Economics of equipment cost and energy recovery in VOC thermal oxidation for industrial air pollution control applications

Transcription

1 Economics of equipmen cos and energy recovery in VOC hermal oxidaion for indusrial air polluion conrol applicaions Aruna S. K. Warahena, Yew Khoy Chuah Deparmen of Energy, efrigeraing and Air-condiioning Engineering, Naional aipei Universiy of echnology aipei 0608, aiwan. Absrac VOC (Volaile organic compound) emissions are harmful o human healh and environmen. Many indusries rely on VOC hermal oxidaion as a proven echnique of VOC abaemen. In VOC hermal oxidaion, boh he hermal oxidaion and hea recovery are equally imporan. Hea recovery reduces he energy cos while he hermal oxidizaion fulfills he necessary VOC desrucion. ecuperaive hermal Oxidizer and egeneraive hermal Oxidizer are capable o fulfill boh funcions of hermal oxidaion and hea recovery in VOC hermal oxidizaion process. his work discussed he equipmen coss of ecuperaive and egeneraive hermal Oxidizers, and also emphasized he imporance of hea recovery in hermal oxidaion process. hree parameers namely, Equipmen Cos Facor (ECF), Hea ecovery Facor (HF), and Equipmen Cos agains Hea ecovery (), were presened in he analysis of economics of equipmen cos and hea recovery, in VOC hermal oxidaion. he case sudy conneced wih ecuperaive and egeneraive hermal Oxidizers, provided a beer undersanding of he usage of he parameers ECF, HF, and in he evaluaion and selecion of hermal oxidizer equipmens.. INODUCION Volaile organic compounds (VOC), which are harmful o human healh and environmen, are formed in he manufacuring processes in indusries such as semi conducor indusries, chemical plans, pain boohs, prining indusries, ec. Environmenal auhoriies enforce sringen regulaions on VOC air polluion conrol. As a resul, indusries are forced o adop VOC abaemen processes and sysems, o effecively rea VOC emissions. hermal oxidaion is popular among indusries as a proven echnique of VOC abaemen. In VOC hermal oxidaion process, VOC or VOC laden air is hermally-reaed (burned, oxidized, decomposed) by exposing hem o higher emperaures of abou C. aising he emperaure of VOC laden air o such higher emperaures needs enormous amoun of energy, especially when he airflow raes are in housands of NCMH (normal cubic meers per hour). In such siuaions he cos of process operaion becomes exremely expensive, unless a subsanial amoun (above 60%) of wase hea is recovered. herefore energy (hea) recovery in hermal oxidaion of VOC laden air becomes very much imporan. When hea recovery is imporan, equipmens named ecuperaive, and egeneraive hermal Oxidizers are used for VOC hermal oxidaion. ecuperaive hermal Oxidizers (ecuperaive O) are used for he hea-recovery-efficiency generally up o 60-70%, whereas regeneraive hermal oxidizers (egeneraive O) are used for hea-recovery-efficiency even exceeding 90%. hough he hermal oxidizer is he key equipmen mean for VOC desrucion, here are oher peripheral componens inegraed wih he hermal oxidizer, as for he process viabiliy & opimizaion. In VOC abaemen sysems in semiconducor indusries, peripheral equipmens such as concenraor (desiccan, or acivaed carbon made) is used o concenrae he VOC conens, and hereby o reduce he process-air volume. his reduced process air volume resuls in significan lowering of boh equipmen and operaion cos. Because of he presence of he concenraor, addiional hea exchanger is employed, o supply ho air o concenraor. Fig. and Fig. show he schemaics of ecuperaive-o, and egeneraive-o, and oher peripherals (concenraor and fans are no shown).

2 o concenraor HX Air o HX Sack Pre-heaer hea exchanger Combusion chamber From desorbed concenraor ecuperaive O Fig. Schemaics of ecuperaive-o, and oher peripherals (concenraor and fans are no shown). F Combusion chamber o concenraor D B D B HX Air o HX Z Sack E A egeneraive O A: O inles; Hea media inle gas (Hea degeneraion ower) B: Combusion chamber inle; Hea media oule gas (Hea degeneraion ower) D: Combusion chamber oule; Hea media inle (Hea regeneraion ower) E: O oule; Hea media oule (Hea regeneraion ower) F: Ho gas bypass from O combusion chamber for VOC DESOB process. Z: Ceramic (hea media) bed heigh. Fig. Schemaics of egeneraive-o and oher peripherals (concenraor and fans are no shown).

3 . EQUIPMEN COS Equipmens of hermal oxidaion sysem wih hea recovery, and peripheral componens, could easily cos few hundred housands of US$. he high cos is because of he sizes of he equipmen and he expensive pars and maerials used o wihsand higher emperaures. For comparison purposes, able presens some esimaes for he cosing of only ecuperaive and egeneraive O, for differen capaciies (excluding he peripheral componens). able.. Equipmen coss (ecuperaive and egeneraive O) in US$, for differen airflow capaciies. 0.5kW/NCMH* NCMH ecuperaive O Cos/ (US$) egeneraive O of Cos/ (US$) 570,000 45, ,000 69, ,000 8, ,000 03, ,000 8,000 Efficiency(η) η60% η85% * Where, is he Per-NCMH hea requiremen needed o increase emperaure of VOC laden air o hermal oxidaion emperaure. 3. EQUIPMEN HEA ECOVEY oal hea required for hermal oxidaion process If recovered hea is Q ɺ hen Where is he hea addiion (like refill). Q ɺ + Q ɺ () Energy recovery efficiency: η () By rearranging of (), + (3) η (4) hen from () in (3), ( ) Q ɺ, and hen by referring equaion (4), when he For given recovery efficiency η is higher, he hea addiion Q ɺ becomes lower. hen, he energy cos will be lower, since he addiion of new hea by burning fuel or elecriciy becomes lesser, when η is higher. Also, for given Q ɺ, and according o he equaion (), he increasing of recovery efficiency η resuls he lowering of he oal hea Q ɺ. Hence, whaever he opimizing ineres, wheher i is Q ɺ orq ɺ, meeing of desired efficiency η will be he ulimae concern. However, for higher hea recovery efficiency η, hea recovery componens (hea exchangers in ecuperaive O, or ceramic beds in egeneraive O) become larger. his larger hea exchange componen mean for higher hea recovery adds a significan amoun for he equipmen cos. Simply increasing he recovery efficiency by over sizing he equipmens, wihou doing a proper analysis of equipmen cos and hea recovery could even cause loosing finances. 4. MODEL FO HE EVALUAION OF EQUIPMEN COS AND HEA ECOVEY As an evaluaion of equipmen cos and hea recovery, in hermal oxidaion of VOC laden air, he below parameers are suggesed. Equipmen Cos Facor is used o reflec equipmen cos agains he cumulaive amoun of airflow handled during he ime period of, ($/NCM), could be suggesed as, ECF f ($,V ɺ,, ), as ECF $.. (6) Hea ecovery Facor is used o reflec cumulaive hea recovery agains he cumulaive amoun of airflow, during he ime period of, (kwh/ncm), could be suggesed as, HFf ( Q ɺ,V ɺ,, ), as herefore HFf ( HF.. HF V ɺ.. Q ɺ,V ɺ ) becomes η η (7) Where, : Per-NCMH hea requiremen needed o increase VOC laden air emperaure o hermal oxidaion emperaure V ɺ : in NCMH : Equipmen nominal age in hours : Usage ime facor (raio of acual operaed hours o equipmen age in hours ) $: Cos of equipmen in US$ Equipmen Cos agains Hea ecovery (), as o reflecs he cos of equipmen agains he hea recovery payback, ($/kwh), could be suggesed as, ECF $ HF η. qv ɺ. ɺ.. (8) General comparison of for wo hermal oxidizer equipmens can be performed as follows,

4 for Equipmen, for Equipmen, hen, η η $ ECF HF ECF HF $ $ η q If >, or > $ η $.... η qv ɺɺ $.... η qv ɺɺ ɺ, hen >, and hence he equipmen- can be considered as economically more favorable han Equipmen-. Even in such analysis, Equipmens- and should no necessary o be of he same ype. One can be ecuperaive O, and he oher can be egeneraive O. In such evaluaions can be considered as he guaraneed life ime or expeced usage of equipmens. According o he equaion (6) and (8), ECF and improve (reduce) wih he age of he hermal oxidizer, and wih a higher usage ime facor. Also according o equaion (7), higher η and values, improve boh HF (higher) and (lower). 5. CASE SUDY By referring able, for airflow of 680 NCMH, equipmen cos of 60,000 US$, 0.5kW/NCMH, η60%, and for 00%, values of ECF, HF, and for ecuperaive O can be shown as in able. able showing ECF, HF, and for ecuperaive O for airflow of 680 NCMH*, equipmen cos of 60,000 US$*, 0.5kW/NCMH*, η60%*, 00%, and for differen values of. * ef. able. hr(yrs) 8640 ( yr) 780 ( yrs) 590 (3 yrs) (4yrs) 4300 (5 yrs) By referring able, for airflow of 680 NCMH, equipmen cos of 03,000 US$, 0.5kW/NCMH, η85%, and for 00%, values of ECF, HF, and for egeneraive O can be shown as in able 3. able 3 showing ECF, HF, and for egeneraive O for airflow of 680 NCMH*, equipmen cos of 03,000 US$*, 0.5kW/NCMH*, η85%*, 00%, and for differen values of. * ef. able. hr(yr) 8640 ( yr) 780 ( yrs) 590 (3 yrs) (4yrs) 4300 (5 yrs) By referring able, for 5xx30x44300hr, 0.5kW/NCMH, η60%, 00%, ECF, HF, and values for ecuperaive O can be shown as in able 4. able 4 showing ECF, HF, and for ecuperaive O, for 4300hr (5 yrs), 0.5kW/NCMH*, η60%*, and 00%. *ef. able. NCMH* ecuperaive O Cos/ (US$)* 570, , , , , By referring able, ECF, HF, and values for 4300hr (5 yrs), 0.5kW/NCMH, η85%, 00%, for egeneraive O can be shown as in able 5. able 5 showing ECF, HF, and for egeneraive O, for 4300hr (5 yrs), 0.5kW/NCMH*, η85%*, and 00%. *ef. able. NCMH* egeneraive O Cos/ (US$)* , , , , ,

5 able and 3 shows he rend of he improvemen of ECF and wih ime of usage, for boh ecuperaive and egeneraive O. For his paricular se of resuls, and for a same period of equipmen usage, in egeneraive O is beer han ha of ecuperaive O, hough ECF is shown in he oher way. However values of ecuperaive O can be beer han ha of egeneraive O, for differen ime of usages. able 4 and 5 shows he ECF, and variaions for differen equipmen capaciies, for boh ecuperaive O and egeneraive O. Of he se of ecuperaive hermal Oxidizers, one wih of 7850NCMH is he mos economical, where as he one wih of 470NCMH is he leas economical. Of he se of egeneraive hermal Oxidizers, one wih he of 7850NCMH is he mos economical, where as he one wih of 570NCMH is he leas economical. Of all he sizes of ecuperaive and egeneraive hermal Oxidizers, egeneraive O wih he of 7850NCMH is he mos economical, where as he ecuperaive O wih he of 470NCMH is he leas economical. Values of $ and could be adjused o accommodae coss of he peripheral equipmens and he oher energy consumpions. Cos of he peripheral componens could be direcly added o he $ value. Oher energy consumpions could be convered ino cos equivalen burner energy unis, before adding hem o. HF and ECF could be considered as wo independen parameers. By considering only one parameer will no give a proper view of he economy of usage. However he parameer which is he combinaion of wo parameers of ECF and HF would provide a beer insigh for he undersanding of economy of he equipmen. his evaluaion could be useful for he users o choose more economical equipmen, among many design proposals. ACKNOWLEDGEMEN I is acknowledged here his work is parly suppored by DECH, Inc. CONCLUSION Analysis of equipmen cos and energy recovery, in he selecion and evaluaion of hermal oxidizer equipmens, was reviewed.