Alternative fuels; CO2 neutral fuels
Some waste fuels are mixtures of materials of either fossil or biologic nature, ie the fuels are partly CO2 neutral. This is typical of Refuse Derived Fuels (RDF), also called Solid Recovered Fuels (SRF), which are based on Municipal Solid Waste (MSW). Solid Hazardous Wastes (SHW) are typically mixed with saw dust to improve flow, storage and conveying properties; hence SHW is another example of a partly CO2 neutral fuel. Other alternative fuels are of completely fossil (e.g. plastic) or biogenic nature (e.g. animal meal or dried sewage sludge).
Determination of biomass fraction in partly CO2 neutral fuels
Plants burning considerable amounts of partly CO2 neutral fuels need to document the biomass fraction and the fossil fraction, respectively, in these fuels. This is required by the authorities as part of the annual reporting of CO2 emissions.
TUC is carrying out research on different methods that could be applied in the industry to determine the biomass fraction. One method that is being tested is the selective dissolution method (SDM), in which the biomass fraction is solved in sulphuric acied and hydrogen peroxide. The method appears to be reliable, but is quite time and labour consuming.
Calciner technology for optimized use of alternative fuels in cement kilns
In modern cement kiln systems there are two burning zones; one in the precalciner, where most of the carbonate in the raw meal is decomposed into calcium oxide and carbon dioxide, and another one in the rotary kiln outlet. The precalciner, which operates at the lowest temperature (typically 850-1200 °C), offers the greatest potential of using alternative fuels.
Different precalciner designs are available from different suppliers; some of them are more suitable for burning large amounts of alternative fuels.
Residence time, temperature distributions, oxygen availability, turbulence levels and velocity distributions are key parameters in determining the characteristics of a good precalciner design.
Increased use of lumpy alternative fuels may have a negative impact on the internal cycles of sulphur, alkalis and chlorine in the kiln system. By good precalciner designs, such negative impacts can be avoided or considerably reduced.
NOx emission reduction by alternative fuels in the cement industry
Alternative fuels typically contain less fuel nitrogen than coal. Hence, fuel-NOx emissions may be reduced by switching from coal to RDF or other waste-based fuels.
Moreover, if the alternative fuel can be fed into the process in a fuel staging regime, then additional NOx reduction by reburning can be expected. This has been tested in full-scale processes, and up to 40 % NOx reduction has been demonstrated.
Waste fuels containing nitrogen in amine form may in some cases be used for NOx reduction through selective non-catalytic reduction (SNCR). The potential of using amine reclaimer wastes (ARW) from amine-based CO2 capture processes has been demonstrated.
Impact of alternative fuels on emissions to air
Increased use of alternative fuels may have positive as well as negative effects on the emissions of gas pollutants. Typically, CO2 and NOx emissions are reduced, whereas HCl and VOC emissions may increase. The latter can however be avoided or significantly reduced by proper kiln system modifications.
Some alternative fuels (hazardous wastes) may contain Mercury (Hg). As this element is very volatile it will basically escape from a kiln system via the stack, and increased emissions of mercury will be the result. However, the behaviour of mercury in the kiln system is quite complex, being a part of internal as well as external cycles, and understanding this behaviour is crucial if mercury abatement is on the agenda.
Optimized use of CO2 neutral fuels in rotary kiln burners in cement kilns
Currently there is an ongoing PhD project in cooperation with the Norwegian cement manufacturer Norcem AS (part of Heidelberg Cement Group). The main goal of the PhD project is to optimize the utilization of CO2 neutral fuels. Factors impacting (limiting) the use of CO2 neutral fuels will be investigated. Different approaches are applied:
- Testing of different fuels and fuel mixtures in full-scale trials
- Energy balances of the main burner by use of models implemented in Matlab or Excel
- Investigation of flow and combustion behaviour of different types of fuel in the main burner by applying computional fluid dynamics (CFD) simulations (using Fluent)
- Documentation of CO2 neutrality by laboratory testing
The research work is headed by Associate Professor Lars-André Tokheim who has practical experience with the use of alternative fuels in the cement industry (PhD on the impact of staged combustion by alternative fuels in cement kiln systems 1994-1998; kiln engineer at Norcem 1998-2001; head of process development and environmental affairs at Norcem 2001-2006).
Hiromi Ariyaratne is a PhD student at Telemark University College. The title of her project is "Utilization of CO2 neutral fuels in cement kilns". Full-scale trials, laboratory tests and modelling is combined in Hiromi's project.
Professor Dag Bjerketvedt is also active in the field of alternative fuels (but is focusing more on gas explosions, technical safety and hydrogen technology).
Assoc. Prof. Lars André Tokheim