The limited supply of fossil fuels, growing concerns over climate change and rapidly rising energy costs – each of these has intensified the search for alternative materials used to run power plants. In particular, using energy from solid biomass as a fuel presents a major alternative to burning fossil fuels such as gas, oil and coal. While the conversion technology for burning wood is more or less perfected, the applications of other biomass such as rice straw and its use in thermal power plants have not been explored to the same extent.
To be able to use solid biomass as a fuel, it is important to understand its chemical and physical properties. As well as having a significant impact on the type of combustion, these properties also greatly determine how the fuel has to be transported, how systems need to be run, how the exhaust gas has to be cleaned downstream and how ashes can be recycled. Biomass fuels are generally categorized as “delicate” fuels, as experience with the level of energy conversion, ignition, burnout, the risk of slagging, and corrosion has highlighted their differences to fossil fuels. More than anything else, it is slagging and corrosion that impact a machine’s availability, ultimately making single and multiunit incineration systems for biomass less cost-efficient.
Enter a new thermal process, derived from the calcination that occurs during the production of cement and designed along the lines of the flight-stream / cyclone process; this technology poses a genuine alternative to previous industrial methods of burning straw. In the flight-stream/cyclone process, conditioned rice straw which is capable to fly is oxidized in a combustion chamber consisting of a riser and a cyclone. A steam power process then uses the energy released into the flue gas to generate electricity. According to this principle a large-scale plant with a firing thermal capacity of 50 MWth is scheduled to be realised for the first time in Italy.
To obtain basic information about the behaviour of the combustible rice straw in a large-scale plant and to be able to estimate whether the flight-stream/cyclone process is generally suitable for the oxidation of rice straw, the experts of Steinbeis - Transfer Centre for Process, Energy and Environmental Engineering in Heilbronn designed and built a pilot plant to run incineration experiments. The aim: to shed light on the process and to disclose problems that need to be solved.
Since this procedure involves several state of- the-art components – such as steam turbine and waste heat boiler – the pilot plant was designed to build and investigate only the “unknown” part of the flight-stream/ cyclone process. In other words: producing hot gas, preparing and feeding in rice straw, the combustion chamber, the cyclone, the ash discharging and controlling the process.
Multiple series of tests yielded important information about the behaviour of the combustible rice straw in the pilot plant. The tests showed that the flight-stream/cyclone process is generally suitable for the thermal treatment of rice straw. Materials achieved burnout ratings of less than five percent and CO values in the flue gas close to the emission limit of 250 ppm. The process also runs stable over longer periods. Compared to other procedures, maintaining this kind of system should involve much less time and effort because the combustion chamber contains no moving parts. This work has laid the cornerstone for the realisation of a large-scale plant.
Prof. Dr.-Ing. Ewald Pruckner
Steinbeis Transfer Center Process and Power Engineering, Environmental Technology (Heilbronn)
stz97@stw.de