Betriebsoptimierung moderner Steinkohlenstaubfeuerungen zur Vermeidung von Feuerraumkorrosion (English shop)

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Since the liberalisation of the energy market in 1996, the operators of German coal-fired power plants have also been permitted to burn imported coal. Where imported coals are used whose composition and properties differ substantially from the original design coal, this can cause operational problems in the respective firing system.

Differing coal properties in combination with an operating mode that has not been adapted accordingly can lead to increased fouling, slagging and a greater tendency towards corrosion on the furnace walls and on the convective heating surfaces of the firing system. Although furnace-side corrosion on the surface of evaporator tubes is sufficiently well known and the underlying corrosion mechanisms have been investigated, approximately 27 % ³³ of all unplanned outages of pulverised coal firing systems are caused by furnace-side corrosion.

A change in the firing settings, for example in the course of a combustion optimisation, should therefore always be accompanied by oxygen concentration measurements in the near-wall region of the furnace walls in order to ensure that the corrosion potential at the furnace walls is not increased as a result.

Reducing conditions at the furnace walls are regarded as a prerequisite for the occurrence of furnace corrosion. For this reason, the oxygen concentrations in the near-wall regions of the furnace walls are measured in order to identify areas with an increased corrosion potential. In this context, the gas composition in the near-wall region of the furnace walls is also referred to as the wall atmosphere.

Within the scope of this work, the wall atmosphere of a pulverised coal firing system was measured at different firing settings in order to identify parameters that have a decisive influence on the composition of the wall atmosphere. Furthermore, the relationship between the measured wall atmosphere and the material wastage on evaporator tubes due to furnace corrosion was investigated.

The investigations were carried out at Unit 7 of the Rheinhafendampfkraftwerk of EnBW Kraftwerke AG in Karlsruhe. The firing system investigated is designed as an opposed firing system with low-NOx burners. The total of 32 staged-mixing burners of type SM I are arranged on four burner levels. Each burner level is equipped with 8 burners, with four burners installed on the front wall and four on the rear wall. The burners of a burner level are each supplied by one mill. The pulverised coal leaving the mill is distributed to the corresponding burners by means of so-called forced distributors.

In the course of a scheduled overhaul of the plant investigated, wall thickness measurements were carried out throughout the entire furnace. The areas with the highest material wastage determined agreed very well with the areas at which low oxygen concentrations had previously been measured.

The results of these investigations demonstrate that regular monitoring of the wall atmosphere is a suitable method for identifying critical areas with regard to possible furnace corrosion.

In order to avoid furnace corrosion, it is necessary to know by which parameters the wall atmosphere, and thus the oxygen concentrations in the near-wall regions of the evaporator walls, can be influenced.

The following were identified as the decisive parameters in this respect:
_ the excess air,
_ the air staging at the burner,
_ the pulverised coal distribution to the burners
_ and the fineness of the pulverised coal.

To determine the pulverised coal distribution, the pulverised coal mass flow was measured at each burner by means of an isokinetic sampling probe. With this measuring method, a partial flow is extracted from the pulverised coal pipe and the pulverised coal is separated from the carrier gas. The pulverised coal samples thus obtained for each burner were analysed in the laboratory with regard to their particle size distribution. In addition, the pulverised coal pipes of all 32 burners are equipped with a MIC online pulverised coal measuring device, which continuously measures and records the pulverised coal distribution to the individual burners of a mill. On the basis of the signals of this measuring system, the fluctuations of the pulverised coal quantity in each pulverised coal pipe can be recorded and evaluated.

The fluctuations in pulverised coal transport determined in some pulverised coal pipes show a behaviour comparable to the fluctuations of the measured oxygen concentration at the corresponding wall atmosphere measuring points. Owing to the existing fluctuation ranges both in the pulverised coal transport and in the primary and secondary air at the burners, it is very difficult to quantify the influence of the individual parameters on the wall atmosphere. For this reason, the simulation tool AIOLOS, developed by the Institute of Process Engineering and Steam Boiler Technology (IVD) of the University of Stuttgart and the company RECOM Services, was used to carry out parameter studies.

The simulation results of the parameter studies carried out make it possible to verify and quantify the influencing parameters determined with regard to the composition of the wall atmosphere. In addition, information is also obtained regarding the effects of these parameter variations on the flue gas composition and on the loss on ignition.

With the aid of the simulation calculations it can be clearly demonstrated that an uneven distribution of the pulverised coal to the burners has a decisive influence on the composition of the wall atmosphere. By setting a more uniform pulverised coal distribution to the burners, the oxygen concentration in the near-wall region of the furnace walls can be increased. A further positive aspect is evident from a lower loss on ignition and lower primary NOx concentrations in the flue gas. The investigations furthermore show that an uneven pulverised coal distribution in the distributor systems also leads to a particle size separation. Usually, burners with a high pulverised coal loading also receive a higher coarse particle fraction, whereby the negative effects of an uneven pulverised coal distribution are further intensified.

With the aid of continuous and reliable monitoring of the pulverised coal distribution, a burner-specific adaptation of the combustion air quantity to the respective coal quantity is possible. The improvements achievable in this way correspond to those that can be attained with a uniform distribution of the pulverised coal.

The findings obtained show which measures are available for increasing the oxygen concentration at the furnace walls of a pulverised coal firing system. A distinction is made between measures that are based merely on a changed operating mode and measures that require a structural modification of the plant.