The aim of the project – funded by the German Federal Ministry for Economic Affairs and Energy (Bundesministerium für Wirtschaft und Energie; BMWi) – is the development and optimisation of an efficient and reliable micro gas turbine (MGT) based thermal incinerator with low emissions for decentralised power supply. In this case, the volatile organic compounds (VOCs) contained in the exhaust air of technical processes are oxidised by co-firing with natural gas. Applications include the chemical industry in particular, but also industrial processes such as painting facilities.
In the system design examined, the process exhaust air contains up to 9 g/Nm³ of VOCs such as methanol, ethanol and ethyl acetate. This VOC-laden air is fed to the compressor of the MGT. Due to the large amount of excess air in MGT operation, a large amount (50–65 percent) of the VOCs are introduced as mixing air after the primary combustion zone. Depending on the operating point, the VOCs introduced with the mixing air react in a secondary combustion zone (reactor) at temperatures of about 1000 degrees Celsius. The hot gases are then fed to the turbine, which drives both the compressor and an electric generator. The thermal energy contained in the turbine exhaust gases can be used to, for example, provide process heat.
Institute of Combustion Technology (VT) tasks
DLR’s main responsibilities are the development, characterisation and testing of a fuel-flexible, low-emission and reliable FLameless OXidation (FLOX®1) based combustion chamber. The relevant combustion processes were analysed and optimised in atmospheric tests using parametric studies. These serve as a basis for future operations in a pilot plant. The design of the combustion chamber was carried out in close cooperation with the company Dürr. In addition, calculations of the MGT cycle were performed at DLR.
The basis for the MGT operation with VOCs was the two-staged FLOX® combustion chamber developed by the Institute for the Turbec T100 for use with natural gas. The combustor was examined and optimised on an atmospheric combustor test rig for all relevant operating conditions with VOC-laden air. During operation, the natural gas introduced via the main burner is partially replaced by the VOCs premixed with the combustion air. It could be shown that the substitution of natural gas with VOCs has a positive effect on emissions. At preheat temperatures of 650 degrees Celsius, CO emissions were reduced up to 80 percent compared to natural gas only operation. While the influence of the premixed operation on carbon emissions decreases for higher air numbers, nitrogen oxide emissions are reduced by up to 50 percent throughout the operating range. Emissions in unburned hydrocarbons (UHCs) occur below an adiabatic flame temperature of 1671 Kelvin (~1400°C). By partially bypassing the internal recuperator (see Figure 1), the preheat temperature of the combustion chamber Tpre decreases. This leads to an extended operating range when running with VOC-laden air. As shown in Figure 2, the thermal power of the VOC-laden air can be increased with decreasing preheat temperatures. The successful design and experimental characterisation of the FLOX® combustion chamber for VOC-containing exhaust air is the basis for the design of the flow path and residence time of the VOC-laden mixing air in the reactor (see Figure 1).
Funding Reference: 03ET1069B
Duration: 01.04.2012 – 30.09.2014
Project Partner and Coordinator:
Dürr Systems GmbH
1FLOX® is a registered trademark of the company WS Wärmeprozesstechnik GmbH, Renningen, Germany