Results of Long-Term Trials First Large Scale Fluidized Bed Furnaces for Sewage Sludge Worldwide Boosted by Oxygen

Abstract

Oxygen is required in every combustion process, including the incineration of sewage sludge. When air is used to provide oxygen the nitrogen it contains is only ballast which has to be heated up in the furnace and which further increases the volume of flue gas to be purified. The process for partial oxygen combustion (POC) jointly developed and patented by Messer Griesheim and Infraserv Ho¨chst avoids this problem. In this process, oxygen is blown directly and transversally into the fluidized bed at supersonic speed. The first promising test results were already presented at the 16th International Conference on Fluidized Bed Combustion 2001 in Reno. In the meantime we have three years of experience with detailed operating results. Up to 2000 m3/h of pure oxygen is blasted into the two furnaces transversally via the supersonic nozzles built into the walling. This results in a highly turbulent flow which considerably improves the mixing and the combustion in the fluidized bed. Using the POC method it has been possible to increase the combustion capacity of the systems from 130,000 to currently 160,000 t/a. The hourly throughput of waste material suitable for fluidized bed combustion has been increased by up to 40%. At the same time the combustion air volume has been reduced by 15% and the specific natural gas consumption by 35%. This considerably reduces the specific incineration costs (€ / t waste material). Alongside industrial and residential sewage sludges with calorific values of 500 to 11,000 KJ/kg (depending on the original substance) and dry-substance contents between 22 and 60%, other solid and liquid waste material suitable for fluidized bed combustion can also be processed, including bone meal, screenings, plastics granulate, spent active carbon, building timber, contaminated packaging material, organic solvents and urea waste. Calorific values can here fluctuate between 1,500 and 42,000 KJ/kg (depending on the original substance) and the dry-substance content between 5 and 100%. The specific emissions, e.g. carbon dioxide and nitrogen oxides per ton of incinerated waste material are reduced. Thus the POC process gives Infraserv a third virtual furnace at low investment cost, enabling it to respond to the demands of the market. With this technology, the status of “Best Available Technology” (BAT) was conferred upon the sewage sludge incineration plant operated by Infraserv Ho¨chst by the German Ministry for the Environment.

Spent Activated Carbon Combustion in a Fluidized-Bed Combustor

“…However, few studies that focused on SAC combustion in FBC are reported. Ludwig et al used a partial oxygen combustion method to treat solid and liquid waste materials, including spent activated carbon, bone meal, etc., which are suitable for fluidized-bed combustion . Oxygen is blown directly at supersonic speed into the fluidized bed.…”

Control of Pollutants during FBC Combustion of Sewage Sludge

“…Furthermore, it was verified 8 that, for dried sludges, the levels attained without air staging, between 900 and 1300 mg/Nm 3 , were reduced to below 260 mg/Nm 3 when the air was staged. Studies carried out in the large-scale FBC of sludge , have revealed that, under well-controlled bed temperatures, by employing two-stage combustion with recirculation of flue gases, it is possible to maintain the levels of NO below 200 mg/Nm 3 .…”