METHOD AND INSTRUMENTATION FOR DIRECT MEASUREMENT OF CORROSIVE SPECIES FROM COMBUSTION
One of the key elements of battling climate change is the drive to reduce dependence on fossil fuels. This progression can be seen in transport, industry, and energy production sectors. One approach for the energy sector is to replace coal-based solid fuels with renewable or semi-renewable fuels such as biomass and waste (IPCC working group III, 2022). While this approach is benefi cial in terms of greatly reduced “new” CO2
emissions into
the atmosphere, it presents some challenges for the combustion plants. Biomass and waste have a lower energy density compared to coal so the fl ue gas cleaning systems need to deal with an increased amount of dust when fi ring biomass or waste with same power output. Another more diffi cult problem is the increased potential for the generation of corrosive species during combustion (Berlanga et al., 2013). Corrosive species are in vapor form in the combustion chamber but as the fl ue gas cools down the vapor molecules nucleate forming new particles or grow existing particles by condensing on them.
These newly formed particles cause fouling of the heat exchanger surfaces and boiler walls where they deposit due to the large temperature gradient between the fl ue gas and wall. This again reduces the overall effi ciency of the plant and shortens the plant operational time between maintenance operations that require shutting down the combustion unit. It is known that formation of corrosive species can be mitigated with for example reducing combustion temperature, by using additives or a small fraction of coal in the fuel(Aho et al, 2004, 2005). There are issues with each of these mitigation techniques; reduction of combustion temperature reduces the overall effi ciency of the plant while the use of additives or coal in the combustion increases the operation costs of the plant. The most benefi cial approach is to fi nd an operation point for the boiler where the combustion temperature is as high as possible and corrosive species are kept at minimum through optimized use of additives or coal co-fi ring. Thermal NOx formation must be also considered especially in plants operating without an SCR (Selective Catalytic Reduction System). This approach has been hindered in the past by the diffi culty to optimize additive usage or co-fi ring as there hasn’t been a way to assess formation of corrosive species in real-time. In this study, we will present a new aerosol/gas conditioning instrument specifi cally developed to characterize corrosive species into aerosol form.
Methods Sample conditioning
High temperature sampling probe prototypes have previously been used by the Technical Research Centre of Finland (VTT) to study corrosion in biomass and waste fi ring plants (Aho et al., 2008). Dekati Ltd. acquired this technology through a collaboration project with VTT to commercialize the probe and integrate it into an existing sample conditioning and measurement systems. This High Temperature Sampling Probe is an air-cooled dilution probe made of high temperature steel
IET SEPTEMBER 2022
°C . The tip of the probe incorporates a perforated dilution stage which drops the temperature of the sample to ~300 °C (Figure 1.). This temperature decrease is similar to what happens near the boiler walls and especially the heat exchanger surfaces. As the temperature drop is caused by the dilution process, any vapor form compounds in the sample are forced to nucleate or condense onto existing aerosols instead of condensing on the probe walls. The resulting aerosol is transferred into further dilution and subsequent measurement with instruments. High Temperature Sampling Probe also incorporates a heater to
thermophoretic losses subsequent to the dilution. Dekati® High
Temperature Sampling Probe schematic is shown in Figure 1. The High Temperature Sampling Probe is connected to the Dekati®
eDiluter™ Pro that is an automatically controlled two
stage dilution system. eDiluter™ Pro regulates dilution air fl ow into the heated probe and as it is an ejector dilution-based system it also pumps the sample from the probe and further dilutes it. As the eDiluter™ Pro automatically keeps the dilution factor constant regardless of inlet pressure, it is especially well suited to sample from turbulent fi ring zone conditions. eDiluter™ Pro operation has
to allow direct sampling from combustion zone in up to 1200 Figure 1. Dekati®
High temperature sampling probe operation principle
keep the temperature at a minimum of 200 °C to eliminate any
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