Efficient radiation section
The radiation section is a compromise consisting of the thermodynamic, optimum heat transfer and low wear and equipped with highly reliable sealing material
Compared with conventional heaters, which operate with gaseous or fluid fossil fuels, biomass plants use organic solid fuels, e.g. sawdust, wood chips, old timber, or material from landscape management to produce hot flue gas. However, depending on their quality, these solid fuels also put high requirements on the plant. This refers to the combustion process, the components, e.g. heat exchanger or heater, as well as to the legal regulations to be met for emissions.
Since the end of the 90s of the twentieth century, many biomass plants have been built to support alternatives to fossil fuels. However, many plants built in the first years still have some initial difficulties. Therefore, the two- and three-pass heaters, often built at that time, have multiple problems. On the one hand, the large amount of occurring ash is the reason for the need to clean the heaters very often (up to 6 times per year). Each time, this causes the plant to be shut down. On the other hand, this ash is a solid material in the flue gas and at high rates of flow, the flue gas behaves like abrasive paper and little by little, the tube coil will be damaged. This especially applies in positions with high rates of flow, e.g. in return parts of multiple-pass heaters. Over a long term, this abrasion will be the reason for the reduction of the wall thickness of the tube coil and at one time or another there will be holes in it. In the worst case, this can cause a large fire. However, in the best case, a longer standstill and extended repair works will be required.
NESS produces biomass plants together with its strong partners in your respective special field. In this way, we can implement grate-fired, dust-fired or underfeed-fired variants. Our plants are designed in a way that, on the one hand, simple cleaning is possible and on the other hand, abrasion is minimised. Furthermore, a large percentage of the arising ash and pollution in the ash hopper are directly discharged. With this, the plants require little maintenance, are long-living and efficient. Our focus at NESS is on the thermal oil component of the plant, as well as the complete package around the thermal oil system, i.e. expansion and collecting vessel, pumping stations, heat exchangers, economizers, air-preheaters, emergency coolers, switch cabinets and the complete control.
Due to the separation of the heater in radiation- and convection section, the efficient separation of ash and particles is possible. With this, longer periods between the two maintenance intervals are possible because a large amount of the arising dirt can directly be separated, and the plant must be cleaned less frequently.
When planning and producing the radiation section, you benefit from the usual quality of the NESS heaters, the particularly balanced temperature profile and the high-performance refractory materials. For the convection section, we set great store on long lifetime. For this purpose, for instance, special protective shells are installed that protect the heat exchanger from wearing. We also put great store on best possible cleaning possibilities of the radiation sections.
Contrary to the image in the upper part, it can be reasonable also to use a three-pass heater fired with biomass. For this purpose, NESS has optimised the design, so that known problems, e.g. abrasion or pollution, are minimised. For smaller plants up to a performance range of ~4 MW, it can be economically feasible to plan with a three-pass heater, because this solution – seen from the pure investment – approx. 10-15 % cheaper than the solution with separated radiation and convection section. With this, it is however necessary to ponder some items. The better the quality of the biomass, the less ash arises and the less complicated is the cleaning. For small plants with good biomass quality this solution pays. On the contrary, when using, for instance, material from landscape management or other fuels like old timber of category A3, it will be nearly impossible to operate the plant economically for a longer term and here we clearly recommend our second solution.
Contrary to three-pass heaters, NESS radiation sections consist of only one tube coil. The reason is that the highest rates of flow occur on the return parts of the three-pass heaters and therefore, there is the highest wear. Because this often is a problem for older plants, NESS decided on the solution with hooked in convection section to be able to provide the economically optimum solution.
In this concept, optimised for less wear and easy maintenance, the NESS convection sections the second and third pass of a „classic“ three-pass heater. Here, too, it is required to decide on the compromise between good heat transfer and less abrasion that NESS has solved by meander-shaped heater batteries where flue gas flows through only once in one direction. Especially for larger plants or with unclear quality of biomass this two-part concept is recommended because it works much more reliable for a longer term.
Especially for ORC plants, we have designed a two-stage economizer, a high-temperature (HT)-Eco for the HT system of the turbine and a low-temperature (LT)-Eco for the LT-section of the turbine. This results in high efficiency of the OCR process and in high efficiency of the heater.
The NESS high-performance emergency cooler is equipped with a large cooling surface that provides for high plant safety in case the main plant fails. The cooler is equipped with an automatic filling level controller and protected from corrosion. It is integrated in the primary circuit of the thermal oil system and at low load, it can optionally be used as process cooler.
We at NESS set out not only to plan and produce new plants including the engineering but also to examine existing plants for potential improvements to enhance safety, to increase cost-effectiveness or to improve efficiency. We also arrange maintenance works that are to be done regularly. Do not hesitate to contact us.
Functionality Biomass plants:
radiation section
ash hopper
convection sections