Decarbonisation Technology - February 2022 Issue

everywhere – electrical glass furnaces, EAFs for steel making, electrical boilers, electrolysis (not just of water but also CO 2 ), electric calciners, micro- waves technology. Very high temperatures can be reached with concentrated solar power, while for regions with too low solar irradiance, plasma torches are being developed. So clearly the focus should be on decarbonised possible. The electricity that can be generated will only cover a portion of the total electric power consumption of those plants, even in plants such as cement, lime, steel, and glass, where large flows of high temperature flue gases end up in the atmosphere, (for a cement plant this is usually between 10 and 30%, depending on various parameters, such as the moisture content in the raw materials and fuels). However, this is 24/7 fully decarbonised electricity generation, which is also becoming ever more important in terms of resiliency. 1 MWe of WHR installed capacity is equivalent to 5 MWe of either installed PV or wind generation, as those have capacity factors around 20%, while WHR is close to 95% (it does not produce electricity when the plant is down but then it does not need it!). electricity, and waste heat from industrial processes should be harvested wherever An added benefit of a WHR/ORC installation is that it does not have to go through the electricity generation step. The turbine, fed by the working fluid, can directly drive a large fan, mill, or compressor, hence increasing the overall efficiency of the conversion. Furthermore, by not generating electricity, such projects avoid interaction with the local utility and its numerous regulations, which is always a cumbersome undertaking. Waste heat recovery systems WHR systems are generating a lot of interest; for instance, in the US they now qualify for tax credit under the Consolidated Appropriations Act, 2021. Heat recovery is also mentioned by some trade organisations as one of the pathways towards decarbonisation; for instance, the California Nevada Cement Association (CNCA): “California’s ‘stone, clay, glass, and cement’ industrial sector has 204 MW of untapped cogeneration potential alone.” There are two parts in a WHR plant, the actual boilers/heat exchangers that transfer the heat –

directly or indirectly – to the working fluid, either cyclopentane or water/steam. From that point on, a ‘traditional’ power plant with a turbine, a generator, and a dry cooling system complete the Rankine Cycle. WHR boilers are very well known, and in the power industry they are installed downstream from a gas-fired turbine to recover the energy from the turbine exhaust gas to run a steam turbine under a so-called combined cycle (CC) arrangement, with modern CC reaching 63% efficiency. They are called heat recovery steam generators (HRSGs). The big difference is in the flue gas quality, which for heavy industries varies in terms of temperature, composition, and dust content. Each recovery boiler must be specifically designed to match the process conditions and extract the maximum energy from the flue gas while avoiding corrosion, plugging, and fouling. Only a few companies master the full process, so experience matters the most. The following example is from a cement plant in Turkey, a recent installation with ORC. The WHR system was designed, built, and installed by CTP Italy, a flue gas treatment specialist with experience in energy-intensive industries. The WHR boilers consist of dust-removing chambers, followed by a horizontally arranged set of tubular heat exchangers, through which thermal oils circulate. The hot thermal oils from both boilers (one located at the preheater exhaust, the other Aerial view of the Somnez cement plant in Turkey with two waste heat boilers