Combined Cycles with No Heat Transfer at High Temperature

P M V Subbarao

Professor

Mechanical Engineering Department

Eliminate the Intensive Irreversibilities…..

Introductory Remarks

• Hydrogen has been widely recognized as a clean fuel.

• Water decomposition into hydrogen oxygen is becoming more practical by means of various methods .

• A total energy system is one of the main development trends to use energy sources efficiently and reasonably .

• Various types of gas turbines and their total energy systems have been studied and put into operation rapidly.

• Usually most research is focused on the bottoming cycle analysis , for example, the gas turbine topping cycle combined with HRSG Rankine cycle , Kalina cycle or supercritical bottoming cycle to utilize gas turbine exhaust heat effectively.

• Steam injected gas turbine cycles have also been studied.

• Nevertheless research on improvement of the toping cycle is relatively not too common .

• The conventional topping cycle consists of a gas turbine or diesel engine.

• Recently a helium cooled high temperature reactor ( HTR ) had been introduced to the steam cycle and analysed .

Hydrogen as a Synthetic Fuel

• Coal and oil reserved are limited , and the combustion of fossil fuel will result in the atmosphere pollution .

• As the energy crisis occurred , hydrogen has drawn world wide interest as clean fuel.

• There exit three feature in hydrogen utilization.

• First of all, various measures to produce hydrogen from water decomposition are becoming practical now days , this process also generates stoichiometric oxygen which can be reused as an oxidizer.

• The Second but very important of all, the stoichometric oxidation of hydrogen releases a large amount of heat :

H2 + ½ O2 = H2O (g) H = -241.8 MJ kmol-1

Combustion of Hydrogen

• The reaction per kilomole of the hydrogen and half kilomole of the oxygen composes one kilomole of water with very much larger amount of energy released than any other conventional type of fuels.

• The second characteristic is that the product water has no pollution at all.

• It is hence of significance to analyse the hydrogen fuel prime mover .

• It needs to be noted that the existence of a catalyst or steam.

• Therefore the key factor to control the hydrogen and oxygen composition is that there must be a safety combustion chamber to ensure the reaction is not explosive .

• As long as the reaction occurs under some catalyst or ignition, the product volume is reduced compared with those of reactant for the reaction itself, which can considered as a safety factor against the explosion of the remaining hydrogen .

• The small amount of incondensable oxygen at the turbine exit can be evacuated in this case by the conventional method.

The Total Energy System

• Generally speaking, the improvement of the gas turbine total energy system is to utilize effectively the high temprature exaust gas and reduce the heat transfer loss between exhaust gas and working agent of bottoming cycle.

• For a stoichometric reaction between hydrogen and oxygen , the product is water , so the expansion process of working fluid may include that part of the bottoming cycle.

• There is no process of steam heat absorption from the gas turbine exhaust ,which is the reason why this hydrogen and oxygen combined cycle is proposed , and also is the advantage of this kind of cycle.

• The large amount of heat released in the combination reaction of the hydrogen and oxygen is used not only to raise the product temprature itself, but also to superheat the bottoming cycle agent.

• The cycle maximum temprature in the near future will still be far lower than the stoicometeric combustion temprature of the hydrogen and oxygen.

• The H2 and O2 cycle can be classified into two types according to heating patterns direct mixing and surface exchanger reactor.

• If the turbine inlet temprature is allowed to be high enough, the cycle efficiency will reach the maximum value.

Cycle Configuration

• If the hydrogen and oxygen are selected as working fluids on the basis of stoichiometeric proportions, they are firstly compressed in compressors , and then expanded in the turbine to the condensing back pressure .

• The cycle maximum temperature can be kept not be too high, as the reaction energy released is applied partially to heat the recirculating condensate feed water to keep the moderate cycle maximum temperature .

• The reaction product and heated steam are mixed together to expand in the turbine.

The mixing H2 and O2 combined cycle

• One kilogram of H2 and 7.936 kg O2 are compressed with the intercooling, then combined to form 8.936 kg water in the reactor.

• At the same time, the released energy is used to superheat the preheat 8.936 R kg reciculated condensate feed water in which R is the factor to factor to control the maximum steam outlet temprature.

• The reaction product and heated stem are mixed directly to form ( 1 +R ) 8.936 kg superheated and to expand in the turbine.

• Since the pressure ratios of the hydrogen and oxygen are lower, while the cycle maximum temprature is comparatively high , the exit temperature will be high enough at stop back pressure.

• The thermal energy of this part can be used to preheat the condensate feed water .

• The turbine back pressure is steam condensing pressure rather than the normal gas turbine atmospheric one ,so the turbine power output is much larger than that of gas turbine.

• This can be also shown in the cycle process in T-s diagram.

• Although the pressure of the feed water is lower than the conventional steam cycle, which has influence on the cycle efficiency is still high for the high turbine inlet temperature.

• If the cycle maximum temperature is permitted to have a high enough value in the future, the cycle efficiency will reach a maximum point without recirculating the condensate feed water.

The H2 and O2 combined cycle with surface exchange reactor.

H2

½ O2

Reactor

Turbine

Condenser

Feeding Pump

Compressor

Details of Demonstration Plants:

• Mitsubishi Heavy Industries Ltd. have proposed the advanced hydrogen/oxygen combustion turbine system which is an inter-cooled topping recuperation cycle as part of a Japanese government sponsored program WE-NET (ÒWorld Energy NetworkÓ).

• The efficiency of this cycle reaches more than 60% (HHV), not (LHV), with a power capacity of 500MW. This cycle is formed by a compressor, turbines, a combustor and heat exchangers.

• The combustor burns hydrogen/oxygen to make high temperature (17000 C) steam.

• As a result of MHI research in, the topping extraction cycle (A) designed by Jericha was found to be the best cycle.

• MHI also designed the inter-cooled topping recuperation cycle (BI) in 1995 which modified cycle (A) to be suitable for a high combustion temperature (17000 C).

Effects of Topping Pressure Ratio

Effects of Inlet Temperature of Turbine

Schematic diagram of the advanced Rankine cycle

Solar Fuels

• Convertion of concentrated solar energy into chemical energy carriers is called Production of Solar Fuels.

• These can be long-term stored and long-ranged transported.

• By concentrating the sunlight with help of parabolic mirrors, and by capturing the irradiated energy with help of solar receivers, we can obtain heat at high temperatures for carrying out the thermo chemical production of solar fuels.

Solar H2 & O2 Cycle

Hydrogen Oxygen CycleWater

Solar Hydrogen – Thermo-chemical Production

Solar Hydrogen by H2O-splitting thermo-chemical cycle

Solar Hydrogen by Thermal Decarbonization of Fossil Fuels

Hybrid solar/fossil endothermic processes, in which fossil fuels are used exclusively as the chemical source for H2 production,

(a) Thermal craking CnHm nC(gr) + (m/2)H2

(b) Thermal gasification/reforming CnHm + nH2O nCO + (m/2+n)H2

Both routes proceed endothermically at T > 1500 K

Power Cycle

Power Cycle

Demonstration plant

• Solar-Wasserstoff-Bayern GmbH. has constructed a Solar hydrogen cycle plant at Neunburg vorm Wald , Germany, presently the plant is under testing phase and will soon become a commercial plant.

• Research on solar fuels is actively carried out at many international research and academic institutions.

• Swiss Federal Institute of Technology, Zurich has many ongoing projects on solar fuel generation systems.