1. Field of Endeavor
The invention relates to a method and a device for producing an ignitable fuel-air mixture, the fuel fraction of which consists of hydrogen or of a gas mixture containing hydrogen and which is burnt in a burner arrangement for driving a thermal engine, in particular a gas turbine plant.
2. Brief Description of the Related Art
Motivated by the virtually worldwide effort to reduce the emission of greenhouse gases into the atmosphere, not least set down in what is known as the Kyoto Protocol, the emission of greenhouse gases which is to be expected in 2010 is to be reduced to the same level as in 1990. Major endeavors are required to implement this aim, particularly to reduce the contribution of anthroprogen-induced CO2 releases into the atmosphere. About one third of the CO2 released into the atmosphere by humans is attributable to energy generation, in which mostly fossil fuels are burnt in power plants for current generation. Particularly due to the use of modern technologies and because of additional political framework conditions, a considerable saving potential to avoid a further-increasing CO2 emission can be seen in the energy-generating sector.
One possibility, known per se, which can be implemented in technical terms to reduce the CO2 emission in combustion power stations is to extract carbon from the fuels to be burnt, even before the fuel is introduced into the combustion chamber. This presupposes corresponding fuel pretreatments, such as, for example, the partial oxidation of the fuel with oxygen, and/or a pretreatment of the fuel with steam. Fuel pretreated in this way has high fractions of H2 and CO and, depending on the mixture ratios, have calorific values which, as a rule, lie below those of natural gas. Depending on their calorific value, gases synthetically produced in this way are designated as Mbtu or Lbtu gases, which are not readily suitable for use in conventional burners designed for the combustion of natural gases, such as may be gathered, for example, for EP 0 321 809 B1, EP 0 780 629 A2, WO 93/17279, and EP 1 070 915 A1. All the above publications describe burners of the premix combustion type, in which, in each case, a swirl flow consisting of combustion air and of admixed fuel is generated, which widens conically in the flow direction, and which, in the flow direction, after emerging from the burner, as far as possible after a homogenous air/fuel mixture has been achieved, becomes unstable due to the increasing swirl and changes into an annular swirl flow with backflow in the core.
Depending on the burner concept and as a function of the burner power, the swirl flow of liquid and/or gaseous fuel, which is formed inside the premix burner, is fed in to form as homogenous a fuel/air mixture as possible. If, however, as mentioned above, it is appropriate, for the purposes of reduced pollutant, in particular CO2 emission, to employ synthetically prepared gaseous fuels alternatively to or in combination with the combustion of conventional fuel types, then special requirements arise with regard to the design of conventional premix burner systems. Thus, synthesis gases, in order to be fed into burner systems, require many times more fuel volume flow than comparable burners operating with natural gas, thus resulting in markedly different flow momentum conditions. On account of the high fraction of hydrogen in the synthesis gas and the associated low ignition temperature and high flame velocity of the hydrogen, there is a high tendency of the fuel to react, which leads to an increased flashback risk. In order to avoid this, it is appropriate, as much as possible, to reduce the average dwell time of ignitable fuel/air mixture within the burner.
If, furthermore, the intention is to use pure hydrogen as fuel instead of synthesis gases, which, for example, are obtained by coal gasification and typically have a mixture of hydrogen, carbon monoxide, and nitrogen in a mixture ratio of 30:60:10, this being against the background of combustion which is, as much as possible, of reduced emission or is emission-free, then the problems indicated above apply in even more intensified form, especially since hydrogen has a flame velocity which lies by an order to magnitude above that of natural gas and is about 45% higher than the flame velocity of undiluted synthesis gases, such as are also obtained within oil gasification. In addition, hydrogen as fuel has a much greater spontaneous ignitability or reactivity, for example than that of natural gas, so that, with the above hydrogen-specific combustion qualities taken together, the production of an ignitable fuel/air mixture consisting of hydrogen under conditions, such as prevail for the firing of gas turbine plants, is extremely difficult, yet it is still important to avoid, in particular, premature ignitions of the hydrogen before a homogenously intermixed fuel/air mixture for the firing a combustion chamber in order to drive a gas turbine plant, has been formed. In the case of an insufficient intermixing of the fuel/air mixture, pronounced temperature peaks and associated high nitrogen oxide emissions occur on account of combustion inhomogeneities.