In Dinkelsbühl, Bavaria, five entrepreneurs developed a concept that  combines local added value and sustainability in an exemplary way.  Nature, citizens, and each of the founders’ operations benefit equally  from the jointly founded company, TEAtherm GmbH. It all started with  an insolvent combined heat and power station (CHP) run on biomass  with a thermal sewage sludge recycling plant: The five shareholders  with synergistic skills in the areas of waste management, recycling,  and electricity generation purchased it in equal shares. The newly  founded TEAtherm GmbH began operations in April 2013 and now  produces 64,000 megawatt hours of green energy, which supplies  around one third of households in the district. The economic highlight  of the concept, however, is the use of the heat generated as a by product of electricity generation: The largest consumer is one of the  shareholders, a state-of-the-art plant nursery, which uses it to heat its  ten-hectare greenhouse adjacent to the power station. In addition,  some of the heat is fed into the district heating network of Dinkelsbühl  municipal services to supply firms in the Waldeck industrial zone,  houses, and a hotel complex. A further portion of the heat is used in  the company’s own belt dryer for drying municipal sewage sludge from  around 40 different sewage plants. A major contributor to the great  efficiency of the sewage sludge drying plant is the 5099 PPC process  belt from the GKD Group (GKD – Gebr. Kufferath AG) used in the dryer.  It was replaced for the first time after a service life of seven years and  the TEAtherm team was trained in fitting the belt.

With an output power of eight megawatts, the power station generates  electricity for 8,200 hours per year – which corresponds to an availability of  93 percent. The only fuel used is wood from rural conservation – i.e. wood  which is not intended for use in the construction trade or for furniture. This  material, which is generated during clearing in the maintenance of highways,  electrical power lines, or railroad lines, for example, is subject to strict quality  management with certified proof of origin at TEAtherm. Quality control of  every single truckload of prepared fuel takes place upon delivery: The fuel  shipments are sorted into quality classes and stored in separate fractions  based on visual criteria such as grain size distribution and the amount of  needles, leaves, and bark. This incoming control also ensures that no  contaminants or old wood have been delivered. In addition, a sample is  taken from every load delivered in order to determine the water content as  the basis for the calculation. Several photos are also taken to document the  quality classification. This strict quality management is the result of extensive  process optimizations. They ensure not only that the requirements are met  for upholding remuneration under the Renewable Energies Act, but also that  fuel use, plant failure, and ash disposal are minimized. 

90,000 tons of renewable fuel 

The power station receives 90,000 tons of wood from rural conservation per  year from around 80 regional fuel suppliers – 70 to 100 truckloads per week.  TEAtherm uses only fresh, undried wood from rural conservation: To ensure  that the system is able to reach full capacity, the water content and fuel  quality need to be just right. The fuel proceeds to the infeed grate via the  vibration chute, trough chain conveyor, and hydraulic pusher. The combined  heat and power station with steam turbine has a maximum electrical output  of 9.6 megawatts and is fed with 36 tons of steam per hour. The turbine  exhaust steam is condensed at a temperature of 45 degrees Celsius and is  used to heat the neighboring greenhouse. In comparison with conventional 

district heating networks with a temperature of 90 °C, this use of exhaust  steam not only avoids the loss of power caused by steam withdrawal from  the turbine, it also increases the electrical efficiency of the system. With  40,000 megawatt hours per year, the greenhouse is already the largest heat  consumer by far. By the end of 2021, however, the greenhouse area is set to  be expanded by a further five hectares and will then consume another  15,000 megawatt hours of heat.  

Drying 20,000 tons of wet sludge  

TEAtherm uses around 13,000 megawatt hours of heat to operate the  sewage sludge dryer. The belt dryer made by Andritz has a drying capacity  of 22,000 tons of wet sludge per year. The volume of dry material produced  annually is 4,800 tons. At 20 meters in length with a width of ten meters, the  dryer has a throughput of 2.7 tons of wet sludge per hour. For the drying  process, the sludges delivered with a water content of 78 percent are evenly  spread onto a slowly moving process belt made from PPC in an 80  millimeter-thick layer. The air in the dryer is circulated via two circulation  fans, heated in heat exchangers, and channeled through the product for  drying. The dryer is fed with hot steam at a temperature of 200 degrees  Celsius at a pressure of eight bars. The sludge then moves through different  temperature zones on the process belt for 30 to 50 minutes – from 140  degrees Celsius at the start of the dryer to 100 degrees Celsius at the end.  This causes the water in the sludge to evaporate and the water content  drops from 78 to less than ten percent at the dryer outlet. Some of the dry  material generated is mixed with the wet sewage sludge in the mixer at the  dryer infeed in order to achieve a granulate with better drying properties. The  exhaust air is channeled via another fan through a washer and biofilter for  cleaning. Cleaning the exhaust air produces an ammonium sulfate solution of  fertilizer quality.

Seven-year belt life in continuous operation 

When the system was put into operation by TEAtherm, there was still a GKD  5099 PPC process belt on the dryer – around 36 meters long, 3.80 meters  wide, with L-seam and edge coating. Unlike with polyester belts, the material  of this belt type means that no hydrolysis occurs, so that the mesh design  comprised of robust monofilaments permanently withstands the large surface  weights. Woven in 3/2 twilled weave, the belt is particularly resistant to wear  on the underside due to its flatness. With air permeability of 4,300 l/m²/s  optimally tailored to the drying process and a CFM value of 660, it ensures  an efficient drying process despite the very closed surface. At the same time,  the smaller number of direct apertures resulting from the weave type reduces  dust ingression. The belt also stands out with dimensional stability up to 200  degrees Celsius and resistance to chemical influences up to a pH-value of  14. “It is simply a reliable belt that served us well for seven years,” says  Jakob Friedrich, CEO of TEAtherm, in praise of the firm’s first experience  with the GKD belt. He adds: “We know from other system operators that  some of them have to replace their belt every year.” 

All-in service with perfect training  

When a tear appeared in the belt in its seventh year of use, TEAtherm  wanted to fit a spare GKD belt they had found at the power station. However,  mechanical engineer Friedrich and his team lacked the necessary  experience and so asked GKD for help, which was promptly delivered. The  fitter sent by GKD was on site for two days to fit the replacement belt and  train the TEAtherm team in how to fit the belts. He didn’t know that Jakob  Friedrich had joined the team so that he could see the process for himself. “If  you want to form a valid opinion as an engineer, you look at things yourself  and join in with the work so you know what works and what doesn’t when  you’re in contact with your co-workers,” explains Friedrich. GKD supplies  these process belts with a specific unrolling function in the transport crates. 

This makes it possible for the heavy belt to be pulled into the machine  directly from the crate with a special belt feeder and straps. This not only  enables energy-saving fitting without the need for a crane, but is also gentle  on the material, as the belt is not pulled accross the floor into the dryer. “The  GKD employee performed the replacement brilliantly with our staff and  generally provided us with very good support,” says Jakob Friedrich. He was  so impressed with this dedication and service that he ordered a new belt  right away. This support proved its value after just a year: The spare belt  found by TEAtherm turned out to be a used belt, which itself had to be  replaced. “We fitted the new belt ourselves without any problems,” says  Friedrich, who attributes this success to the high-quality training service from  GKD. The 5099 PPC also meets TEAtherm’s specific requirements for a  dryer belt in every respect: Thanks to its low wearability with process-specific  air permeability, tear resistance to contaminants, and long service life, it  guarantees an efficient drying process in the long term. That’s why the  company immediately ordered a new spare belt, as Jakob Friedrich’s opinion  of the process belt and GKD is thoroughly positive: “Fast delivery, reliable  belt, and a service we can rely on.” Perfect conditions for the planned  capacity expansion in sewage sludge drying. 

Manali bhanushali
Author: Manali bhanushali

Manali Bhanushali