i a teel, 5 m high chimney with an outlet diameter of 0.9 m. The chimney i free-tanding and located outide the building.. Fuel policy In the analyed local boiler room, II A fine coal i burnt to produce heat for the purpoe of central heating and hot tap water. An amount of fuel purchaed in pecific month of the year and it baic parameter were preented in Table 1. Month I m Mg Table 1. Fuel conumed and amount of heat produced Qw MJ/kg Q calculated GJ Coat PLN Price PLN/Mg Price PLN/GJ Q meaured GJ Price PLN/GJ , II , III ,40 IV ,57 V ,24 VI ,46 VII , VIII , IX ,91 X , XI , XII ,05 together ,25 2

4 IOP Publihing IOP Conf. Serie: Material Science and Engineering (2017) doi: / x/245/5/ From information received from the boiler room and calculation made on the bai of conumed fuel, amount of produced heat are obtained, a preented in Table no 1. The table how alo amount of produced heat, a reulting from meaurement of intalled meter of thermal energy. A gro annual value of purchaed fuel according to calculation made wa PLN It i a value of purchaed fuel. Cot of tranport were added to it. So, finally, the gro annual cot of fuel wa PLN 8 817, Energetic aement of current condition A meaure of the effectivene of boiler room operation i a total watt-hour efficiency which include all component of primary and ultimate energy. In the cae of the boiler room, it i ufficient to aume thermal efficiency in which electric energy will be diregarded. The baic thermal balance i determined by the following equation: Qu c m Qw (1) Q u ueful heat produced to the need of heating and hot water, η c total thermal efficiency, m ma of burnt fuel, Q w fuel calorific value. On the bai of Table 1, the calculated average annual calorific value of burnt coal wa: Q w = 2,675 MJ/kg. The total amount of coal burnt in that time wa: m = 1120,5 Mg. An amount of produced heat in the analyed year a indicated by heat meter wa: Q u = GJ. It mean that the total average annual efficiency in the analyed year wa: Qu ,64 64% (2) c m Q ,675 w The efficiency achieved i relatively mall taking into account the analyed type of the boiler room. The total efficiency i mainly affected by boiler efficiency which i influenced by thermal loe incurred during operation of the boiler. They include: carry-over (chimney) lo, incomplete combution lo, imperfect combution lo, radiation lo. The balance of loe i determined by the following relation: η k boiler efficiency, wyl carry-over lo, nc fraction of incomplete combution lo, nz fraction of imperfect combution lo, r fraction of radiation lo. k 1 ( wyl nc nz r ) 5. Carry-over (chimney) lo Mot ignificant in the thermal balance of boiler i a carry-over lo and, therefore, mot attention i given to reducing that lo. The carry-over lo reult from conveying to the environment exhaut gae whoe temperature i higher than the ambient temperature. Such thermal lo reult from the following relation: Q wyl V c T T ) (4) p ( o ()

5 IOP Publihing IOP Conf. Serie: Material Science and Engineering (2017) doi: / x/245/5/ Q carry-over lo, wyl V amount (volume) of exhaut gae, denity exhaut gae, c pecific heat of exhaut gae with contant preure, p T temperature of exhaut gae, T ambient temperature. o An amount of exhaut gae i connected with an amount of burnt fuel which i determined by the following relation: V - amount of exhaut gae received from a unit of fuel. 1 V V 1 m (5) If the thermal balance of the boiler i not made uing a meauring method, an amount of exhaut gae may be determined computationally in a ufficiently precie way [1]. For coal, a theoretical unit amount of exhaut gae may be calculated uing the following formula: t V 1 0,212 Qw 0,5 m / kg An actual unit amount of exhaut gae include exce air and i determined by the following formula: t V t 1 V 1 ( 1) V t a Va 0,242Qw 1,65 m / kg i a theoretical unit amount of air. The above-mentioned quantitie refer to condition of reference, i.e. for preure po = 1 bar = 10 5 Pa and temperature To = 0 o C = K. For a calorific value of burnt coal Q w = MJ/kg, uch quantitie were calculated and they are, repectively: t V m 1 6,699 / kg V t a 1 6,229m / kg. An exce air number decribing air upplied in exce of what i theoretically neceary affect the total amount of exhaut gae and, conequently, the carry-over lo. Therefore, to determine the carry-over lo, only the temperature of exhaut gae and the exce air number are neceary. Other quantitie may be aumed without making a major error. The denity of exhaut gae reult from the following formula: p M ~ R T (6) (7) (8) (9) 4

6 IOP Publihing IOP Conf. Serie: Material Science and Engineering (2017) doi: / x/245/5/ p 10 5 Pa preure, ~ R 8.15lJ / kmol K - ga contant, M 0.5 kg/kmol molar ma of exhaut gae. Alo, pecific heat of exhaut gae with contant preure c p depend on the exce air number. To determine the exce air number, the knowledge of the exhaut ga compoition i neceary, though it i ufficient to know either the fraction of carbon dioxide in exhaut gae or the fraction of oxygen in exhaut gae [2]. The fraction bear the following relation: y fraction of carbon dioxide, co2 y fraction of carbon monoxide, y co o2 fraction of oxygen, y co2 yco yo2 yn 2 yn 2 fraction of nitrogen. The fraction of carry-over lo Swyl i defined in the following formula: wyl Qwyl m Q w 1 (10) (11) Outlet lo Share of loe outlet % l=1,0 l=1,1 l=1,2 l=1, l=1,4 l=1,5 l=1,6 l=1,7 l=1,8 Flue ga temperature o C Figure 1. Fraction of carry-over lo depending on the temperature of exhaut gae and the exce air number Such fraction may be alo determined uing the following approximate formula: wyl T T y co2 o (12) 5

7 IOP Publihing IOP Conf. Serie: Material Science and Engineering (2017) doi: / x/245/5/ = 0.68 factor of proportionality for coal. In view of the above introduction, the fraction of carry-over lo wa calculated depending on the temperature of exhaut gae and the exce air number. Such relation with reference to a unit of burnt coal wa preented in Figure 1. It wa alo aumed that the ambient temperature i 10 o C. From the reult of meaurement made by the Provincial Environmental Protection Inpectorate Laboratory, it appear that the content of oxygen in exhaut gae i 18.8 % and the content of coal dioxide i 1.9 %, which correpond to a very high exce air number = 10. When the operation i proper, the number for that type of boiler hould not exceed 2. With the preence of an exce air number and a meaured temperature of exhaut gae of 110 oc, the fraction of carry-over lo i 26.4 %. Taking that into conideration and auming the abovementioned total efficiency of 0.64, the remaining loe, apart from the carry-over lo, are: ( ) 1 1 0,64 0,264 0,096 nc nz r k wyl (1) Therefore, the remaining loe contitute only 9.6 % and are lightly changed with a changing load of the boiler. 6. Reduction of fuel conumption A tated above, the larget lo in uing fuel in the boiler i the carry-over lo and, therefore, ome effort hould be made to reduce uch lo. From the analyi made in the previou chapter it appear that two quantitie have the greatet influence on reducing the carry-over lo: the exce air number and the temperature of exhaut gae Exce air number Such number i a meaure of the amount of air upplied for combution in exce of the amount which i theoretically needed, a reulting from toichiometric equation of combution. The amount of air hould be a little a poible but it hould aure a proper proce of combution. It mean that the condition of compete combution hould be met, i.e. a little a poible combutible particle (not burnt particle of coal) hould be left in lag. At the ame time, combution hould be perfect, i.e. there hould be no carbon monoxide in exhaut gae. The fraction of carbon monoxide hould not exceed %. If coal i burnt on a moving grate, uch condition i met when the exce air number i , which correpond to the content of oxygen in exhaut gae of % [].The taff operating the boiler i reponible for etablihing a proper exce air. A meter of oxygen and/or carbon dioxide content in exhaut gae may be helpful. A carbon dioxide content for the abovementioned condition hould be %. It i recommended to fix a meter or, even better, a recorder of oxygen content in exhaut gae for each boiler to achieve proper combution in the boiler Temperature of exhaut gae The temperature of exhaut gae i a reult of exchange of heat between exhaut gae and a heated factor (water). It depend on a heat exchange area, that i a tructure of the boiler. Alo, the temperature of exhaut gae i affected by the amount of upplied air which doe not take part in combution and mut be heated from the ambient temperature to the outlet temperature. Auming that combution i made properly and the exce air number i minimal, the temperature of exhaut gae may be reduced only by fixing an additional heat exchange area on the road of exhaut gae, mot frequently, outide the boiler (of the recuperator). Thu, it i poible to additionally cool exhaut gae and ue recuperation heat in an amount expreed by the following formula: Q rek V c p ( T Trek ) (14) 6

8 IOP Publihing IOP Conf. Serie: Material Science and Engineering (2017) doi: / x/245/5/ T temperature of exhaut gae in front of the recuperator (without recuperation), Trek temperature of exhaut gae behind the recuperator, other notation a above. In fact, the final temperature of exhaut gae hould not be lower than the temperature of the dewpoint of exhaut gae. The temperature of dew-point i connected with the content of water in exhaut gae [4]. Taking into account the average moiture of coal and the mot frequent propertie of air, the temperature of the dew-point of exhaut gae i: TR 65 o C. Auming that exhaut gae are cooled down to the temperature of the dew-point TR, unit heat gain obtained from combution of 1 kg of coal were calculated. Calculation reult were placed in Figure 2. Heat gain in the recuperator 1,0 1,2 1,4 1,6 1,8 2,0 kj/kg Qrek kj/kg Flue ga temperature o C Figure 2. Relation between unit heat gain in the recuperator, the outlet temperature of exhaut gae and the exce air number 7. Modernization guideline Organizational and technical action hould be undertaken to reduce fuel conumption. Organizational action include the aurance of the proper proce of combution in the boiler with a minimal amount of air. Technical action aume intallation of an additional feedwater heater in the exhaut duct behind boiler to after cool exhaut gae, Figure. A reduction of the temperature of exhaut gae affect a reduction of the carry-over lo. Approximately, it may be aumed that the reduction of the temperature of exhaut gae by 15 o C will reult in an increae of efficiency by approx. 1%. When chooing the ize of the heater, the temperature of exhaut gae in front of and behind the heater and the boiler power hould be taken into account. In front of the heater, there i uually a pa-through dut extractor which i a firt tage of dut extraction and which protect the heater againt ah eroion. Exhaut gae may be alo cooled uing a blat air heater. However, that i a le beneficial arrangement. The heat exchanger fixed in the exhaut duct i intended for preheating anitary water [5]. 8. Economic effect of modernization The economic effect of applying the recuperator for heat recovery (economizer) from exhaut gae reult from reducing heat conumption due to additional cooling of exhaut gae. A meaure of economic effectivene i a Simple Pay Bank Time (SPBT) decribed in the following relation: 7

9 IOP Publihing IOP Conf. Serie: Material Science and Engineering (2017) doi: / x/245/5/ SPBT N K N outlay on implementing the undertaking, K annual cot aving caued by reduction of heat conumption. (15) Figure. Proce diagram of the boiler room with recuperator. Financial outlay on implementing the undertaking include the purchae and aembly of equipment. A yearly cot aving reult from the reduction of heat conumption due to application of a recuperation exchanger. The cot reduction i determined by the following formula: K Q O 12 Q O Q a - annual reduction of heat conumption, O z - unit price of heat, Qa - thermal power, O m - monthly contant fee for heat production. a z m (16) Heat cot and heat production were determined on the bai of data from the boiler room. Yearly variability of heat production wa preented in Figure 4. On the bai of the diagram (Figure 4), the maximum power for heating and hot water purpoe wa determined, auming that in the ummer heat i ued only for preparing hot water. From the diagram, it appear that the required power for hot water purpoe i approx. 00 kw. For heating purpoe, it i 896,75 kw. After taking into account all data, the following wa obtained: O z = 18,48 PLN/GJ - unit heat price, O = PLN/(MW monthly) - monthly contant fee for heat production. m 8

10 IOP Publihing IOP Conf. Serie: Material Science and Engineering (2017) doi: / x/245/5/ Table 2. Determination component of the levy fixed and variable rate of heat Depreciation cot Ka 2706,00 PLN/year The cot of repair Kr 5000,00 PLN/year The cot of ervicing Ko 18024,00 PLN/year The annual fixed cot Kt 14570,00 PLN/year Monthly fixed cot M-y contant fee for heat production Om 10147,62 PLN/MW m-t Heat demand c.o. 896,75 kw Heat demand c.w. 00,00 kw Heat demand Σ 1196,750 kw boiler efficiency ηk 80 % calorific value of burnt coal Qw kj/kg The annual heat conumption Qa =jśr * Q * ta 7525,2 GJ/year The annual heat conumption c.w. 9460,8 GJ/year The annual heat conumption Σ 16986,0 GJ/year The annual conumption of coal 1120 Mg Annual cot the carbon tax 27596,0 PLN/year Annual electricity conumption in the boiler room En 99661,0 kwh/year Cot of electricity Ke = ke * En 7871,18 PLN/year The average price of electricity tax ke 0,80 PLN/kWh Average price of heat Oz = kc = ( Ko + Ke ) / Qa 18,48 PLN/GJ Heat production ( meaurement) 700 Q [ kw ] Month Figure 4. Yearly variability of heat production 9

11 IOP Publihing IOP Conf. Serie: Material Science and Engineering (2017) doi: / x/245/5/ To determine effect, it wa aumed that the exce air number = 1.6 and the reduction of the temperature of exhaut gae wa made by 50 o C, the maximum thermal power of the boiler room i 1200 kw. An annual heat aving i determined by the following relation: Q V1 m Q wyl carry-over lo, V m / kg unit amount of exhaut gae, m amount of burnt fuel, C p T C p 1.kJ / m - volumetric thermal capacity of exhaut gae with contant preure, T reduction of the temperature of exhaut gae, t a = 8760 h annual time of operation. After replacing the above with the value, the following wa calculated: fuel conumption Qu 1200 m 0,0780kg / k Qw 0, (18) annual heat aving (19) Q a 6 V1 m C T t 10,407 0,0780 1, /10 p a t a (17) 166,5GJ / a thermal power of recuperator Q V1 m Cp T 10,4070,07801, 50 52, 75kW (20) Thu, annual aving are K Qa Oz 12 Q Om 166,5 18, , ,62 716,85zl / a (21) An expected cot of regeneration exchanger (recuperator) with the indicated thermal power will be approx. PLN Taking into account aembly cot, an expected cot of modernization (financial outlay) will be: N = PLN The Simple Pay Back Time (SPBT) will be: N SPBT , year K 716, Concluion On the bai of data obtained from the boiler room uer and the analyi performed, it wa found that combution in the boiler i with too high exce air number, which reult in low efficiency of the boiler room. Meaured temperature of exhaut gae are relatively low; however, it i caued by an exceive amount of air and not ufficient cooling of exhaut gae. Therefore, it i poible to increae the efficiency of boiler after undertaking the following action: - reducing the amount of the exce air during coal combution in the boiler to the value not exceeding (meter of oxygen content in exhaut gae outide the boiler), - applying additional heat exchanger (economizer) with the power of KW in the exhaut duct outide the boiler, intended for preheating hot water. An expected increae of the efficiency of the boiler approx. 4 %, wherea an expected increae of the average annual efficiency of boiler room operation i approx. 9.5 %. 10

12 IOP Publihing IOP Conf. Serie: Material Science and Engineering (2017) doi: / x/245/5/ Reference [1] Roman Zarzycki; Heat exchange and ma movement in environmental engineering. Wydawnictwo Naukowo-Techniczne, Warzawa, [2] Bukowka M., Nowak K., Rabczak S., Emiion of air pollutant in the hot water production, Technologia Wody, volume 51, iue 1, page 8-44, 2017." [] Krytyna Mizielińka, Jaroław Olzak Ga and oil heat ource of low power. Oficyna Wydawnicza Politechniki Warzawkiej, [4] Kazimierz Buczek Steam and water boiler operator. Wydawnictwo Kabe, [5] Wojdyga K., Niemyjki O., Hydraulic analyi for a ditrict heating ytem upplied from two CHP plant. Energy and Building 54 (2012) p Elevier