Products and solutions, 2006-06-28, 10:17 AM
More efficient combustion, lower emissions and much lower operating and maintenance costs-all this has been achieved by Bremer Abfallbehandlung Nord GmbH (ANO) at its waste-fed heating plant by means of systematic modernization. The centerpiece of the new firing strategy is a feed grate with patented water cooling which facilitates higher waste throughput and better waste burnout. And, more importantly, it also permits the combustion of fuels with a high calorific value.
Due to tougher environmental regulations, changes in the waste industry and hence changes in the waste composition, the roller grate firing of the four natural-circulation saturated steam boilers at Bremen's waste-fed heating plant (built in 1969) was no longer up to the mark. What is incinerated as waste today has no other useful purpose and usually has a high calorific value. This not only results in higher combustion temperatures, but also produces different, exceptionally corrosive flue gas components, from which the environment as well as the plant itself have to be protected.
Optimizing existing plants, i.e. modernizing and balancing the combustion process, firing system and steam generator, is one of the specialties of Duisburg-based ThyssenKrupp Xervon Energy, formerly LLS Standardkessel Service. The tradition-steeped company joined industrial service provider ThyssenKrupp Xervon in January and supplies all the required services - planning and detailed engineering through to commissioning - from a single source. And of these services there are many. Modernization, finally, affects the entire boiler design and its ancillary units. Plant components and processes such as the boiler pressure unit, waste feed, combustion grate, slag removal unit, combustion air system, hydraulics, electronics/measuring & control/automation equipment as well as firing power regulation have to be modified or totally newly installed.
The Duisburg specialists were initially awarded the contract to replace the existing roller grate firing with water-cooled feed grate firing (LLS process) on boilers 2 and 3. The operator was particularly impressed by the modernization concept developed by LLS, because the anticipated key process data had already been supplied at the planning stage. For this, the firing technologists got together with the Chair of Environmental Process Engineering and Plant Engineering at the University of Essen-Duisburg in simulating optimized grate combustion in boilers 2 and 3 - based on experience with already installed systems.
The centerpiece of the new waste incineration system is the modular water-cooled feed grate. With rabbling action, it feeds the fuel through the various combustion zones and conveys the residues to the slag pit. This is a high-temperature operation in which the primary air required for combustion-controlled separately for each grate module and supplied to the individual combustion zones-serves additionally as cooling for the grate coatings exposed to extreme thermal stressing.
While most grate types and above all older ones have to make do with this air cooling alone, the LLS feed grate firing has an additional patented water cooling system. Cooling tubes for the circulating cooling water are integrally cast in the interior of every single grate bar. The relatively small amount of cooling water is cooled in a closed circuit with the aid of a heat exchanger. This ensures that the heat is discharged in a controlled fashion and the gained waste heat is returned to the overall process.
On the basis of findings from current plant operation, the sealing concept for the water supply to the cooled grate elements was modified in Bremen. The interfaces between the movable and immovable elements were replaced by low-maintenance sealing elements.
Optimal air supply
But it is not only the unusual cooling principle that makes the system highly efficient. An ingenious combustion air distribution arrangement also plays its part. The feed grates employed in Bremen each consist of two grate lines. Each of these is divided into three zones. Zones 1 and 2 are water-cooled, while grate zone 3 serving as the burnout zone is air-cooled. There are two primary air supplies per grate zone, which means that a total of twelve ducts supply the entire grate surface. Each of these is equipped with a control flap and flow meter. This way, the primary air supply to all grate modules can be controlled independently and precisely adapted to the constantly changing combustion process. The secondary air, which amounts to about 30 percent of total air, is introduced at the entrance of the first line via several nozzles distributed in the front and rear walls. This ensures thorough mixing of the waste gases and optimal afterburning.
Different speeds through the furnace
For the optimal open- and closed-loop control of the combustion process, the various grate stage modules are operated at different speeds. For predrying and ignition at the start (grate zone 1), main combustion (zone 2) and burnout (zone 3), the desired process conditions are created in each individual zone by skillfully balancing the air supply and feed rate. This ensures efficient burnout and a high energy yield from the fired fuel. The outcome is generated energy and environmentally safe waste disposal.
Once the first two combustion lines had been modernized on schedule, the orders to replace boiler 1 and modernize combustion line 4 by the end of 2006 soon followed. As a result of optimization, the Bremen incineration plant can now process fuels with a high calorific value more efficiently. These can be substitute fuel, municipal waste, industrial waste, lightweight shredder waste from automobile recycling and, mixed with other fuels, sewage sludge. The emitted flue gases not only safely comply with the current ceilings, but in fact fall well short of them.