The present invention is directed to a process for operating a fuel fired reactor, wherein fuel is introduced into a reactor and burned therein by means of at least one main burner, wherein the relevant parameters of the process are monitored and wherein the main burner is shut down if one or more of the relevant parameters leaves a predetermined critical operating range. In a multitude of the thermal processes the process equipment is heated up by a preheat burner installation before the start of the main burner system needed for the continuous and designated plant operation. When the main burner system is operated, the preheat burner is shut down. All modes of burner operation are regulated by national or international regulations such as the EN-746-2 or equivalent. To avoid hazardous operation resulting from incomplete combustion, the violation of the parameters stipulated in such regulations, which define a permissible or hereinafter called critical operating range, results in mandatory shut downs of the burner system. The currently used modes of operation and processes result in an enforced shut down of the burner system and subse- quently the entire process if the safety relevant parameters, in particular temperature, air/fuel ratio, 0 ₂ -concentration etc. in the reactor, are violated. In addition, the reactor often has to be purged resulting in a significant loss of temperature, time, heating fuel and production since reheating for a restart of the reactor is required.

It is, therefore, the object of the present invention to avoid unnecessary shut downs of the reactor in order to reduce the loss of energy and production.

With the present invention this object is achieved by a process comprising the features of claim 1 . In particular, it is provided that within the predetermined critical operating range for the enforced shut down of the reactor, a secondary, more stringent operating range for the shut down criteria is implemented, wherein the main burner is shut down if one or more of the relevant parameters leaves the secondary operating range, while at least one pilot burner continues to be operated as long as the relevant parameters are maintained within the critical operating range. The pilot burner(s) itself preferably is/are monitored by a burner management system separate from the main burner system, complying with the relevant legal requirements, such as EN-746-2. Practice has shown that most of the fuel fired reactors usually are operated within a rather small range of the operating parameters. Upon external influences, such as fluctuations in the fuel composition, the reactor pressure, etc. , one or more of the relevant parameters leaves not only the normal operating range but also the critical operating range leading to an enforced shut down of the complete reactor. In most cases the operating parameters could be very quickly returned to the permissible operating range by, e.g. , adapting the fuel or O2 supply, but due to the official regulations, the shutdown of the plant is necessary as soon as the critical operating range is violated. By providing the secondary, more stringent operating range the main burner is shut down well before the critical operating range is reached so that in most cases the reactor can be stabilized and returned to normal operating conditions by adapting the input variables. In this time period the pilot burner still is operated to maintain the temperature within the reactor. Accordingly, it is not neces- sary to reheat the reactor after the normal operating range is reaccomplished. Further, a purge of the reactor is not necessary as long as the critical operating range is not violated. In a preferred embodiment of the invention the main burner is restarted as soon as all relevant parameters have returned to be within the secondary operating range, so that normal operation of the reactor can be quickly resumed. Preferably and depending on the respective parameter, the upper limit of the secondary operating range is about 5 to 50%, preferably about 7,5 to 30% and more preferably 10 to 20% lower than the upper limit of the critical operating range, and the lower limit of the secondary operating range is about 5 to 50%, preferably about 7,5 to 30% and more preferably 10 to 20% higher than the lower limit of the critical operating range. Normal operation, therefore, is performed in a smaller operating range preferably corresponding to 60 to 80% of the critical operating range defined by the relevant national or international regulation. In most cases this will ensure a continuous operation of the reactor while the secondary operating range provides for an easy possibility to readjust the operating conditions within the reactor without requiring a complete shutdown.

Usually, the relevant parameters of the process are the temperature, the air/fuel ratio and/or the 02-concentration within the reactor. Depending on the process and the relevant official regulations, however, other parameters may be considered.

It is present standard procedure that, except for a brief period of time during the switch over from preheating to normal operation, the preheat burners and main burners are not operated in parallel. Rather, in the prior art, the preheat burners are shut down once the main burners begin their operation. Contrary to that, the present invention provides for a parallel operation of the pilot burner and the main burner as long as the process parameters are maintained within the secondary operating range. Thereby, a smooth transition is secured when the main burners are shut down upon leaving the secondary operating range. The pilot burner can easily maintain the temperature in the reactor in order to allow for a smooth restart without requiring to purge the reactor.

In the prior art, the preheat burner are usually operated with Diesel fuel, while the main burners are operated with heavy fuel oil or natural gas. This requires two separate fuel supply infrastructures and thus increases equipment and maintenance costs. By operating the main burner and the pilot burner with the same fuel as the main burner, in particular with heavy fuel oil or natural gas, the infrastructure can be facilitated and costs can be reduced.

The invention now will be described in more detail on the basis of a preferred embodiment and the drawing.

In the drawing:

Fig. 1 is a simplified process flow diagram of a plant implementing the process of the present invention, and

Fig. 2 is a diagram schematically showing the operating ranges used in the process of the present invention.

In the plant shown in Fig. 1 , a reactor 1 , in particular a furnace, comprises a main burner system 2 and a start up or pilot burner system 3. The main burner system 2 and/or the pilot burner system 3 each may consist of one or more, in particular four burners. The main burner 2 and the pilot burner 3 are supplied with a fossil fuel, in particular heavy fuel oil, stored in a fuel tank 4 through main supply line 5 and pilot supply line 6, respectively. A fuel pump 7 provides for the required fuel pressure, while the required fuel temperature is regulated by a fuel preheater 8. Instead of heavy fuel oil natural gas or any other suitable fossil or synthetic fuel may be used. Combustion air is introduced into the reactor 1 through line 9 as primary air. Additionally, secondary air may be introduced if required. The flue gas is withdrawn from the reactor 1 through line 10.

In the main supply line 5 and the pilot supply line 6 shut off valves 1 1 , 12 are provided which are operated by a safety and interlock system 13 and a pilot burner safety system 14, respectively. Upon operation the relevant parameters in the reactor 1 and/or the off-gas line 10, such as the temperature within the reactor, the air/fuel ratio and/or the oxygen concentration within the reactor, etc, are detected by respective detectors (not illustrated) and fed into a control system, in particular the safety and interlock system 13 and the pilot burner safety system 14, where they are compared with predetermined process parameters defined by the operating personnel and/or official regulations.

If the comparison between the actual operating parameters in the reactor 1 and the predetermined process parameters reveals that one or more of the parame- ters exceeds an upper or lower limit of a secondary operating range defined for the process, the safety and interlock system 13 will close the shut off valve 1 1 so that the main burner 2 is shut off. The pilot burner 3, however, still is supplied with fuel through the pilot supply line 6 so that it continues to operate and maintains the temperature within the reactor 1 . If during further operation of the reac- tor 1 the actual operating parameters measured within the reactor 1 return to the permissible operating range within the secondary operating range, the shut off valve 1 1 is opened again and the main burner 2 is reignited so that the reactor 1 returns to normal operation. If, however, the comparison between the actual process parameters in the reactor 1 and the predetermined process parameters reveals that the actual process parameters exceed critical limits defining a critical operating range, the reactor 1 has to be shut down completely in order to comply with official regulations. In this case, not only the shut off valve 1 1 leading to main burner 2 but also the shut off valve 12 leading to pilot burner 3 are closed so that all burners 2, 3 and thereby the complete reactor 1 are shut down. In this case, the reactor 1 has to be purged, which normally is carried out through the primary air line 9 and/or a secondary air line. Thereafter, preheating of the reactor 1 via the pilot burner 3 may be resumed, and when a predetermined temperature of the reactor 1 is reached also the main burners 2 are put back into operation.

In Fig. 2 the measured process parameters during normal operation are shown. The curve T indicates the temperature within the reactor, while the curve C indicates the 02-concentration within the reactor. These parameters are only examples of possible relevant process parameters necessary to monitor a controlled process in the reactor 1 .

As evident from Fig. 2 both curves T and C are well within the area of normal operation. The area of normal operation is limited by an upper and lower level TSH and TSL defining thresholds for a controlled shutdown of the reactor if this threshold is crossed by the curve T or C. In case of such a controlled shutdown the main burner 2 is shut down as described above while the pilot burner 3 continues to be operated. Thereby, the temperature within the reactor 1 is main- tained at a desired level.

In case the temperature or 02-concentration leaves a critical operating range as defined by upper and lower levels TSHH and TSLL, a complete and uncontrolled shutdown of the reactor 1 has to be performed in order to comply with official regulations. In this case also the pilot burner 3 is shut down as described above. In most cases, however, it will suffice to shut down the main burner 2 if the controlled shut down threshold is exceeded, so that the reactor 1 can return to normal operating conditions. Then, the main burner 2 is reignited.

The process according to the invention allows for a quick restart minimizing downtimes of the reactor and boosting availability. Thereby, energy losses can be minimized and production figures can be increased. As the main burner 2 and the pilot burner 3 are operated by the same fuel, it is possible to reduce the investment costs. As the process according to the invention can be included in the operation of existing plants, such plants can be revamped so that the operating efficiency can be improved.

Example - Alumina Calcination

Alumina calcination often is effected in a circulating fluidized bed reactor. The preferred operating temperature T of such a reactor is about 950°C. If heavy fuel oil is used as fuel, the admissible minimal combustion temperature is 750°C. If the temperature in the reactor falls below this limit, the reactor has to be shut down. Due to material constraints the maximum operating temperature of the calcining reactor is 1 .100°C. Usually, the burner system is shut off, if the content of free oxygen (O ₂ ) falls below about 1 vol.-%. There is no upper limit for the oxygen concentration as it is not relevant for plant safety. Based on the critical operating range of the temperature 750°C < T < 1 .100°C the respective lower limit of the secondary operating range would be set at e.g. 800°C or 825°C, while the upper limit of the secondary operating range would be set at e.g. 1 .050°C. If the upper limit is not relevant for the plant safety (no mandatory shut-down of the plant in case of violation of this limit), no upper limit needs to be defined for the secondary operating range. The lower limit of the secondary operating range of the oxygen concentration C could be set at e.g. 1 .5 vol.-%. There is no critical upper limit of the oxygen concentration as it is not relevant for plant safety. Consequently, no upper limit of the respective secondary operating range is defined.

This results in the following operating ranges:

T: critical operating range: 750°C < T < 1 .100°C

secondary operating range: 800°C < T (< 1 .050°C)

C: critical operating range: C > 1 vol.-%

secondary operating range: C > 1 .5 vol.-%

The above values are intended to exemplify the present invention based on a preferred embodiment only but do not limit the scope of protection of the appended claims. The adequate relation between the limits of the critical and secondary operating ranges will have to be adapted to the respective parameter, process and plant.

Reference numbers

1 reactor

2 main burner

3 pilot burner

4 fuel tank

5 main supply line

6 pilot supply line

7 fuel pump

8 fuel preheater

9 line

10 line

1 1 shut off valve of main burner 2

12 shut off valve of pilot burner 3

13 safety and interlock system

14 pilot burner safety system

T temperature

C 0 ₂ -concentration

TSL lower threshold of secondary operating range

TSH upper threshold of secondary operating range

TSLL lower threshold of critical operating range TSHH upper threshold of critical operating range