Multi-Stage Control of Waste Heat Recovery from High Temperature Slags Based on Time Temperature Transformation Curves

Abstract

This paper presents a significant method and a basic idea of waste heat recovery from high temperature slags based on Time Temperature Transformation (TTT) curves. Three samples with a fixed CaO/SiO 2 ratio of 1.05 and different levels of Al 2 O 3 were designed and isothermal experiments were performed using a Single Hot Thermocouple Technique (SHTT). The TTT curves established through SHTT experiments described well the variation of slag properties during isothermal processes. In this study, we propose a multi-stage control method for waste heat recovery from high temperature slags, in which the whole temperature range from 1500 °C to 25 °C was divided into three regions, i.e., Liquid region, Crystallization region and Solid region, based on the TTT curves. Accordingly, we put forward an industrial prototype plant for the purpose of waste heat recovery and the potential of waste heat recovery was then calculated. The multi-stage control method provided not only a significant prototype, but also a basic idea to simultaneously extract high quality waste heat and obtain glassy phases on high temperature slags, which may fill the gap between slag properties and practical waste heat recovery processes.

Integrated carbon dioxide/sludge gasification using waste heat from hot slags: Syngas production and sulfur dioxide fixation

“…Additionally, the heat capacity of hot slags was $1.15 kJ kg À1 K À1 (Barati et al, 2011;Sun et al, 2014b), and consequently the sensible heat of hot slags was 0.23 MJ/kg slag (denoted as QD T ) at 1000-800°C, which supplied the heat for sludge gasification (Q ext ). Accordingly the mass ratio of sludge to slags (R sludge/slag ) could be derived as follows: Therefore the calculated optimum mass ratio of sludge/slag was 1:9 for gasification in the temperature range of 800-1000°C.…”

“…To meet this challenge, we have analyzed the cooling path of BFS (Sun et al, 2014a) and proposed a multi-stage control method (Sun et al, 2014b), based on which a chemical method at temperatures lower than 950°C was theoretically reasonable. Amongst the chemical methods, coal gasification and methane reforming have been extensively investigated recently.…”

Heat Recovery from High Temperature Slags: A Review of Chemical Methods

“…The liquid slags should be first granulated into small droplets and rapidly quenched to glassy phases and then the heat transfer of chemical reactions is conducted. Indeed, a two-step heat recovery process has been forecasted by Barati et al [8] based on operational processes; moreover, based on the inherent crystallization properties of slags [28,32], the heat recovery from slags should be divided into at least three stages, as depicted in Figure 6. Therefore the foregoing studies pose the main R&D trend, i.e., combination of mechanical granulation and chemical reactions.…”

“…From high temperature to low temperature, the thermal conductivity greatly changes [7,8,31], and as a result, the difficulties of heat recovery vary, which makes up the first discontinuity. Furthermore, if the liquid slag follows a cooling path with a small cooling rate, crystals could form inside the slag and therefore the physical properties of the slag including the thermal conductivity and viscosity would change greatly; accordingly the cooling path of the slag could be divided into at least three regions, i.e., liquid region, crystallization region and solid region [32], which made up a great discontinuity of the heat recovery from slags. Besides, the molten slags are produced from steel factories intermittently [20] and if physical methods are used to generate electricity, the obtained electricity is not continuous, which will impact the power grid and accounts for another discontinuity of heat extraction, so the transformation from intermittently emitted heat of slags to a stable heat source is a significant challenge for heat recovery from molten slags, from the viewpoint of which, a chemical method is reasonable.…”

“…As the gasification temperature was 1473 K, the gas yield and H2 content achieved could be up to 1.28 N· m 3 /kg and 46.54%, respectively. However, from the viewpoint of material recycling of slags, the gasification should be less than 1173 K [32], and therefore, recently Sun et al proposed an integrated method to use the waste heat of BFS, i.e., low temperature biomass gasification (523-773 K) [66]. They found that the kinetic mechanism of biomass gasification changed from an Avrami-Erofeev model to a three-dimensional diffusion model because of the addition of slag.…”