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Closed Dual Fluid Gas Turbine Power Plant without Emission of CO2 into the Atmosphere
Copyright © IFAC Energy Systems. Management and Economics, Tokyo. J apan 1989
ENERGY AN D ENVI RONMENT (Ill )
CLOSED DUAL FLUID GAS TURBINE POWER PLANT WITHOUT EMISSION OF CO 2 INTO THE ATMOSPHERE P. S. Pak, K. Nakamura and Y. Suzuki Department of Electrical Engineeri ng, Osaka University, Suita, Osaka 565, J apan Abstract. This paper describes a construction and characteristics of a coal-gas - burned h i gh efficiency power plant which emits no ca r bon dioxide (C0 ) into the atmosphere. I n 2 the p l an t, CO gas and s uper heated steam a re used a s the main a nd sub work i ng f l u i d , 2 respectively , of a c l osed dua l fluid g a s tur bine power generation system . It is ass umed t hat a coa l gas whose principal compositions are CO , H , CO and CH is burnt in a 4 2 2 combustor using oxygen , and t h at CO gas and superheated steam are used as the ma i n and 2 s ub working fluid of a tu r bine , respec ti ve l y . Consequently , the constituent gases of the combustion gas become CO and H 0 . Thus , CO gas included in the exhaust gas can be easily 2 separated at the conde~ser ou€let f r om the condensate (H 0) . Most of recovered CO is 2 2 recycled as t he main working fluid of the turbine . In the plant , high - temperature turbine e xh aust gas is uti l ized in a waste heat boiler to produce superheated steam whi ch is injected into the combustor in order to improve power generation efficiency . It has been estimated that an electeic power of 208MW can be generated with gross thrmal efficiency of 51 . 3% . Power generating efficiency has been est i mated to be 43 . 7% since the electric power of 31.1MW is required for producing the oxygen for combustion . The problem of liquefying CO recovered has also been dealt with in the paper . It has been shown that the resultant 2 powe r generat i ng efficiency is estimated to be 39 . 0% if the power for liquefact ion of recovered CO is taken into account. 2 Keywords .
Carbon dioxide ; greenhouse effect; coal gas ; power plant ; gas turbine fluid cycle ; closed cycle ; high efficiency . INTRODUCTION
removing CO should be able to be reused 2 r ecyclically . Alkanolamine-based solvent , which has a chemica l property of absorbing CO gas at low 2 temperature about 40 · C and of discharching it at high temperatu re about 1 50· C, is one of chemical products satisfying this requirement . Table 1 shows estimated energy and cost which are required for treatment of CO, when a process for removing and recover i ng CO irom the s tack gas using the solvent 2 is adopted in a 100MW coal fired steam turbine powe r p l ant. In Table I energy and cost required for liquefaction and disposal of th e recovered CO are 2 also shown. As shown in Table I, the energy and the cost required for removing and recovering CO are 2 est i mated to be much larger than those for l iquefaction and disposal of the recovered CO , On 2 account of this l arge quantity of energy req u 1re~ent for removing and recovering CO , power generating 2 e fficiency has been estimated t o be reduced from 38% to 15 . 9%2 When the low pressure steam at about 7 . 03kg/cm and 204 . C is extracted from the low pressure section of the turbine to supply the heat , the efficiency has been estimated to be improved from 15 . 9% to 31 . 4%, being still lower by 6 . 6 percentage points compared with the origina l efficiency (Pak et al ., 1989a) . To realize a power plant without the emission of CO into the 2 a tmosphere , the r efore , it is considered to be more efficient to construct a completely new power plant so that the method of recovering CO becomes much easier than that of incorporating a removal-and2 r ecovery proc e ss using the solvent into a conventional power plant .
As a resu lt of large scale use of fossil fuels , a g reat quantity of carbon dioxide (C0 ) is being 2 exh a u s t ed into the atmosphere , and CO density is 2 inc rea sing a ll over the worl d. It is afra i d that the a tmo s ph eric temperature might be increased on a ccou nt of the greenhouse effect by CO , and t h at 2 th e c l i mate might be changed on g l obal scale . Hence it i s req u ired to reduce the emission of CO into 2 the a tmosphere . Exh austed amount of CO is extremely enormous 2 compa r ed with that of a i r pollutants such as n 1trog en oX1des (NO ) ' and CO has been considered x 2 to be a c l ean gas unt11 t he present . Hence , t echnolo i ges on r emova l of CO from t he s t ack ga s of 2 a bo il er a r e compl etely different from convent i onal air pollution control technologies , and it is con s idered to be very difficult to develop t ech no l og i es for recovering CO efficien tl y . 2 Therefore , if it is required to realize a power g e ne ra tion p l ant which emits no CO into the 2 a tmo sphere , it would be easier to construct a c omp l ete l y new power generation system than to remove a nd recover CO from the stack gas from a c o n v en tional power plan€ . Th is paper describes a construction and c h ara cter istics of a new power generating system whi c h ut ilizes coal as its fuel and emits no CO into the atmosphe r e . The reason a coal-utilize~ power p l ant is inve st igated is that coal is the most af fl uen t fossi l fuel but has the most infe ri or pro perty in CO emi ssion cha r acteristics among 2 f o ssil fuels used for thermal power plants . The d e gr ee of effect which will be achieved by reducing th e emi s s i on of CO can thus be expected to be 2 largest.
RECOVERY OF CO
2
FROM A CONVENTIONAL POWER PLANT
Ste i nberg e t al . has already investigated a problem o f remo v ing and recovering CO from a stack gas of a 2 c o n venti o nal coa l f i red s t eam turbine power pl a nt (Steinbe r g , 1984 ) . Since t he amount of an exha ust ga s is ext r e me l y enor mous , it becomes indispensable c o ndition tha t t h e c h emi cal product used for
dual
c6
Tabl e 1 Energy and cost required f or treatment of CO2 Required energ y'! Req uired cost" · and recovery 1241 50 I Liquefact ion 254 23 Disposal 8.64 5 *1: The energy requ ired for t reating CO: of 1000kg is des ignated by us ing the un it of Heal . where e lectr ic energy of 1 kWh is converted into 2. 45 Mea l. *2 : The cost produc ing electricity of 1 kWh at a lOOMW coa l fi red power plant is adopted as the ref er ence va lue of 100.
I Remova I
229
p, S, Pa k, K, Na ka mura an d y , Su zu ki
230
POWER GENERATING SYSTEM WITHOUT EMISS I ON OF CO Necessity Protection
of
Gasifying
Coal
For
2
Environmental
Feed vater ~
If coal is directly burned in a boiler a t a the rmal power plant , it is not ea sy to control t h e emission of air po llutant s such as dust , sulpher oxides ( SO ) and fuel - der ived nitrogen o xides (NO) , not ~o mention the r emoval of CO. Therefore , ~n th e power plant to be inv estigated f n this study it is assumed that coal is gasified and substances which causes air po l l utants such as dust , SO and f ue l NO are removed in th e process of gasifi~ation and i~ the following gas refining p rocess .
Condensate
G Generator
Turbine Construction of a Proposed System
Re.oval of
excess 0)2
~----------------------------------~--~
Figure 1 shows a schematic construction of a power generating syst em which emits no CO into the 2 a t mosph ere . Th e fo llowing is a brief expl anation how electric power is gene rat ed with emitting no CO 2 into the atmosphere . The proposed system can be called a col sed dual fluid gas turbine power generating syst e m in which CO gas is used as the ma in working fluid and supe rheated steam as the sub working f l u i d . It is assumed in t he system that the coal gas ifi ed by using oxygen is burned in a combustor using oxygen instead of the air , and that both CO gas and superheated steam a re used a s the working 2fluid of a turbine . Consequently , the constitue nt gases of the combustion gas become CO and H 0 , since the principal composi tions of coaf gasifi~d by using oxygen are CO , H , CO , and CH 4 • 2 Th e combustor outlet high-temperature com~ustion gas is first used to ge nerate electric power by driving a t u rb i ne . The turbine exhau st gas , which has still a considerab l e heat energy , is then utilized to produce superheated steam at a waste heat boiler to inc r ease power output as well as to improve power generating efficiency . The low-temperature outlet gas from the waste heat boiler is lastly led into a condenser , where the steam included in the exh aust gas is complete ly conden sated into water. The condensate is used as th e feed water of the waste heat boiler to generate steam. The p rincipal component of the condense r outlet gas is CO , although a smal l quantity of oxygen and nitrog~n g3S i s included . Oxygen gas inclusion is caused by inJecting excess o xyg en into the combustor for realizing complete combustion of the fuel gas , and nitrogen gas i nclus i on is caused by the fact t hat i n general an ext reme l y small quantity of nitrogen gas is included in a coal gas , even if coal is gas if ied by using oxygen gas instead of the air . The condense r o utl et gas is recycled as the main working fluid of the gas turbine . The quantity of CO gas not reused as th e working fluid , which is 2 equal to th e quantity of CO gas generated by 2 burning the coal gas , can directly be removed from the system at a wate r separator outlet . Thus , no CO gas 1S emitted from this power plan t into the 2 atmosphere . The removed CO gas can be used as the 2 raw material of CO gas . In the case CO reuse is 2 impossible , the r~covered gas should be liquefied and disposed into a deep ocean or a large scale storage suc h as depleted oil we ll s . It shou l d be noted that no thermal NO is in this power plant in addition to n~ CO character1st lc,
SInce
generated
emission combustIon reactIon is taken
2
p l ace in the combustor whe re practical ly no nitrogen gas exists.
Fig . 1. Construct ion of proposed system (closed dual fluid gas t urbine power generating SyStell). the va l ues of entha lpy of the gas at inlet and The e nthalpy va l ue is outlet of the turbine . dependent upon gas temperature and gas compositions , which are affected by the combu s tor inlet gas tempe rature and by fuel flow rate , and these values are in turn depend e nt upon the value of entha l py of the turbine outlet gas , etc . Hence , trial-and - error procedure is necessary for est ima ting characteristics . A simulation mode l has been constructed t o estimate various characteristi cs of the system. In the model , the variables and parameters s hown in Table 2 are assumed to be the e xogenous variables and parameters , r espective ly. The detailed explanation of the constructed model is omitted in this paper . An e xample of simulation results is shown in Fig . 2 , where the values shown in Table 2 have been assumed to be u sed as the reference values of the exogenous variables and the parameters . As s hown in Tab l e 3 , the lower calor ific value of the c oa l g as , which is assumed to be used as the fuel, is 2489 kcal / kg . Figure 2 shows th e estimated results when the maximum steam quanti ty that can be generated in the waste heat boile r is injected into th e combustor . AS
shown
in
Fig . 2, t h e syst em ca n generate steam of 218t/h whose pressure is 34 . 0 kg/cm and temperature 888K . By increasing fuel gas flow rate from 105 t/h , in case of no superheated steam injection , to 140 t/h in case o f the above mentioned maximum steam quantity injection , for the purpose of keeping turbine in l et temperature to be the specified value of 1523K, generated power correspondingly increases from 100MW t o 208MW . The temperature of the e xhaust gas flo wing into the waste heat boiler is 928K a nd that flowing into the condenser is 382K. Flow rate of the gas which is not condensated in the condenser is estimated to be 962 t/h , and the weight percentag e of the composition of this gas is estimat ed as fo ll ows : CO ~ 97.5% , 02 = 1 . 8 4%, and N2 = 0 .65% . The 2 quant1ty of the gas r eci rculated as the main working fluid is 780 t /h , and thus the gas quantity which shou ld be r emoved from the system i s estimated to be 181 t/h, and the quantity of CO recovered is 2 , calculated t o be 1 7 7t / h . Since the quant1ty of CO 2 gene r ated by burning 140 t/h fuel gas is 181 t/h , the removal rate of CO is 98 . 0% . The reason all 2 the amount of generated CO can not be recovered iG 2 that flow loss has been assumed at the various parts of the system, as indicated i n Table 2 . super~eated
PERFORMANCE CHARACTERISTICS OF THE PROPOSED SYSTEM Results of Characteristics Estimation To estimate various characteristics o f the proposed power gene rating system , it is required to estimate
Gross thermal efficiency has been estimated to be 51 . 3%. The e s timated value is remarkably high compared with that of a conventional gas turbine power generating system . This is because: ( a ) the heat energy inc l uded in the tur bine exhaus t gas is
Closed Dual Fluid Gas Turbine Power Plant
231
Table 2 Exogenous variables and parameters of silulation model (a) Exogenous variables Reference value SYlbol Definition T. Turbine inlet telperature 1523K 30kg/C1 2 Turbine inlet pressure P. COlPOsition of gaseous fuel and its volume percentage Sce Table 3 Vf. i Telperature of fuel and oxygen gas 573K:298K Tf :T o2 Excess oxygen ratio in colbustor a 1.2 Condenser outlet te~perature 303K Te 0.5kg/CIl 2 Comdenser outlet pressure Pe Steam injection rate standardized by G3 MaximulII value Rs determined Generator output when Rs = 0.0 100MII W. (b) Exogenous parameters Symbol Definition nk Adiabatic efficiency of C02 compressor E k Flow loss rate at C02 co.pressor Adiabatic efficiency of turbine nt Flow loss rate at turbine Et Combustion efficiency of coabustor nb Pressure loss rate at combustor rh Flow loss rate at co~bustor E b (5. n Pressure loss rate at fuel gas nozzle Pressure loss rate at oxygen injection nozzle /5 on Pressure loss rate at steam injection nozzle (5 s n Enthalpy exchange efficiency of waste heat boiler 6TT Waste heat boiler terminal telPerature difference (incolli ng exhaust gas te~perature outgoing steal temperature) Waste heat boiler pinch point telPerature difference TPP 6TD Dry telPerature condetion for waste heat boiler outlet exhaust gas (outgoing exhaust gas tellPerture - staturated steall tellPerature) Exhaust gas pressure loss at waste heat boiler Steam pressure loss rate at waste heat boiler Exhaust gas flow loss rate at waste heat boiler Eo. (5 Exhaust gas pressure loss at condenser Preassure loss rate at water separator /5 w s Generateor efficiency n ••
c.
Reference value 85:\: O.l:\:
85% O.l:\:
98:\: 2% O.l:\:
10:\: 10% 10% 90:\:
40K 20K
10K
o. lkg/cm2 5% 0.1%
O.lkg/c l1l < 2:\:
98%
Exhaust gas
,.==.:.::.;:....;:.=-----,T ,'382.24
T: TelPerature [K) p: Pressure [kg/CIl 2) I: Enthalpy [kcal/kg) G: Flow rate It/h)
P?,=8 . bE)
17 "22.1 5 G7 z 1ZZ5 . b,s
Waste heat .11383 , :5b ."'31,1)1
Is ·Ub . 75
r=-=-_+-",·-2 18 . 22 TJI-753 .Z8
P2 ")G . bl 12 =11 •. qJ
w· 35 . 80 w"218.22
Combustor ,
G.'77Q . 35
Fuel
w- 31 3.15
Condenser
gaS T.=57J .1 5 Pr- z3 '1 . il
C02 cOIIPressor G," JQ.qJ r,,=b2 .~7
Oxygen
Wc::IIZ87.7~
T,=313.15
Poa "'3"1,lil
P, =1 . 1Q J1=',(JQ
Go2 zQl . S8
nc=51.'3~
Turbine excess C02
Fig. 2.
Estimated characteristics of proposed power generating system.
232
P. S. Pa k, K. Na kam ll ra a nd Y. SlI zu ki
Table 3
Co.position of ref ined coal gas
COIIPositi on Volu.e per centag e (~l Weight percentage (%l
CO CO2 H2 43 .53 17.39 38 .33 58. 57 36.77 3.71
CH. 0. 11 0. 08
N2 0.64 0.86
Air
recovered in the waste heat boler ; (b) it is possible to make the turbine outlet pressure lower than the atmospheric pressure .
Generator
G However ,
since
the proposed system uses oxygen
to
burn fuel gas , the electric power of 21 . 9MW has been estimated to be consumed as a station se r vice power,
because the electr~c power of 0 . 34 kWh is required for producing I Nm oxygen gas , or 0 . 2379 MWh to produce 1000kg oxygen gas. The electric power of 9 . 21MW is also consumed for compressing the gener~ted oxygen to the required pressure of 34 . 0 kg/cm . Thus , power generating efficiency of the proposed system has been calculated to be 43 . 7%.
Turbine
Air co.,,-essor (a) Construct i on
2ee
0 ..
l
Generated power \.
Consequently ,
when
the
thermal efficiency of
the it
75% ,
has been shown that the resultant thermal efficiency of 32 . 8% of the proposed system is estimated to be efficient
more
than 1 . 3 percentage points
on
the
basis of the calorific value of the coal than that of the conventional coal-fired steam turbine power p l ant with wh i ch the process using the solvent for recovering CO from the stack gas is integrated . 2 Comparison
CO
2
with Gas Turbine Power Plant into
Which
Recovery Process Using Solvent is Inco r porated
In this section we will compare the estimated characteristics of the proposed system wi th that of a power plant utilizing gas turbines into which a CO recovery process using the solvent is 2 incoporated . The construction of a power plant utilizing a conventional gas tu r b i ne i s shown in Fig . 3(a) , in which the air is used not on l y for fuel combustion but also as the main working fluid of a gas turbine. This system is a system called a dual fluid open cycle gas turbine power gene r ating system . Figure 3(b) shows estimated chara c t eristics which have been obtained by using a mode l constructed by the authors (Pak et al ., 1988) • In estimating characteristics, the pressure of the exhaust gas at the waste heat boiler outlet has been assumed to be equal to the atmospheric pressure , since the exhaust gas is all emitted di r ectly to the atmosphere . The other conditions have been assumed to be the same as shown in Table 2 . In Fig . 3(b) , steam quantity injected into the combustor is taken as the axis of abscisa , and generated e le ctric powe r and gross thermal efficiency are plotted on the axis of ordinates . As seen from Fig . 3(b) , generatea power increases with the increase of t he steam injected, and gross thermal efficiency has been estimated to increase from 35.2% up to 47 . 9% . However, it is required in this system to remove and recover CO from the stack gas . According to a research by St~inberg et al . (1984) , it is possible to remove and recover CO from the e ffluen t by 2 utilizing alkanolamine-based solvent with the CO 2 removal efficiency of 90%, in which the heat energy of 1 . 21 Gcal is estimated to be required to recover 1000kg CO. In the present system shown i n Fig . 3(a) , heat energy can be supplied f rom the waste heat boiler by producing a hot water with temperatu r e of 180 oC, although this hot wat er production reduces the amount of steam produced . The reduction rate of power generating efficiency due to the reduction in the steam quanti t y inje c ted into the combustor has been estimated as explained in the fol l owing:
Generating characteristics of the hot wa t er has also been estimated as shown in Fig . 3(b) . The maximum
100
..•
0
o ' .0 ..
..0
'
"
c ..~ .. o ·
150
coal gasification process is assumed to be
0 .. .0 ..
0
3
0 ' ,0"
EI " s"
ij..:.: . " .. 3 · "" ' 0
Therma I output of hot water
.
......... £
Power generati~g ' ''O~ ' o()~
50
eff ic iency ~ ~:-=-!:'::~~_~M_'>r-)t-)(-+< ":"~*"_,,*=,=:::~':'-:,-x'- ~ ' '0-.. ' -0. , .. Heat generat ing:;»---+--- +---+-- _+__-+-_-+_ _ - ·&· ~ ~ · .o err ic iency -+- --+-- +,,-t--.-+._-+ 26
<0
bO
80
100
120
"0
Ib0
180
200
Steam quant i ty injected into combustor (tlhl (b) Esti.ated char acteristics when CO2 is not recoverd
Fig. 3. Cons truction and characteri sti cs of an open-cycle dual fluid gas turbine power generat ing syste•.
the r ma l output of the hot water can be obtained at the oper ating condition that no steam is generated in t he was t e heat biler , at which the electric power o f 100MW and the hot water having thermal output of 110 Gcal/h can be generated . The fuel consumption is 98 . 7 t/h , producing 127 t/h CO , Accordingly , 2 the hea t quantity required for recovering 90% of CO 2 gene ra ted fr om the stack gas is calculated to be 163 Gcal/ h, if thermal efficiency of a reboiler fo r r ecove r ing CO is assumed to be 85% . Consequently , th e he a t quantity of 56 Gcal/h is in shortage , even if t h e obtainable maximum quantity of waste heat ene r gy is all utili z ed for CO recovery . 2 Th is s hows that it is impossible for a power gene r ating system using a gas turbine to construct a dua l fluid cycle (Cheng cycle) system in the case whe r e the CO generated must be recovered . 2 Simila r ly , und e r CO emission constraint it is 2 impo ssi b l e to construct a h1gh efficiency combined cycle power generating system in which the turbine exhaust gas is utilized to produce superheated st e am which is used to drive a steam turbine generator . Henc e, in recovering CO in the system shown in Fig . 2 3 , an ext r a fuel becomes necessary to meet the s hor tage of t he r equi r ed heat quantity . It should be no ted here that the more extra fuel should be requi r ed, since the us e o f the fuel gas to genera te heat for recovery of CO generates also CO ' The 2 2 lower calor1 f 1c value of the fuel gas 1S 2 . 49 Mcal/kg , and t he genera t ion rate of CO is 1 . 29 kg 2 CO pe r unit kg fuel gas . Thus , the ne t lower ca t orif1c value reduces to 0 . 344 Mcal/kg , if thermal effici e ncy of a hot water generating boiler is assumd to be 90% . Thus , powe r gene r ating eff i ciency of the system shown in Fig . 3 has been estimated to be 13 . 5% when generated CO. should be recovered by utilizing the solven t. L
Closed Dual Fluid Gas T u rbine Powe r Plan t Conseque n tly , it can be seen that powe r generating efficiency of the p r oposed system has been estimated to be noti ceab l y higher than that of the gas turbine power p l ant into which CO removing a nd r ecovery 2 p r oces s ut il izing alkano l a mine-ba sed solvent is incor pora ted .
233 Exhaust gas (Oh+H,o)
reed
water
PUlP
Condenser
Compar i son with Aalate r native System Condensate
As a power plant without the em i ssion of CO int o 2 the atmo s phere, the author s proposed a power plant shown in Fig . 4 so far . Th e system shown in Fig . 4 is a sys tem wh i ch can be called a cl osed d ual f l u i d regenerative gas turbine power generat i ng system in which CO and H 0 gas are respectively used as the 2 2 main and sub working fluid of a turbine . Let us abbr eviate thi s system as a regenerative CO turbine 2 system hereafter . In this section we will compare the r e g enerative CO tu r b i ne system wi th the sys t em 2 proposed i n the present paper .
G Generator Turbine
R_ vel
of
excess Oh
Fig , 4, Alternative power plant without emission of Uh into the at.osphere (Closed dual fluid regenerative gas turbine power generat ing system. regenerati ve ah turbine system) . the
The difference of the proposed system with regenerative . CO turbine system is that e 2 regenerator 1S not 1ncorporated 1n the proposec system . The r egenerator is a heat e x changer to recover heat energy from the turbine exhaust gas and the heat energy recovered is uti l ized to r a i se the tempera t ure of the main work ing f l uid of the turbin e . It is we l l known that incor porating 3 r e generator makes it possible to noticiably improve thermal efficiency of a gas turbine power generating system , although this increases the pressure loss of the turbine exhaust gas (Pak and Suzuki , 198&) . Gross thermal effi ciency and power gene rating efficiency of th e regenerative CO turbine system 2 have been estimated to be 50 . 3% and 42 . 7% , respec tive ly (Pak e t al ., 1989b) . That is , both effici enc ies of the proposed system have been estimated to be higher by 1 . 0 percentage point. The reason the proposed system has a high efficiency characteistic is considered as follows :
for CO
2
liquefaction h"" been tak en into account .
Consequently, it can be ascertained from t h e abovementioned simulation results that we can constr uct a coal-gas - burn ed power generating system without the emission of CO into th e atmosphere with the power 2 gen e rating eff1ciency of 39 . 0% , although the ext ra equipments for producing oxygen and for liquefying t h e recovered CO are required . Figure 5 shows the 2 es t imated total mass and electric power fl ow in the proposed system . proposed economical aspects of the Concerning system , the detail will be described in another paper.
CONCLUSION Th e steam quantity which can be produced in the waste heat boiler (WHB) in the proposed system is . estimated to be larger by 35 . 3t/h than that in the regenerative CO turbine system , since the inlet gas 2 temperature of the WHB in the proposed system i s higher by lllK than that in the regenerative CO 2 turbine system . In both systems , the higher the ratio of H 0 quantity to CO quantity in the exhaust 2 2 gas , the lower the pressure at the condenser outle t. Hence , the turbine outlet pressure in the proposed syste~ wa s able to be made t~ be lowe r by' 0 . 15 kg/cm compared with the case where the regenerator was incorporated . This indicates that the more work can be done in the turbine in the proposed system compared with the case of the regeneratorincorporat ed system . Thus , it can be concluded that th e proposed system without the reg enerator is not only simpler in the construction but also more efficient compared with the regenerative CO turbine system shown in Fig . 4 . 2 DISPOSAL OF RECOVERED CO
2
This chapt er deals with the problem of disposing the recovered CO , The fl~w rate of the recovered CO 2 2 i s equal to 93 , 400 Nm /h and this enormous gas quan t i t y should be l i quefied for the purpose of vo lume r eduction in general . Since electric power to liquefy CO gas has been estimated to be 0 . 1036 kWh/lkg CO (steinberg et al ., 1984 ) , the electric 2 power of lB . 3MW is required to liquefy only CO gas 2 included in the recovered exhaust gas . However , a sma ll quant i ty of 02 and N2 are included i n thi s recovered gas , it has thus been estimated t h at the e l ectr i ca l power of 19 . 0 MW is r equi r ed for liquefaction of the recovered gas in total . Hence, power gen erating effic i ency of the proposed system has been estimated to be 39.0% in the case the power
In this paper , a power plant without the emiss ion of CO into the atmosphere has been proposed , by taking 2 a power p l ant using coal ga s as an example of a power plant . The proposed system has such an advantage that it is easy to be realized , Slnce it can be constructed by combining the ex isting t echno l og i es , and that it emits no thermal NO as x well as no CO , It has been shown from the 2 simulation results that it s power generatins; efficiency is 43 . 7%. It has also been shown for a gas turbine system in which the coal gas and the air are used as its fuel and working fluid , r espectively , that const r uction of n e ither a dual fluid power plant nor a combined cycle power plant is possible under the constraint of no CO emission . 2 Thus, its power generating eff ici e ncy has bee n estimated to become 13 . 5% under the const raint of no CO emission . 2 It has been shown that on the basis of the calorific value of the coal the thermal efficiency of the proposed system ha s been estimated to be higher by 1 . 3 percentage points than that of a conventional coal - fired steam turbin e power generating plant wit~ which a process using the solvent for recovering CO 2 from the stack gas is integrated . The problem of liquefying CO gas recovered has also 2 been dealt with , and the power generating efficiency of a coal - gas-burned power generating system has been estimated to be 39 . 0% and 29 . 3% on the basis of the calorific value of the coal gas and the coal , respectively , if the power for liquefaction of recovered CO has been taken into account . 2 It is needl es s to say that any hydrocarbonic fue l such as liquefied natural gas (LNG) and fuel oil can be used in the proposed system . In thi s case the energy loss generate d in the process of gasification
234
P. S. Pak. K. Naka mura and Y. Suzuki Generat ion of power "uel gas =139.9tIh ==l Power plant (Closed dual Generated power fluid gas turbine power ='(jJ7.8M\/ CoIpressed " generat ing syStem) 02=91 .9tIh Pro
02
of
coal
Liquefaction of CO2 CO2 .':::: : COlI'I"essor
I I
Liquefied CO:(. =177.0tIh
pressor
j 9.21M\/
121. 9H\/
and refining accoun t.
l;
COII-
Recovered CO2 =181.5t1h
Suppliable power =157.7M\/
19.011W
"ig. 5. Total
ESS
and electric power flow in the proposed syste•.
is no t
be
taken
to
into
It was not long the researchs began for i nvestifgat i ng power ge nerating systems wi t hout the e mi ss i o n o f CO i nto the atmosph e r e . I t i s hoped 2 that va r iou s k~nds of r esea r c h es are made fo r realizing high efficien c y power genera t ing systems fr om whi c h hazardous subs t ances for the ambient such as SO . NO and CO are not emitt ed into t h e 2 a t mospl'ie r e . x
REFERENCES Pak . P . S . and Y. Suzuki (1988) . Evaluation of ther modynamical . economica l and environmental characteri s tics of high efficien cy ga s turbine cogeneration systems . T. lEE . Japan . 108- D. 895 - 902 . ( i n Japanese) • Pak. P.S •• K. Nakamura and Y. Suzuki ( 1989a) . Const ruction and characteristics of a thermal power plant which recovers ca r bon dioxide gene rated . Cont r i buted to En e rgy and Resources . (i n Japanese) • Pak . P . S •• K. Nakamura and Y. Suzuki (1989b) . Coalu tilized high e fficiency power p l ant wi thout emission of CO into the atmosphe r e . Procee ding of 2 8th Simulation Technology Conf e r e nce . pp . 129/132 . Japan Soc i ety for Simulation Technology (i n Japan ese) • Steinberg . M•• H. C. Cheng and F . Horn (1984) . A system study for the removal . reco v e ry and di s posa l of ca r bon dioxide from fossil fu e l power p l ants in the U. S . BNL 356 66 . Brookhaven National Lab .
ENERGY AN D ENVI RONMENT (Ill )
CLOSED DUAL FLUID GAS TURBINE POWER PLANT WITHOUT EMISSION OF CO 2 INTO THE ATMOSPHERE P. S. Pak, K. Nakamura and Y. Suzuki Department of Electrical Engineeri ng, Osaka University, Suita, Osaka 565, J apan Abstract. This paper describes a construction and characteristics of a coal-gas - burned h i gh efficiency power plant which emits no ca r bon dioxide (C0 ) into the atmosphere. I n 2 the p l an t, CO gas and s uper heated steam a re used a s the main a nd sub work i ng f l u i d , 2 respectively , of a c l osed dua l fluid g a s tur bine power generation system . It is ass umed t hat a coa l gas whose principal compositions are CO , H , CO and CH is burnt in a 4 2 2 combustor using oxygen , and t h at CO gas and superheated steam are used as the ma i n and 2 s ub working fluid of a tu r bine , respec ti ve l y . Consequently , the constituent gases of the combustion gas become CO and H 0 . Thus , CO gas included in the exhaust gas can be easily 2 separated at the conde~ser ou€let f r om the condensate (H 0) . Most of recovered CO is 2 2 recycled as t he main working fluid of the turbine . In the plant , high - temperature turbine e xh aust gas is uti l ized in a waste heat boiler to produce superheated steam whi ch is injected into the combustor in order to improve power generation efficiency . It has been estimated that an electeic power of 208MW can be generated with gross thrmal efficiency of 51 . 3% . Power generating efficiency has been est i mated to be 43 . 7% since the electric power of 31.1MW is required for producing the oxygen for combustion . The problem of liquefying CO recovered has also been dealt with in the paper . It has been shown that the resultant 2 powe r generat i ng efficiency is estimated to be 39 . 0% if the power for liquefact ion of recovered CO is taken into account. 2 Keywords .
Carbon dioxide ; greenhouse effect; coal gas ; power plant ; gas turbine fluid cycle ; closed cycle ; high efficiency . INTRODUCTION
removing CO should be able to be reused 2 r ecyclically . Alkanolamine-based solvent , which has a chemica l property of absorbing CO gas at low 2 temperature about 40 · C and of discharching it at high temperatu re about 1 50· C, is one of chemical products satisfying this requirement . Table 1 shows estimated energy and cost which are required for treatment of CO, when a process for removing and recover i ng CO irom the s tack gas using the solvent 2 is adopted in a 100MW coal fired steam turbine powe r p l ant. In Table I energy and cost required for liquefaction and disposal of th e recovered CO are 2 also shown. As shown in Table I, the energy and the cost required for removing and recovering CO are 2 est i mated to be much larger than those for l iquefaction and disposal of the recovered CO , On 2 account of this l arge quantity of energy req u 1re~ent for removing and recovering CO , power generating 2 e fficiency has been estimated t o be reduced from 38% to 15 . 9%2 When the low pressure steam at about 7 . 03kg/cm and 204 . C is extracted from the low pressure section of the turbine to supply the heat , the efficiency has been estimated to be improved from 15 . 9% to 31 . 4%, being still lower by 6 . 6 percentage points compared with the origina l efficiency (Pak et al ., 1989a) . To realize a power plant without the emission of CO into the 2 a tmosphere , the r efore , it is considered to be more efficient to construct a completely new power plant so that the method of recovering CO becomes much easier than that of incorporating a removal-and2 r ecovery proc e ss using the solvent into a conventional power plant .
As a resu lt of large scale use of fossil fuels , a g reat quantity of carbon dioxide (C0 ) is being 2 exh a u s t ed into the atmosphere , and CO density is 2 inc rea sing a ll over the worl d. It is afra i d that the a tmo s ph eric temperature might be increased on a ccou nt of the greenhouse effect by CO , and t h at 2 th e c l i mate might be changed on g l obal scale . Hence it i s req u ired to reduce the emission of CO into 2 the a tmosphere . Exh austed amount of CO is extremely enormous 2 compa r ed with that of a i r pollutants such as n 1trog en oX1des (NO ) ' and CO has been considered x 2 to be a c l ean gas unt11 t he present . Hence , t echnolo i ges on r emova l of CO from t he s t ack ga s of 2 a bo il er a r e compl etely different from convent i onal air pollution control technologies , and it is con s idered to be very difficult to develop t ech no l og i es for recovering CO efficien tl y . 2 Therefore , if it is required to realize a power g e ne ra tion p l ant which emits no CO into the 2 a tmo sphere , it would be easier to construct a c omp l ete l y new power generation system than to remove a nd recover CO from the stack gas from a c o n v en tional power plan€ . Th is paper describes a construction and c h ara cter istics of a new power generating system whi c h ut ilizes coal as its fuel and emits no CO into the atmosphe r e . The reason a coal-utilize~ power p l ant is inve st igated is that coal is the most af fl uen t fossi l fuel but has the most infe ri or pro perty in CO emi ssion cha r acteristics among 2 f o ssil fuels used for thermal power plants . The d e gr ee of effect which will be achieved by reducing th e emi s s i on of CO can thus be expected to be 2 largest.
RECOVERY OF CO
2
FROM A CONVENTIONAL POWER PLANT
Ste i nberg e t al . has already investigated a problem o f remo v ing and recovering CO from a stack gas of a 2 c o n venti o nal coa l f i red s t eam turbine power pl a nt (Steinbe r g , 1984 ) . Since t he amount of an exha ust ga s is ext r e me l y enor mous , it becomes indispensable c o ndition tha t t h e c h emi cal product used for
dual
c6
Tabl e 1 Energy and cost required f or treatment of CO2 Required energ y'! Req uired cost" · and recovery 1241 50 I Liquefact ion 254 23 Disposal 8.64 5 *1: The energy requ ired for t reating CO: of 1000kg is des ignated by us ing the un it of Heal . where e lectr ic energy of 1 kWh is converted into 2. 45 Mea l. *2 : The cost produc ing electricity of 1 kWh at a lOOMW coa l fi red power plant is adopted as the ref er ence va lue of 100.
I Remova I
229
p, S, Pa k, K, Na ka mura an d y , Su zu ki
230
POWER GENERATING SYSTEM WITHOUT EMISS I ON OF CO Necessity Protection
of
Gasifying
Coal
For
2
Environmental
Feed vater ~
If coal is directly burned in a boiler a t a the rmal power plant , it is not ea sy to control t h e emission of air po llutant s such as dust , sulpher oxides ( SO ) and fuel - der ived nitrogen o xides (NO) , not ~o mention the r emoval of CO. Therefore , ~n th e power plant to be inv estigated f n this study it is assumed that coal is gasified and substances which causes air po l l utants such as dust , SO and f ue l NO are removed in th e process of gasifi~ation and i~ the following gas refining p rocess .
Condensate
G Generator
Turbine Construction of a Proposed System
Re.oval of
excess 0)2
~----------------------------------~--~
Figure 1 shows a schematic construction of a power generating syst em which emits no CO into the 2 a t mosph ere . Th e fo llowing is a brief expl anation how electric power is gene rat ed with emitting no CO 2 into the atmosphere . The proposed system can be called a col sed dual fluid gas turbine power generating syst e m in which CO gas is used as the ma in working fluid and supe rheated steam as the sub working f l u i d . It is assumed in t he system that the coal gas ifi ed by using oxygen is burned in a combustor using oxygen instead of the air , and that both CO gas and superheated steam a re used a s the working 2fluid of a turbine . Consequently , the constitue nt gases of the combustion gas become CO and H 0 , since the principal composi tions of coaf gasifi~d by using oxygen are CO , H , CO , and CH 4 • 2 Th e combustor outlet high-temperature com~ustion gas is first used to ge nerate electric power by driving a t u rb i ne . The turbine exhau st gas , which has still a considerab l e heat energy , is then utilized to produce superheated steam at a waste heat boiler to inc r ease power output as well as to improve power generating efficiency . The low-temperature outlet gas from the waste heat boiler is lastly led into a condenser , where the steam included in the exh aust gas is complete ly conden sated into water. The condensate is used as th e feed water of the waste heat boiler to generate steam. The p rincipal component of the condense r outlet gas is CO , although a smal l quantity of oxygen and nitrog~n g3S i s included . Oxygen gas inclusion is caused by inJecting excess o xyg en into the combustor for realizing complete combustion of the fuel gas , and nitrogen gas i nclus i on is caused by the fact t hat i n general an ext reme l y small quantity of nitrogen gas is included in a coal gas , even if coal is gas if ied by using oxygen gas instead of the air . The condense r o utl et gas is recycled as the main working fluid of the gas turbine . The quantity of CO gas not reused as th e working fluid , which is 2 equal to th e quantity of CO gas generated by 2 burning the coal gas , can directly be removed from the system at a wate r separator outlet . Thus , no CO gas 1S emitted from this power plan t into the 2 atmosphere . The removed CO gas can be used as the 2 raw material of CO gas . In the case CO reuse is 2 impossible , the r~covered gas should be liquefied and disposed into a deep ocean or a large scale storage suc h as depleted oil we ll s . It shou l d be noted that no thermal NO is in this power plant in addition to n~ CO character1st lc,
SInce
generated
emission combustIon reactIon is taken
2
p l ace in the combustor whe re practical ly no nitrogen gas exists.
Fig . 1. Construct ion of proposed system (closed dual fluid gas t urbine power generating SyStell). the va l ues of entha lpy of the gas at inlet and The e nthalpy va l ue is outlet of the turbine . dependent upon gas temperature and gas compositions , which are affected by the combu s tor inlet gas tempe rature and by fuel flow rate , and these values are in turn depend e nt upon the value of entha l py of the turbine outlet gas , etc . Hence , trial-and - error procedure is necessary for est ima ting characteristics . A simulation mode l has been constructed t o estimate various characteristi cs of the system. In the model , the variables and parameters s hown in Table 2 are assumed to be the e xogenous variables and parameters , r espective ly. The detailed explanation of the constructed model is omitted in this paper . An e xample of simulation results is shown in Fig . 2 , where the values shown in Table 2 have been assumed to be u sed as the reference values of the exogenous variables and the parameters . As s hown in Tab l e 3 , the lower calor ific value of the c oa l g as , which is assumed to be used as the fuel, is 2489 kcal / kg . Figure 2 shows th e estimated results when the maximum steam quanti ty that can be generated in the waste heat boile r is injected into th e combustor . AS
shown
in
Fig . 2, t h e syst em ca n generate steam of 218t/h whose pressure is 34 . 0 kg/cm and temperature 888K . By increasing fuel gas flow rate from 105 t/h , in case of no superheated steam injection , to 140 t/h in case o f the above mentioned maximum steam quantity injection , for the purpose of keeping turbine in l et temperature to be the specified value of 1523K, generated power correspondingly increases from 100MW t o 208MW . The temperature of the e xhaust gas flo wing into the waste heat boiler is 928K a nd that flowing into the condenser is 382K. Flow rate of the gas which is not condensated in the condenser is estimated to be 962 t/h , and the weight percentag e of the composition of this gas is estimat ed as fo ll ows : CO ~ 97.5% , 02 = 1 . 8 4%, and N2 = 0 .65% . The 2 quant1ty of the gas r eci rculated as the main working fluid is 780 t /h , and thus the gas quantity which shou ld be r emoved from the system i s estimated to be 181 t/h, and the quantity of CO recovered is 2 , calculated t o be 1 7 7t / h . Since the quant1ty of CO 2 gene r ated by burning 140 t/h fuel gas is 181 t/h , the removal rate of CO is 98 . 0% . The reason all 2 the amount of generated CO can not be recovered iG 2 that flow loss has been assumed at the various parts of the system, as indicated i n Table 2 . super~eated
PERFORMANCE CHARACTERISTICS OF THE PROPOSED SYSTEM Results of Characteristics Estimation To estimate various characteristics o f the proposed power gene rating system , it is required to estimate
Gross thermal efficiency has been estimated to be 51 . 3%. The e s timated value is remarkably high compared with that of a conventional gas turbine power generating system . This is because: ( a ) the heat energy inc l uded in the tur bine exhaus t gas is
Closed Dual Fluid Gas Turbine Power Plant
231
Table 2 Exogenous variables and parameters of silulation model (a) Exogenous variables Reference value SYlbol Definition T. Turbine inlet telperature 1523K 30kg/C1 2 Turbine inlet pressure P. COlPOsition of gaseous fuel and its volume percentage Sce Table 3 Vf. i Telperature of fuel and oxygen gas 573K:298K Tf :T o2 Excess oxygen ratio in colbustor a 1.2 Condenser outlet te~perature 303K Te 0.5kg/CIl 2 Comdenser outlet pressure Pe Steam injection rate standardized by G3 MaximulII value Rs determined Generator output when Rs = 0.0 100MII W. (b) Exogenous parameters Symbol Definition nk Adiabatic efficiency of C02 compressor E k Flow loss rate at C02 co.pressor Adiabatic efficiency of turbine nt Flow loss rate at turbine Et Combustion efficiency of coabustor nb Pressure loss rate at combustor rh Flow loss rate at co~bustor E b (5. n Pressure loss rate at fuel gas nozzle Pressure loss rate at oxygen injection nozzle /5 on Pressure loss rate at steam injection nozzle (5 s n Enthalpy exchange efficiency of waste heat boiler 6TT Waste heat boiler terminal telPerature difference (incolli ng exhaust gas te~perature outgoing steal temperature) Waste heat boiler pinch point telPerature difference TPP 6TD Dry telPerature condetion for waste heat boiler outlet exhaust gas (outgoing exhaust gas tellPerture - staturated steall tellPerature) Exhaust gas pressure loss at waste heat boiler Steam pressure loss rate at waste heat boiler Exhaust gas flow loss rate at waste heat boiler Eo. (5 Exhaust gas pressure loss at condenser Preassure loss rate at water separator /5 w s Generateor efficiency n ••
c.
Reference value 85:\: O.l:\:
85% O.l:\:
98:\: 2% O.l:\:
10:\: 10% 10% 90:\:
40K 20K
10K
o. lkg/cm2 5% 0.1%
O.lkg/c l1l < 2:\:
98%
Exhaust gas
,.==.:.::.;:....;:.=-----,T ,'382.24
T: TelPerature [K) p: Pressure [kg/CIl 2) I: Enthalpy [kcal/kg) G: Flow rate It/h)
P?,=8 . bE)
17 "22.1 5 G7 z 1ZZ5 . b,s
Waste heat .11383 , :5b ."'31,1)1
Is ·Ub . 75
r=-=-_+-",·-2 18 . 22 TJI-753 .Z8
P2 ")G . bl 12 =11 •. qJ
w· 35 . 80 w"218.22
Combustor ,
G.'77Q . 35
Fuel
w- 31 3.15
Condenser
gaS T.=57J .1 5 Pr- z3 '1 . il
C02 cOIIPressor G," JQ.qJ r,,=b2 .~7
Oxygen
Wc::IIZ87.7~
T,=313.15
Poa "'3"1,lil
P, =1 . 1Q J1=',(JQ
Go2 zQl . S8
nc=51.'3~
Turbine excess C02
Fig. 2.
Estimated characteristics of proposed power generating system.
232
P. S. Pa k, K. Na kam ll ra a nd Y. SlI zu ki
Table 3
Co.position of ref ined coal gas
COIIPositi on Volu.e per centag e (~l Weight percentage (%l
CO CO2 H2 43 .53 17.39 38 .33 58. 57 36.77 3.71
CH. 0. 11 0. 08
N2 0.64 0.86
Air
recovered in the waste heat boler ; (b) it is possible to make the turbine outlet pressure lower than the atmospheric pressure .
Generator
G However ,
since
the proposed system uses oxygen
to
burn fuel gas , the electric power of 21 . 9MW has been estimated to be consumed as a station se r vice power,
because the electr~c power of 0 . 34 kWh is required for producing I Nm oxygen gas , or 0 . 2379 MWh to produce 1000kg oxygen gas. The electric power of 9 . 21MW is also consumed for compressing the gener~ted oxygen to the required pressure of 34 . 0 kg/cm . Thus , power generating efficiency of the proposed system has been calculated to be 43 . 7%.
Turbine
Air co.,,-essor (a) Construct i on
2ee
0 ..
l
Generated power \.
Consequently ,
when
the
thermal efficiency of
the it
75% ,
has been shown that the resultant thermal efficiency of 32 . 8% of the proposed system is estimated to be efficient
more
than 1 . 3 percentage points
on
the
basis of the calorific value of the coal than that of the conventional coal-fired steam turbine power p l ant with wh i ch the process using the solvent for recovering CO from the stack gas is integrated . 2 Comparison
CO
2
with Gas Turbine Power Plant into
Which
Recovery Process Using Solvent is Inco r porated
In this section we will compare the estimated characteristics of the proposed system wi th that of a power plant utilizing gas turbines into which a CO recovery process using the solvent is 2 incoporated . The construction of a power plant utilizing a conventional gas tu r b i ne i s shown in Fig . 3(a) , in which the air is used not on l y for fuel combustion but also as the main working fluid of a gas turbine. This system is a system called a dual fluid open cycle gas turbine power gene r ating system . Figure 3(b) shows estimated chara c t eristics which have been obtained by using a mode l constructed by the authors (Pak et al ., 1988) • In estimating characteristics, the pressure of the exhaust gas at the waste heat boiler outlet has been assumed to be equal to the atmospheric pressure , since the exhaust gas is all emitted di r ectly to the atmosphere . The other conditions have been assumed to be the same as shown in Table 2 . In Fig . 3(b) , steam quantity injected into the combustor is taken as the axis of abscisa , and generated e le ctric powe r and gross thermal efficiency are plotted on the axis of ordinates . As seen from Fig . 3(b) , generatea power increases with the increase of t he steam injected, and gross thermal efficiency has been estimated to increase from 35.2% up to 47 . 9% . However, it is required in this system to remove and recover CO from the stack gas . According to a research by St~inberg et al . (1984) , it is possible to remove and recover CO from the e ffluen t by 2 utilizing alkanolamine-based solvent with the CO 2 removal efficiency of 90%, in which the heat energy of 1 . 21 Gcal is estimated to be required to recover 1000kg CO. In the present system shown i n Fig . 3(a) , heat energy can be supplied f rom the waste heat boiler by producing a hot water with temperatu r e of 180 oC, although this hot wat er production reduces the amount of steam produced . The reduction rate of power generating efficiency due to the reduction in the steam quanti t y inje c ted into the combustor has been estimated as explained in the fol l owing:
Generating characteristics of the hot wa t er has also been estimated as shown in Fig . 3(b) . The maximum
100
..•
0
o ' .0 ..
..0
'
"
c ..~ .. o ·
150
coal gasification process is assumed to be
0 .. .0 ..
0
3
0 ' ,0"
EI " s"
ij..:.: . " .. 3 · "" ' 0
Therma I output of hot water
.
......... £
Power generati~g ' ''O~ ' o()~
50
eff ic iency ~ ~:-=-!:'::~~_~M_'>r-)t-)(-+< ":"~*"_,,*=,=:::~':'-:,-x'- ~ ' '0-.. ' -0. , .. Heat generat ing:;»---+--- +---+-- _+__-+-_-+_ _ - ·&· ~ ~ · .o err ic iency -+- --+-- +,,-t--.-+._-+ 26
<0
bO
80
100
120
"0
Ib0
180
200
Steam quant i ty injected into combustor (tlhl (b) Esti.ated char acteristics when CO2 is not recoverd
Fig. 3. Cons truction and characteri sti cs of an open-cycle dual fluid gas turbine power generat ing syste•.
the r ma l output of the hot water can be obtained at the oper ating condition that no steam is generated in t he was t e heat biler , at which the electric power o f 100MW and the hot water having thermal output of 110 Gcal/h can be generated . The fuel consumption is 98 . 7 t/h , producing 127 t/h CO , Accordingly , 2 the hea t quantity required for recovering 90% of CO 2 gene ra ted fr om the stack gas is calculated to be 163 Gcal/ h, if thermal efficiency of a reboiler fo r r ecove r ing CO is assumed to be 85% . Consequently , th e he a t quantity of 56 Gcal/h is in shortage , even if t h e obtainable maximum quantity of waste heat ene r gy is all utili z ed for CO recovery . 2 Th is s hows that it is impossible for a power gene r ating system using a gas turbine to construct a dua l fluid cycle (Cheng cycle) system in the case whe r e the CO generated must be recovered . 2 Simila r ly , und e r CO emission constraint it is 2 impo ssi b l e to construct a h1gh efficiency combined cycle power generating system in which the turbine exhaust gas is utilized to produce superheated st e am which is used to drive a steam turbine generator . Henc e, in recovering CO in the system shown in Fig . 2 3 , an ext r a fuel becomes necessary to meet the s hor tage of t he r equi r ed heat quantity . It should be no ted here that the more extra fuel should be requi r ed, since the us e o f the fuel gas to genera te heat for recovery of CO generates also CO ' The 2 2 lower calor1 f 1c value of the fuel gas 1S 2 . 49 Mcal/kg , and t he genera t ion rate of CO is 1 . 29 kg 2 CO pe r unit kg fuel gas . Thus , the ne t lower ca t orif1c value reduces to 0 . 344 Mcal/kg , if thermal effici e ncy of a hot water generating boiler is assumd to be 90% . Thus , powe r gene r ating eff i ciency of the system shown in Fig . 3 has been estimated to be 13 . 5% when generated CO. should be recovered by utilizing the solven t. L
Closed Dual Fluid Gas T u rbine Powe r Plan t Conseque n tly , it can be seen that powe r generating efficiency of the p r oposed system has been estimated to be noti ceab l y higher than that of the gas turbine power p l ant into which CO removing a nd r ecovery 2 p r oces s ut il izing alkano l a mine-ba sed solvent is incor pora ted .
233 Exhaust gas (Oh+H,o)
reed
water
PUlP
Condenser
Compar i son with Aalate r native System Condensate
As a power plant without the em i ssion of CO int o 2 the atmo s phere, the author s proposed a power plant shown in Fig . 4 so far . Th e system shown in Fig . 4 is a sys tem wh i ch can be called a cl osed d ual f l u i d regenerative gas turbine power generat i ng system in which CO and H 0 gas are respectively used as the 2 2 main and sub working fluid of a turbine . Let us abbr eviate thi s system as a regenerative CO turbine 2 system hereafter . In this section we will compare the r e g enerative CO tu r b i ne system wi th the sys t em 2 proposed i n the present paper .
G Generator Turbine
R_ vel
of
excess Oh
Fig , 4, Alternative power plant without emission of Uh into the at.osphere (Closed dual fluid regenerative gas turbine power generat ing system. regenerati ve ah turbine system) . the
The difference of the proposed system with regenerative . CO turbine system is that e 2 regenerator 1S not 1ncorporated 1n the proposec system . The r egenerator is a heat e x changer to recover heat energy from the turbine exhaust gas and the heat energy recovered is uti l ized to r a i se the tempera t ure of the main work ing f l uid of the turbin e . It is we l l known that incor porating 3 r e generator makes it possible to noticiably improve thermal efficiency of a gas turbine power generating system , although this increases the pressure loss of the turbine exhaust gas (Pak and Suzuki , 198&) . Gross thermal effi ciency and power gene rating efficiency of th e regenerative CO turbine system 2 have been estimated to be 50 . 3% and 42 . 7% , respec tive ly (Pak e t al ., 1989b) . That is , both effici enc ies of the proposed system have been estimated to be higher by 1 . 0 percentage point. The reason the proposed system has a high efficiency characteistic is considered as follows :
for CO
2
liquefaction h"" been tak en into account .
Consequently, it can be ascertained from t h e abovementioned simulation results that we can constr uct a coal-gas - burn ed power generating system without the emission of CO into th e atmosphere with the power 2 gen e rating eff1ciency of 39 . 0% , although the ext ra equipments for producing oxygen and for liquefying t h e recovered CO are required . Figure 5 shows the 2 es t imated total mass and electric power fl ow in the proposed system . proposed economical aspects of the Concerning system , the detail will be described in another paper.
CONCLUSION Th e steam quantity which can be produced in the waste heat boiler (WHB) in the proposed system is . estimated to be larger by 35 . 3t/h than that in the regenerative CO turbine system , since the inlet gas 2 temperature of the WHB in the proposed system i s higher by lllK than that in the regenerative CO 2 turbine system . In both systems , the higher the ratio of H 0 quantity to CO quantity in the exhaust 2 2 gas , the lower the pressure at the condenser outle t. Hence , the turbine outlet pressure in the proposed syste~ wa s able to be made t~ be lowe r by' 0 . 15 kg/cm compared with the case where the regenerator was incorporated . This indicates that the more work can be done in the turbine in the proposed system compared with the case of the regeneratorincorporat ed system . Thus , it can be concluded that th e proposed system without the reg enerator is not only simpler in the construction but also more efficient compared with the regenerative CO turbine system shown in Fig . 4 . 2 DISPOSAL OF RECOVERED CO
2
This chapt er deals with the problem of disposing the recovered CO , The fl~w rate of the recovered CO 2 2 i s equal to 93 , 400 Nm /h and this enormous gas quan t i t y should be l i quefied for the purpose of vo lume r eduction in general . Since electric power to liquefy CO gas has been estimated to be 0 . 1036 kWh/lkg CO (steinberg et al ., 1984 ) , the electric 2 power of lB . 3MW is required to liquefy only CO gas 2 included in the recovered exhaust gas . However , a sma ll quant i ty of 02 and N2 are included i n thi s recovered gas , it has thus been estimated t h at the e l ectr i ca l power of 19 . 0 MW is r equi r ed for liquefaction of the recovered gas in total . Hence, power gen erating effic i ency of the proposed system has been estimated to be 39.0% in the case the power
In this paper , a power plant without the emiss ion of CO into the atmosphere has been proposed , by taking 2 a power p l ant using coal ga s as an example of a power plant . The proposed system has such an advantage that it is easy to be realized , Slnce it can be constructed by combining the ex isting t echno l og i es , and that it emits no thermal NO as x well as no CO , It has been shown from the 2 simulation results that it s power generatins; efficiency is 43 . 7%. It has also been shown for a gas turbine system in which the coal gas and the air are used as its fuel and working fluid , r espectively , that const r uction of n e ither a dual fluid power plant nor a combined cycle power plant is possible under the constraint of no CO emission . 2 Thus, its power generating eff ici e ncy has bee n estimated to become 13 . 5% under the const raint of no CO emission . 2 It has been shown that on the basis of the calorific value of the coal the thermal efficiency of the proposed system ha s been estimated to be higher by 1 . 3 percentage points than that of a conventional coal - fired steam turbin e power generating plant wit~ which a process using the solvent for recovering CO 2 from the stack gas is integrated . The problem of liquefying CO gas recovered has also 2 been dealt with , and the power generating efficiency of a coal - gas-burned power generating system has been estimated to be 39 . 0% and 29 . 3% on the basis of the calorific value of the coal gas and the coal , respectively , if the power for liquefaction of recovered CO has been taken into account . 2 It is needl es s to say that any hydrocarbonic fue l such as liquefied natural gas (LNG) and fuel oil can be used in the proposed system . In thi s case the energy loss generate d in the process of gasification
234
P. S. Pak. K. Naka mura and Y. Suzuki Generat ion of power "uel gas =139.9tIh ==l Power plant (Closed dual Generated power fluid gas turbine power ='(jJ7.8M\/ CoIpressed " generat ing syStem) 02=91 .9tIh Pro
02
of
coal
Liquefaction of CO2 CO2 .':::: : COlI'I"essor
I I
Liquefied CO:(. =177.0tIh
pressor
j 9.21M\/
121. 9H\/
and refining accoun t.
l;
COII-
Recovered CO2 =181.5t1h
Suppliable power =157.7M\/
19.011W
"ig. 5. Total
ESS
and electric power flow in the proposed syste•.
is no t
be
taken
to
into
It was not long the researchs began for i nvestifgat i ng power ge nerating systems wi t hout the e mi ss i o n o f CO i nto the atmosph e r e . I t i s hoped 2 that va r iou s k~nds of r esea r c h es are made fo r realizing high efficien c y power genera t ing systems fr om whi c h hazardous subs t ances for the ambient such as SO . NO and CO are not emitt ed into t h e 2 a t mospl'ie r e . x
REFERENCES Pak . P . S . and Y. Suzuki (1988) . Evaluation of ther modynamical . economica l and environmental characteri s tics of high efficien cy ga s turbine cogeneration systems . T. lEE . Japan . 108- D. 895 - 902 . ( i n Japanese) • Pak. P.S •• K. Nakamura and Y. Suzuki ( 1989a) . Const ruction and characteristics of a thermal power plant which recovers ca r bon dioxide gene rated . Cont r i buted to En e rgy and Resources . (i n Japanese) • Pak . P . S •• K. Nakamura and Y. Suzuki (1989b) . Coalu tilized high e fficiency power p l ant wi thout emission of CO into the atmosphe r e . Procee ding of 2 8th Simulation Technology Conf e r e nce . pp . 129/132 . Japan Soc i ety for Simulation Technology (i n Japan ese) • Steinberg . M•• H. C. Cheng and F . Horn (1984) . A system study for the removal . reco v e ry and di s posa l of ca r bon dioxide from fossil fu e l power p l ants in the U. S . BNL 356 66 . Brookhaven National Lab .