Renovation of the flue gas scrubbing at the central combined heat and power station

Everything ok above Brunswick

The BS│ENERGY central combined heat and power station forms the core of Brunswick's energy and heat supply. Electricity and district heating has been produced there since 1928, according to the combined heat and power principle. After being almost completely renewed and expanded in 1986, a large part flue gas scrubber awaited renovation. A job under immense deadline pressure: only 21 days remaining for the challenging task.

It is almost the same as the Brunswick landmark: A chimney towering 200 metres above the city centre at the energy provide BS│ENERGY's central combined heat and power station.

The combined heat and power (CHP) central station operates on principle of recovering and utilizing waste heat. The energy source can be used twice more efficiently than it would have been without CHP. However, not only is the fuel efficiency exemplary but also the combined heat and power coupling is an enormous help towards saving the environment: District heating is the most CO2-saving type of heat supply that uses conventional fuels. Already 40 percent of the heating requirement in Brunswick is supplied by district heating.

Resolutely on a path of environmental protection

The German Federal Government has rated the upgrading to combined heat and power as being the most important of all climate protection measures. According to the latest amendment to CHP legislation, a doubling of the CHP contribution to 25% of the total energy generation is required up until 2020. This is no problem for BS│ENERGY at Brunswick: At present they already produce over 30 percent of electricity using the CHP method. And it shall shortly be significantly more. Because in the spring of 2009, the building of a new gas and steam turbine plant in the combined heat and power central station was started.

It should start operation for the first time at the end of 2010 and then from 2011 onwards produce electricity and heat for Brunswick. The plant will be able to generate 75 megawatts of power and produce up to 65 megawatts of heat.

This plant increases the capacity of the combined heat and power central station. The previous capacity was 78 megawatts. BS│ENERGY delivers electricity over a 2100 kilometre supply network to 161,000 domestic, business and industrial customers.

Eight boilers fired-up

Coal, natural gas and heavy fuel oil are used to heat eight boiler systems in the combined heat and power central station. The most important plant and the one that is in continuous base-load operation is the coal-fired boiler. Connected downstream are powerful flue gas scrubbing units, which ensure that the air remains clear over Brunswick.

Since its installation in 1986, and with 23 years in operation it is now time for a complete renovation of the flue-gas desulfurization system. "Time" – a factor that also plays a very central role during the renovation of the flue gas scrubbing unit. However for different reasons, because when carrying out this job practically every second counts. After all the central combined heat and power station couldn't just simply be disconnected from the net.

Instead it was necessary to carry out the renovation while the power station was running. It was only possible to replace the coal station with many smaller natural gas plants for a short space of time.

Many jobs

Many tasks needed to be carried out in order to complete the renovation. Work was required on the absorber, the unscrubbed gas duct, textile filter truncated wedge, textile filter and the scrubbed gas duct.

In the summer of 2008, these jobs were contracted out by BS│ENERGY. One important requirement was an extremely short period of completion. Just 21 days were available for die complete renovation.

Another important point: Those responsible at BS│ENERGY thought it could be a great advantage, if the renovation company for the final coating material were also both developer and manufacturer at the same time. In Brunswick it was said "Quite simply to bundle the responsibility for the work, getting it done by one contractor – therefore also bundling the guarantee undertaking".

The choice of tender finally focussed on the GBT Bücolit GmbH from Marl in Westphalia. The middle-sized structured GBT Group, with 140 employees generated an annual turnover of about 25 million euros. GBT is also specialised in the subject of air pollution control, whereby flue gas scrubbing systems for power plants and waste incineration plants are the main areas of work. As desired by BS│ENERGY, the Westphalian renovation experts are also the developer and manufacturer of the coating material – in this case BÜCOLIT V590 G.

Special job under particular conditions

Work was started on the 7th of August, the 28th of the same month circled in red and kept in mind – the immovable, fixed deadline for all jobs.

First the absorber, scrubbed gas duct, unscrubbed gas duct and textile filter truncated wedge, were professionally separated from all media and control cables. Then the disassembly of the installed and attached parts. There weren't any exact technical drawings of the existing six compensators, which meant that they had to be measured before they could be removed.

The next step was to clean all areas, whereby cleaning the absorber proved to be a special challenge: This involved the removal of 50 tonnes of incrustation. The absorber is a circular cylinder-shaped container with at flat roof and with a tapered cone construction. Its dimensions are 10.6 metres internal shaft diameter, 7.3 metre shaft height and 8.5 metre cone height. The angle of the cone against the vertical is 60 degrees.

50 tonnes of baked-on deposits in the absorber

A travelling cradle was installed specifically for taking away the baked on deposits. The 50,000 kilograms of material rejected from here was disposed of using a single manhole within only 24 hours. A great feat beyond comparison.

All other sectors of the plant needed to be cleaned at the same time as the absorber. In addition, the entire area to be renovated of about 1,500 square meters was divided up into single action segments and closed off from each other.

Following the basic cleaning of the inside surfaces of the areas being worked on were abrasive blasted to a roughness of SA 2.5. A close-knit wall thickness measurement was conducted in order to assess the scope of the renovation necessary. This showed that 95 percent of the steel area was damaged to such an extent that it could no longer be restored by sanding or weld-deposit cladding. Consequently the areas must be completely doubled using five millimetre thick steel sheeting. In the absorber cone, it was even necessary to completely replace almost half of the steel area.

Ready-made steel plates save the time schedule

However, being an experienced renovator of flue gas scrubbing systems, we were at GBT very well prepared for this "surprise": As a precaution, all the plates for the absorber and the textile filter truncated wedge had been prefabricated in advance. All other plates were available for use on demand. Not a minute could be lost – the schedule remained stably on course.

Welding was carried out according to DIN 14879, weld seam classification B. This meant that all welders had to pass a test by successfully submitting a sample weld before starting their work. After the welding work, all areas were completely given a second blasting to grade SA 2.5.

Then a complete coating of the steel surface using the BÜCOLIT V590 G coating system was carried out. In the especially exposed absorber an additional antistick coating was applied, which functions as a sealant, preventing as best as possible any future baking on of deposits.

Meanwhile, during last stage of the job, newly manufactured compensators were installed according to the technical drawings made at the start.

On time and exactly to the minute, GBT manager Martin Konzack handed over the renovated, ready to operate flue gas scrubber unit to the power station operator. Until then, up to 100 workers had been simultaneously ensuring cleanliness, building in over 80 tonnes of steel sheet and applying 1,500 square meters of BÜCOLIT V590 G.

The coating system

The BÜCOLIT V590 G system is a spray coating based on Novolac vinyl ester resin.

System is mainly used for process engineering plant equipment such as storage and process tanks as well as concentrated acids and alkalis. Where this spray system can reveal its advantages, especially for large, geometrically simple surfaces.

BÜCOLIT V590 G was developed use in flue gas scrubbers in power stations and waste incineration plants, especially for use in ducts. Where this spray system can fully reveal its advantages, especially for large, geometrically simple surfaces.

The system, because of its properties, is used particularly where higher temperatures and aggressive chemicals occur. It has been specially designed for corrosion protection in the ducts and heat exchangers of flue gas desulphurisation systems, in which strong H2SO4 concentrations occur at high temperatures. The limit for continuous thermal loading, in comparison against chemicals, up to 80 degrees Celsius (wet) and in the gas phase up to 180 degrees, for a short time up to 220 degrees Celsius.