Pergamon Energy Cotwers. Mgmt Vol. 38, No. 10-13, pp. 1093-1100, 1997

© 1997 Elsevier Science Ltd All fights reserved. Printed in Great Britain

P l I : S0196-8904(96)00139-2 0196-8904/97 $17.00 + 0.00

E F F E C T S O F P R E S S U R E O N F U E L - R I C H C O M B U S T I O N O F

M E T H A N E - A I R U N D E R H I G H P R E S S U R E

TSUYOSHI YAMAMOTO, i NORIYUKI KOBAYASHI, I* NORIO ARAU and TADASHI TANAKA ~

~Research Center for Advanced Energy Conversion, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan and 2Electric Power Research and Development Center, Chubu Electric Power Company

Inc., Kitasekiyama, 20-I, Ohdaka-cho, Midori-ku, Nagoya, 459 Japan

Al~tract--We have proposed a new and innovative gas turbine system, named chemical gas turbine system. It would improve the thermal efficiency more than 10% compared to conventional ones. This gas turbine system is based on promising developments in fuel-rich combustors with a carbon fiber reinforced carbon composite (C/C composite) being used as the turbine blades. As one of fundamental researches to develop this system, we designed a 4 MPa-scale combustor with methane-air. Flammability limit, components of combustion gases and combustion temperature were measured between 1.1 and 4.1 MPa in pressure. Results from these measurements were as follows: (1) stable combustion could attain between the equivalence ratio ~b = 0.7 and 1.3 at 4 MPa in pressure; (2) there was little effect of the pressure on the components of combustion gases; and (3) flammability limit extended with increasing the pressure in the fuel-rich region while it was almost constant in the fuel-lean one. © 1997 Elsevier Science Ltd.

Fuel-rich combustion High pressure Chemical gas turbine Methane Air

I N T R O D U C T I O N

In many countries, thermal power generation depends largely on fossil fuels. To raise the thermal efficiency in thermal power generation plants is almost equal to developing a new energy resource. The gas turbine technology as one of the power generation methods has been steadily advancing, causing a rapidly growing demand for electricity and energy crisis in these +days.

The thermal efficiency of a gas turbine for the power generation is improved about 2% with every rise of 100°C the turbine inlet temperature (TIT). It is expected that the thermal efficiency may be easily more than 50% when TIT becomes 1500°C. As far as the metal materials are used for the turbine blades, however, TIT may be limited to 1500°C. Therefore, further efforts to raise the thermal efficiency should be directed to develop better heat-resistant materials and a new gas turbine system as a breakthrough in existing gas turbine technology.

Given these circumstances, we have proposed a new and innovative gas turbine/steam turbine combined cycle system [1, 2]. A schematic diagram of the proposed system is shown in Fig. 1. This system would suppress the formation of NOx as well as the deterioration of C/C composite being used as the turbine blades of the first gas turbine (GT0, by adopting fuel-rich combustion as the first combustor (CB,). Exhaust gas from the first gas turbine would still contain chemical energy as H2 and CO, and would be mixed with compressed air prior to be burned with fuel-lean mode at the second combustor (CB~,). As fuel-lean combustion is well-known to be a low-NOx method, the formation of NOx would be also suppressed at the second combustor (CBH). Exhaust gas from the second combustor would drive the second gas turbine (GTH). Heat from turbine exhaust gases would be used to operate the third steam turbine. For these features, the proposed combined system was named the "Chemical Gas Turbine" system. Figure 2 concerns the thermal efficiency of the chemical gas turbine system calculated semi-empirically. It was found from calculations that the total thermal efficiency of the proposed system becomes 66% when the equivalence ratio of the first, fuel-rich combustion is 2.0.

*Corresponding author.

1093

1094 YAMAMOTO et al.: FUEL-RICH COMBUSTION OF METHANE-AIR UNDER HIGH PRESSURE

[ Air

CP, I I I CP,, ~ GT,,

Fuel Heat exchan¢

GTI

Low calorific fuel

ST

Exhaust

• Condenser

CP: : Air C o m w e n o r GT, : First ~ Turbine

CP,, : Fuel Compreuo r GT, : Second Gall Turbine

CB= : Fuekdch Combustor ST : Steam Turbine

C B , : Fuel-lean Combustor

Fig. I. Schematic diagram of the proposed gas turbine/steam turbine combined cycle system (chemical gas turbine combined cycle system).

As the first step in a research project to examine the validity of the proposed system, we have focused on the development of a pressurized fuel-rich combustor. A large number of experimental and theoretical studies has been made on fuel-lean combustion of methane-air. However, little is known about pressurized fuel-rich combustion of them. In this work, we designed a 4 MPa-scale model combustor to develop highly pressurized fuel-rich combustor applicable to the chemical gas turbine system. Flammability limit, components of combustion gases and combustion temperature were measured between 1.1 and 4.1 MPa in pressure.

=-'~ 70 , 70

F ........ ....... _I,o

T ~,,~- ............ ~ .............. :..._~0

i i ' 2 7 3 ~ " ~ ~ " [ : ~ [ " : : " ' i ................. ~ .............. ~10

Equivalence ratio [ - ]

Fig. 2. Thermal efficiency of chemical gas turbine calculated semi-empirically.

YAMAMOTO e t al.: FUEL-RICH COMBUSTION OF METHANE-AIR UNDER HIGH PRESSURE 1095

Air Methane ~) (~ (~ (~) (~

Cooling water Drain

(~ Igniter (~) Cooling pipe (~ Quartz window (15 t) (~ Exhaust throats (~) Quarlzwindow(15x35) (~ Cooling chamber (~ Probe holes (~ Thermocouple ® Injector

(a)Details of combustor

(~ Control valve (~ Water reservoir ~ Silencer (~ Stop valve (~ Igniter ~ Gas sampler (~) Mass flow meter (~ Thermocouple (~) Console (~ Pressure transducer (~ Combustor

(b)Total layout of experimental appratus

Fig. 3. Schematic drawing of lab-scale combustor: (a) details of combustor; (b) total layout of experimental apparatus.

E X P E R I M E N T A L A P P A R A T U S AND P R O C E D U R E S

Figure 3(a) is a schematic drawing of the lab-scale combustor employed in the present work. It consists primarily of an injector, an igniter, a combustion chamber and an exit nozzle. The size of the combustor is 50 mm in inside diameter and 626 mm in length. The combustor is made of stainless steel with a pure copper liner inside. The injector is a coaxial type. Methane was supplied through the inner nozzle and air was fed through the outer nozzle. The inner tube is set inside the recess of the outer tube to promote mixing of methane and air to achieve easy ignition. The recess distance between the injection exit of methane and that of air is 2 mm. The exhaust nozzle consists of 4 holes of 1.7 mm in diameter and 4 holes of 1.0 mm. Combustion gas sampling and temperature

1096 YAMAMOTO et al.: FUEL-RICH COMBUSTION OF METHANE-AIR UNDER HIGH PRESSURE

Q.

t - O

3 .JD E o t -

. m

== 3 (/) ( / )

I1.

3

0

stop

!

Pressure in supply line of methane , I

Pressure in combustor 7 / ',

Temperature "

Pressure in supply line of air

Pressure gniter working gas sampling I 4.1 MPa

I

L. J I J

_ _ _ |

0 10 20 T ime [s]

Fig. 4. A typical example of time-changes of pressure and

1500

tooo

E

500

temperature during combustion.

measurement were done at a point 541 mm from the front of the injector at the central portion across. The thermocouple used was the Pt/Pt-Rd(16%) type and 0.1 mm in diameter.

The drawing of the experimental setup of the gas supply system is shown in Fig. 3(b). Air and methane were supplied from gas cylinders connected in parallel. Control was maintained by a sequencer. Each experiment began by supplying air and the igniter was run with 2 s interval. After 1 s, methane started to be fed. In all experiments, the igniter worked for 6 s. Combustion gas was exhausted out through a silencer. The pressures and temperatures were continuously recorded and the components of combustion gases were analyzed by gascromatographs with a TCD detector and

150

~ '1oo

.I-

Combustion pressure

01.1MPa 03.1MPa A2.1MPa N4.1MPa

0.5 2

' I ' ' ! '

I :" ""

I ' • I , , , , I ,

1 1.5 Equiva lence ratio [-]

Fig. 5. Flammability limits of the pressurized combustion of methane-air by using the model combustor.

YAMAMOTO et al.: FUEL-RICH COMBUSTION OF METHANE-AIR UNDER HIGH PRESSURE 1097

1600 ~: lean

~l,mO 1200 •

I~ Combustion pressure 1000 01.1MPa 03.1MPa

A2.1MPa 114,1MPa 8OO

0.8 1 1.2 1.4 1.6 1.8 Equivalence ratio [-]

Fig. 6. Effects of pressure and equivalence ratio on temperature of combustion gas.

10 - ' ' ' I ' ' ' I ' ' ' I ' ' ' i ' ' ' I ' ' ~ I ~ ' ' I ' ' ' I ' ' 7 :

.g C ~ -

0

~2 ::...,..

0.8 1 1.2 1.4 1,6 Equivalence ratio [-]

10 ~ , ' 1 ' ' ' I ' ' ' 1 ' ' ' 1 ' ' ' 1 ' ' ' 1 ' ' ' I ' ' ' 1 ' '

8 ~ r e ~

~2 o 0.8 1 1.2 1.4 1.6

Equivalence ratio [-]

Fig. 7. Calculation results for relationship between major products and equivalence ratio in methanHir combustion at chemical equilibrium state.

1098 YAMAMOTO et al.: FUEL-RICH COMBUSTION OF METHANE-AIR UNDER HIGH PRESSURE

a NOx meter with a chemical luminescence sensor• Experimental conditions were the equivalence ratio of about 0.7 to 1.75 and the pressure of 1.1 to 4.1 MPa.

R E S U L T S A N D D I S C U S S I O N

Figure 4 shows a typical example of the time-changes of the pressure and temperature in the combustor, the pressure in the gas supply lines and the combustion temperature. This figure indicates that the highly pressurized combustion became steady at about one second after starting. Exhaust gas was sampled at from 10 to 15 s intervals during the stable combustion as illustrated in this figure.

Figure 5 indicates the flammable region of the pressurized combustion of methane-air, where all measured points were determined after continuous combustion had been confirmed to take place at the given equivalence ratio and input load. The lower limit was almost the same under any pressure, whereas the upper limit expanded with increasing the pressure. The combustion temperatures were measured as indicated in Fig. 6. It was found that the temperature became higher with increasing the pressure because the heat load was higher with increasing the pressure.

Prior to the experiments, chemical equilibrium was theoretically calculated by using CHEMKIN code developed by Sandia National laboratory [3]. These calculations were based on 54 chemical species and 234 elementary reaction mechanisms. Figure 7 shows two representative examples of

• CO •H2 •C02 •CH4 []02 I I •cO •H2 •COz •CH4 []Oz i . , , , i , , , i , , , i , . , i , , , i , , , I , , , i , , , i , , , i , , , i , , , ,

: " " l " " ' " r " ~ ' " l ' " r . . ~ , . . l . , . ~ . . . . . . . ~ C o m b u s t i o n pressure: I.IMPa -.'J 10 ~_ Combustion pres,sure: 2.1MPa :,

' ° i , • i

6 .o 6

~ / . oTr~.. .- .-~ D [] -

O I l l . . . l i i i l i i 0 ~ i = l . | i I . u l i . . l | i i l i ,.:

E

, 1 , ,

0.8 1 1.2 1•4 1.6 1.8 . 8 Equivalence ratio [-]

riCO • H I • C 0 2 •CH4 1-102 . ' ' ' I " ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' I ' ' ' 1 ' ' ' I ' ' ' I ' ' ' I ~ ' ' ,

10 : Combustion pressure: 3 . ~ " : 10

• . . J i i Z g

,~ 4 ~ 2

2 3 D i

0 _ 0 O.S 1 1.2 1.4 1.e 1.8 0.8 1 1.2 1.4 1.6 1.8

Equivalence ratio [-] Equivalence ratio [-] Fig. 8. Effects of pressure and equivalence ratio on the combustion gas components.

YAMAMOTO et al.: FUEL-RICH COMBUSTION OF METHANE-AIR UNDER HIGH PRESSURE 1099

I I ICO OHz ACOz QCH4 D 0 z 10

8 • t,

io \ "

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Equivalence ratio [-]

Fig. 9. Comparison of components of combustion gases between experimental results and calculations under the pressure of 4.1 MPa. Solid lines mean the results obtained by the theoretical calculations in

equilibrium state. Keys are the results by the experiments.

the relationship between the equivalence ratio and the calculated equilibrium concentrations of major product gases at 0.1 and 4.1 MPa, respectively. There may be almost no difference between their results. The concentration of 02 decreases with an increase in the equivalence ratio and leads to almost zero over q~ = 1.1. On the other hand, CO and H2 generate above ~b = 1.0 and increase remarkably with an increase in the equivalence ratio.

Figure 8 concerns the experimental results of the measurement of combustion gas components except NOx at two different high pressures, 1.1 and 4.1 MPa. There seems to be little difference among them within the range of the pressure employed. Carbon monoxide and hydrogen were produced above tk = 0.8 and their mole fractions increased with increasing the equivalence ratio. The mole fractions of CO and H2 became equal to each other at around ~b = 1.1 for 4.1 MPa and q~ = 1.3 for 1.1 MPa while at ~b = 1.5 by the equilibrium calculation results. The oxygen and methane were detected even over q5 = 1.1, whereas and very small amounts of them were found in the calculation.

The results obtained from experiments and equilibrium calculations for 4.1 MPa were compared to each other in Fig. 9. Concerning the major products of CO, CO2 and H2, there was a fair

10000 . ' I I I I I I I I I I I I I I I I I I I l l l l l l l l l | ~ l l l l l l l l l l l l l | l l l l | l l l

Combustion pressure

1000 - ~ ,~ A2.1MPa BI4.1MPa

! ~ ~ k ~ 0"lMPa(calc')

100 "

10

1 0.8 1 1.2 1.4 1.6 1.8

Equivalence ratio [-] Fig. 10. Experimental results for emission characteristics of NO.~ and theoretical calculation results in

equilibrium state. Gothic lines mean the results obtained by the theoretical in equilibrium state.

1100 YAMAMOTO et al.: FUEL-RICH COMBUSTION OF METHANE-AIR UNDER HIGH PRESSURE

agreement among them on the effect of the equivalence ratio on the concentration, although the calculation results were obtained with the assumption of an equilibrium state that did not take the heat loss or residence time into consideration. The reason for the agreement is that the reaction rates of the major products may be high enough to reach almost chemical equilibrium state.

Figure 10 shows the experimental results of NOx emissions from the pressurized combustion. Under the fuel-rich condition where the first combustion will take place in the chemical gas turbine system, the decrease in the NOx emissions resulted from the increase of the combustion pressure. The concentration of NOx at 4 MPa in pressure decreased to 2.65 ppm at ~b = 1.48. On the other hand, there may be an increase in the NOx emissions with the pressure under a fuel-lean condition. The results of the dependence of NOx emissions on the equivalence ratio were held by those from the theoretical calculations. More detailed discussion concerning the formation of product gases including NOx in the present combustor should be made after theoretical simulation which takes into account fluid mixing, heat transfers etc.

These results tell us that the total emission index of NOx in this new combined cycle system could be considerably decreased by using fuel-rich and pressurized combustion at the first combustor (CB~ in Fig. 1) and by applying a fuel-lean combustion at the second combustor (CBn in Fig. 1) as well.

C O N C L U S I O N

A new concept of a gas turbine combined cycle system, the "Chemical Gas Turbine" system, was proposed in order to raise the total thermal efficiency. The chemical gas turbine system consists of a combustor operated under fuel-rich and highly pressurized conditions and of C/C composite materials being used as the turbine blades.

We designed a 4 MPa-scale combustor with methane-air to develop a combustor operated under fuel-rich and highly pressurized conditions applicable to the proposed system. The combustion characteristics of the combustor were examined experimentally. It was found from these experiments that: (1) stable combustion could attain between equivalence ratio q~ = 0.7-1.3 at 4 MPa in pressure; (2) there was little effect of the pressure on the components of combustion gases; (3) flammability limit extended with increasing the pressure in the fuel-rich region while it was constant in the fuel-lean one; and (4) the NOx emissions decreased with an increase in the pressure under the fuel-rich condition.

R E F E R E N C E S

1. Arai, N., Teramae, N. and Kobayashi, N., Energy WorM, 1995, 226, 16-17. 2. Kobayashi, N., Nakano, K., Matsunami, A. and Arai, N., Proceedings of 25th International Symposium on Combustion,

WIP-25-042, 1994, Irvine, U.S.A. 3. Sandia Report, SAND 89-8009D. 1989, UC-706.