When methanol is intended to be used as fuel for a gas turbine, it is very important to enhance overall thermal efficiency of the gas turbine system, and to make it competitive with conventional oils or gas fuel. There are many ways to accomplish this. Combined cycle is not, however, a proper way, as this could also be applicable to conventional fuel. Noting the unique characteristics of methanol, the steam reforming regenerative cycle was investigated by many institutions. In this scheme, wasted heat of the gas turbine exhaust gas is transferred to reformed gas and is recycled back to the gas turbine as a part of fuel, thus resulting in increased overall efficiency of the gas turbine. Thermal decomposition of methanol is also an endothermic reaction and may be applied to the regenerative cycle. In either case, however, only a part of the waste heat is recovered. Hence, the hybrid system with combined cycle was proposed to achieve additional heat recovery. But, this is a complex system.
As an alternative means of achieving high efficiency with a simpler system, we propose a steam reforming regenerative cycle with reactor by-pass water where waste heat recovery is conducted by steam reforming and steam generation. A part of the generated steam is used for steam reforming and the rest is reinjected into combustor outlet gas by-passing the reactor. Current state of the art limits the maximum inlet temperature of the turbine. Therefore, it is a common practice to use excess air to control combustion temperature. When the steam is injected into the combustion gas, the excess air can be reduced, and as a result, net power output is increased. High thermal efficiency will be achieved by properly installing a methanol evaporator, boiler, reactor and superheater, in such a way as to minimize exergy loss. Many case studies have been conducted for various combinations of turbine inlet temperature and pressure ratio. Calculated thermal efficiencies were compared with those of other systems.
1. Introduction
It is very important to develop as much as possible substitute fuel sources like methanol to diversify supply sources and assure a stable supply of energy. Because of these reasons, the utilization of methanol has a long history of study by many institutions. To be a successful substitute fuel sources in the near future, its cost competitiveness against petroleum oil or LNG is of prime importance.
Methanol could be easily synthesized from natural gas or coal which otherwise would not be utilized due to remote locations. As shown in Fig- 1, energy shrinkage of methanol synthesis is approximately 65% based on a low heating value (hereinafter called LHV base). Energy shrinkage of oil refineries and the LNG plant are approximately 90% and 85% respectively. When combined cycle is used for power generation, overall energy shrinkage becomes 41% for oil, 39% for LNG and 30% for methanol. Very few gas producers can justify accepting larger energy shrinkage of methanol than LNG. These are the reasons why methanol utilization as a fuel is still very limited to specific situations.
To make methanol competitive against LNG, a technology is needed to compensate energy shrinkage at production side by enhancing thermal efficiency up to 60% on the consumption side. Combined cycle, steam injection gas turbine (hereinafter called STIG) or cogeneration could be considered to improve thermal efficiency on the consumption side, but this could also be applicable to conventional fuel like oil and LNG and would not give any advantage to methanol.
Noting the unique characteristics of methanol, the steam reforming regenerative cycle was investigated by many institutions. In this scheme, the wasted heat of the gas turbine exhaust gas is transferred to reformed gas and is recycled back to the gas turbine as a part of fuel, and this results in increased overall efficiency of the gas turbine. Thermal decomposition of methanol is also an endothermic reaction and it may be applied to the regenerative cycle. In either case, however, only a part of the waste heat is recovered, so that a hybrid system with combined cycle was proposed to achieve additional heat recovery. But, this is a complex system.
2. Review of Conventional Gas Turbine Cycle
Before presenting our proposed cycle, it may be helpful to review conventional gas turbine technologies now available in published literature.
2.1. Simple Cycle Gas Turbine
As shown in Fig.-2, the simple cycle gas turbine consists of an air compressor, combustor and turbine. Net power output is power generated by the turbine less the power consumed by the air compressor. The thermal efficiency of the gas turbine is determined primarily by the turbine inlet temperature and secondarily by compression ratio. Optimal pressure exists for each inlet temperature. The higher the inlet temperature, the higher attainable thermal efficiency. Normally, material available for the turbine construction fixes the highest inlet temperature. The highest inlet temperature now available is 1,200 deg. C level for heavy-duty industrial gas turbines. This could be further increased up to 1360 deg. C by turbine blade cooling using inter cooled compressed air. 1) When petroleum oil is used for fuel and turbine inlet temperature is 1,100 deg. C, Overall thermal efficiency based on high heating value (hereinafter called HHV base) is 32%. 2)
2.2. Combined Cycle
In this system, waste heat of the simple cycle gas turbine is utilized in generating steam of Rankine cycle. When petroleum oil is used and turbine inlet temperature is 1,100 deg. C, overall thermal efficiency reaches 43%
(HHV base). 2) In this case, two levels of inlet pressure i.e. l0 and 70 atmospheric pressure are adopted for the Rankine cycle. An admission turbine is needed to expand two different pressure levels of steam down to condensing pressure. High temperature side pinch temperature difference of the waste heat boiler is 30 deg. C.
2.3. STIG
When steam generated by waste heat of the gas turbine is injected into the combustor or to its outlet, turbine inlet temperature is reduced. This means that at given turbine inlet temperatures, excess air ratio could be reduced and the net power output would be increased. In other words, by adding extra capacity to the turbine, this extra capacity could be used as if it is a steam turbine.
Elimination of the steam turbine, surface condenser and cooling water system simplifies the overall system and may provide an advantage over the combined cycle.
Disadvantages of this system are the special gas turbine design and higher boiler feed water (hereinafter called BFW) consumption. Although this system is at its infancy, industry acceptance is gradually spreading.
STIG can achieve shaft thermal efficiency of 45% (LHV base) with an inlet temperature of 1,100 deg. C and compression ratio of 14. 3)
2.4. Endothermic Reaction Regenerative Cycle
All conventional cycles described in para. 2.1. to 2.3. adopted waste heat recovery by sensible or latent heat of water. But, in this case, an endothermic reaction of methanol is used for the heat recovery. There are two kinds of endothermic reactions of methanol i.e.
Catalytic decomposition
CH30H ---> CO + 2H2 -21.7 kcal/kg mol
Steam reforming reaction
CH30H + H20 ---> C02 + 3H2 -11.8 kcal/kg mol
Those two reactions take place at a temperature below 300 deg. C with the aid of a catalyst.
Steam reforming reaction is not a single reaction. The reaction takes place in 2 stages as follows;
Decomposition
CH30H ---> CO + 2H2 -21.7 kcal/kg mol
Shift reaction
CO + H20 ---> C02 + H2 +9.9 kcal/kg mol
Therefore, decomposition and steam reforming are not separate reactions. On the contrary, as shown in Fig.-5, conversion by shift reaction will be proportional to the amount of water added to the methanol. When no water is added, it is called a decomposition reaction. When 1 mol. of water is mixed with 1 mol. of methanol, it is called a steam reforming reaction. As shown in Fig.-5, to make complete conversion by shift reaction, excess steam up to 2 mol. of water/l mol. of methanol must be added.
As shown in Fig.-6, carbon monoxide and hydrogen gas generated in the reactor are sent to the combustor.
Because of recycling of recovered heat to the combustor, overall thermal efficiency increases.
When excess water is added to the reactor, thermal efficiency will be increased but reformed gas calorific value will be reduced and combustion in the combustor becomes difficult.
On the other hand, when no water is added, it is called catalytic decomposition. This has a higher rate of heal of absorption due to non-existence of shift reaction, which is exothermic. This high rate promises better performance of heat recovery. However, it has a disadvantage of coke formation by Boudourd reaction as shown below;
2CO ---> C + C02
The reaction takes place at the temperature range of 300 to 500 deg. C. Because of this; it would be advisable to avoid industrial application of this reaction.
When steam reforming is used and turbine inlet temperature is at 1,176 deg. C, shaft thermal efficiency is 44% (LHV base). 4)
2.5. Hybrid of Endothermic Reaction Regenerative Cycle and Combined Cycle
As shown in Fig.-7, when the steam reforming regenerative cycle is integrated with the combined cycle, shaft thermal efficiency reaches 53% (LHV base) which is the highest of all proposed systems. 4) This is based on the fact that waste heat recovery at the high temperature end is conducted by steam reforming and the low end by combined cycle, thus minimizing exergy loss of waste heat recovery. The major disadvantage is the complexity of the hybrid system.
Performances of the conventional gas turbine cycles as discussed are summarized in Table-1. Depending on the sources, definition of the thermal efficiencies are different. Therefore, all figures are adjusted on the same basis i.e. overall thermal efficiency as defined in Fig.-2 and turbine inlet temp. of 1,100 deg. C. Heating values used in this paper are listed in Table-2.
3. Proposed Cycle
As shown in Fig.-8, for the purpose of achieving high efficiency with a simple system, we propose a steam reforming regenerative cycle with reactor by-pass water where waste heat recovery is conducted by steam reforming and steam generation. A part of the generated steam is used for steam reforming and the rest is reinjected into combustor outlet gas by-passing the reactor.
Due to excess water by-passing the reactor and a combustor, a miss fire problem will be avoided. Instead of water bypassing the reactor and the combustor, catalytic combustion of low calorific value gas may possibly be adopted, but this will add additional complexities.
As discussed in para. 2.1., current state of the art limits the maximum inlet temperature of the turbine.
Therefore, it is a common practice to use excess air to control combustion temperature. Similar to the STIG, when steam is injected into the combustion gas, the excess air can be reduced, and as a result, net power output is increased.
High thermal efficiency will be achieved by properly installing a methanol evaporator, boiler, reactor and superheater, in such a way as to minimize exergy loss.
The only disadvantage will be special design of the gas turbine i.e. low excess air ratio and high BFW consumption. This system could be applicable to both catalytic decomposition and steam reforming.
4. Performance Evaluation of Proposed Cyc1e
Performance evaluation of the proposed system needs a comparison with different systems.
4.1. Case Definitions
The following 8 cases were selected for the evaluation.
Case- 1 Steam Reforming Regenerative Cycle w/ Reactor By-pass Water
Case-2 Decomposition Regenerative Cycle W/ Reactor By-pass Water
Case-3 Steam Reforming Regenerative Cycle
Case-4 Decomposition Regenerative Cycle
Case-5 Methanol Evaporation Regenerative Cycle
Case-6 STIG w/ Methanol Evaporation
Case-7 STIG w/ LNG Superheating
Case-8 Oil Fired STIG
Case- 1 is our proposed system. All cases except Case-8 have fuel superheating. Intercooling by methanol injection and combination with regenerator was not included because these are studied by others. 5)
4.2. Assumptions and Performance Calculation
The assumptions made for the performance calculation are listed in Table-3. The same figures were applied for all cases.
Energy and material balance calculation and thermodynamic property estimation were conducted by an in house process simulator called CAPES . In the CAPES program. Redlich Kwong type PVT equations are used for thermodynamic property estimation.
4.3. Results of Performance Calculation
Results of performance calculation are summarized in Table-4. After comparison with Table-1, it could be said that the proposed cycle has approximately the same efficiency as the hybrid of steam reforming regenerative cycle and combined cycle. It is also true that the proposed system has superior performance than the combined cycle and the STIG for methanol fuel. Although
Case-2 showed higher efficiencies than Case- 1; this should be discarded because of coke formation in the superheater.
It was found out that, for all cases, optimal pressure ratio is 8, 20 and 30 for turbine inlet temperatures of
800, 1,100 and 1,400 deg. C respectively. This optimal compression ratio is higher than the commercially proven compression ratio which may be around 8, 14, and 20. It could be explained that the lower compression ratio was favored due to higher construction cost of a gas turbine at higher compression ratio.
In Fig.-9, thermal efficiency vs. turbine inlet temperature was plotted for 1 cases. Commercially proven compression ratios were selected for this plot.
When the time comes that allows us to use turbine inlet temperatures of 1,400 deg. C, the proposed cycle can achieve approximately 60% overall thermal efficiency (LHV base).
4.4. Temperature Profile of Waste Heat Recovery
Temperature profile in the waste heat recovery section clearly shows us how well the waste heat was recovered.
In Fig.-10, temperature profile of Case-1 is shown. There was a small temperature difference i.e. small exergy loss was attained by lineally installing a water preheater for deaerator, methanol evaporator, boiler, reactor, reformed gas superheater and by-pass steam superheater in series from lower temperature side towards higher temperature side.
High thermal efficiencies were attained by heat recovery at a higher temperature level by steam reforming which takes place at 300 deg. C. This temperature corresponds to steam pressure of 88 atm. This steam pressure is higher than the highest steam pressure of 70 atm. of the conventional combined cycle. This is the reason why Case- 1 achieved higher thermal efficiencies than that of the conventional combined cycle.
In Case- 1, total water rate was determined to keep turbine inlet temperature at 1,100 deg. C. The amount of water added to methanol was I to 1 on mol. basis. Balance of water was directly sent to the combustor outlet by-passing the reactor. Actually, the amount of by-pass water shall be adjusted to control temperature in the combustor so that stable combustion could be maintained and nitrogen oxides (NOX) generation could be controlled. 6)
In Case-1, boiler pressure is fixed by a compression ratio of 14. Therefore, boiler temperature is fixed at 196 deg. C.
5. Conclusion
In this paper, we proposed a new gas turbine cycle, which could achieve high efficiency with a simple system. Our system has the advantages of having the highest efficiency, simplest system without steam turbine, surface condenser and cooling water system and easy control of NOX emission. High efficiency will compensate energy shrinkage in methanol synthesis side and increase viability of utilization of remote natural gas or coal.
The disadvantage of our system is the need of special design of the gas turbine i.e. low excess air rate, high BFW make up rate and high fuel gas holdup in the system. A special design needs a longer lead-time for turbine development. A high BFW make up rate will not deteriorate economic feasibility, because of the elimination of evaporation toss of the cooling tower system which is grater than the consumption of BFW of the proposed cycle. High fuel gas holdup may need safety precautions.
6. Acknowledgment
The authors are grateful to Chiyoda Corporation for permission to publish this work, which was performed several years ago. At that time, valuable contributions were made by T. Kurosu (now Yamamoto), and H. Sugiyama. The authors are also grateful to J. Itoh for reviewing the manuscript and giving valuable comments.
References
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