Article pubs.acs.org/EF
Comprehensive Analysis of the Coal Particle in Molten Blast Furnace Slag To Recover Waste Heat Wenjun Duan,*,† Qingbo Yu,† and Zhimei Wang‡ †
Northeastern University, Post Office Box 345, No. 11, Lane 3, Wenhua Road, He Ping District, Shenyang, Liaoning 110819, People’s Republic of China ‡ Shenyang Metrology Testing Institution, No. 43, Shi Ji Road, Hun Nan District, Shenyang, Liaoning 110179, People’s Republic of China ABSTRACT: In this paper, molten blast furnace slag was used as a heat carrier to make the coal gasification reaction run smoothly. The characteristics of coal particle motion, heat transfer, and gasification reaction in molten slag were investigated in detail. It could be divided into four forms of coal particles in molten slag, and the coal particle could move across the bubble when the velocity and diameter of coal particle were 4.20 m s−1 and 75 μm, respectively. Meanwhile, with the coal particle motion, a large amount of waste heat of molten slag transferred it to ensure the process of the gasification reaction. The slag could enhance the heating rate of the coal particle, promote the gasification reaction, and increase the carbon conversion rate. Molten slag was also beneficial to the syngas production, which improved the fraction of CO and enhanced the combustible gas component in the process of coal/CO2 gasification. Moreover, the problem of sulfide emission in the coal gasification process was solved effectively using molten blast furnace slag as the heat carrier. The total sulfide emission decreased dramatically from 0.21 to 0.04 mol kgcoal−1. Overall, not only was waste heat of molten slag recovered by this method, but also the quality of coal gasification and reduced hazardous gas emissions improved.
1. INTRODUCTION The iron and steel manufacturing industry was developed rapidly along with the development of the social economy and science and technology in China. The average annual growth rate of iron and crude steel was about 12.5% from 1998 to 2014.1 In 2015, the production of iron and crude steel reached 6.91 × 106 and 8.04 × 106 tons, respectively.2 Along with the speedy development of the iron and steel industry, a large amount of energy was consumed, which accounted for approximately 17% of the total energy consumption of China.3 However, under the dual pressure on energy and the environment, it was essential to reduce the energy consumption of the integrated iron and steel industry to achieve the goals of energy savings and emission reduction. As a main kind of byproduct, blast furnace slag (BFS) was exhausted at an extreme temperature of 1773−1873 K, and thus, the energy of about 1700 MJ tonslag−1 corresponded to 13.0% of consumption in the blast furnace process carried out by the slag.4 Therefore, reasonable development and utilization of waste heat of BFS would help to realize energy savings of the iron and steel industry. In the recent years, numerous advanced chemical methods had been exploited to recover the high-quality waste heat of BFS.5−20 Maruoka et al.5 studied the possibility of using hot slag waste heat as a heat source to conduct a methane−steam reforming reaction. The exergy loss of the proposed system was only 15%. Purwanto and Akiyama6 established a new hydrogen production system by decomposition of CH4−CO2 over hot BFS in a packed bed at the temperature range from 973 to 1273 K. The slag acted as not only thermal media but also a good catalyst, and the largest methane conversion reached about 96%. The possibility of combustible gas production from © XXXX American Chemical Society
municipal solid waste using solid BFS was studied by Zhao et al.7 In the experiments, BFS acted as the catalyst and the heat carrier, which promoted the reactivity of the municipal solid waste. The volume fraction of CO, H2, and CH4 at different atmospheres increased in the order of N2 < air < steam. Luo et al.8 designed a continuous moving bed biomass gasification reactor to generate hydrogen-rich gas by BFS waste heat from 1073 to 1473 K. The results showed that BFS performed a catalytic action in the process of tar cracking, char gasification, and hydrocarbon reforming. The H2 content of the syngas reached 46.54% when the temperature of slag was 1473 K and its size was less than 2 mm. Series experiments were conducted on this system to use the slag waste heat.9−11 The integrated system of sludge gasification using slag waste heat was explored with the aim of syngas production, waste heat recovery, and sewage sludge disposal by Sun et al.12−14 The slag could act as a good heat carrier for sludge gasification and enhance the production of CO and H2 when the slag temperature was higher than 773 K. The results approved a potential way of reasonable disposal of sewage sludge and efficient recovery of BFS waste heat. In the whole temperature range of BFS, Li et al.15,16 conducted coal gasification with molten BFS at atmospheric pressure in a molten bath reactor. The results showed that the method of using a coal gasification reaction to recover molten slag waste heat was possible and effective. The kinetic models of coal gasification with/without molten BFS were established to provide the theory guide in the application.17 Some studies about thermodynamic analysis Received: June 6, 2017 Revised: July 12, 2017 Published: July 17, 2017 A
DOI: 10.1021/acs.energyfuels.7b01610 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
agent (CO2). The velocity of flow decreased dramatically when it left from the nozzle mouth. However, the velocity of the coal particle was still higher than that of the gasification agent. Therefore, it was necessary to obtain the motion characteristic of the coal particle in molten BFS to recover the high-quality waste heat. On the basis of the results of Guo et al.,21 the velocity versus time of the coal particle was expressed as follows:
and experiments using molten BFS as the heat carrier in the gasification process were also conducted by Duan et al.18−20 On the basis of the above studies, it was an effective way to use BFS waste heat using a chemical method. Therefore, to master the process of gasification and heat recovery, conducting a comprehensive analysis of coal gasification in molten BFS to recover waste heat was necessary. In the present study, the existing form of coal particles in molten slag was analyzed and a model for estimating the motion characteristic of coal particle was established to judge the coal particle in bubbles or slag. The heat transfer of the coal gasification in molten slag was also investigated by theoretical analysis. Ultimately, the investigation of the coal/CO2 gasification characteristic, including the reaction rate, syngas production, and sulfide emission, was conducted.
g (ρc + ρg ) duc 3u = − φCs c dt ρc 4dc
(1) −1
where uc was the coal particle velocity (m s ), ρc and ρg were the densities of the coal particle and bubble, respectively (kg m−3), Cs was the friction coefficient, φ was the surface roughness of the coal particle (φ = 1), and dc was the coal diameter (m). The velocity versus motion distance of the coal particle was expressed as follows:
2. MODEL In this work, a model of the coal particle in molten BFS was proposed. Coal particles mainly existed in four forms in molten slag, as shown in Figure 1a. First, the coal particle was wrapped completely in the
uc =
dy dt
(2)
where y was the motion distance (m). The critical velocity of the coal particle through the bubble into molten slag could be expressed as follows: ⎛ 12σ cos θ ⎞1/2 ⎟⎟ uc* = ⎜⎜ s ⎝ ρc dc ⎠
(3)
where uc* was the critical velocity of the coal particle through the bubble into molten slag (m s−1), σs was the surface tension of molten slag (N m−1), and θ was the wetting angle between the coal particle and molten slag (deg). The coal particle would break through the bondage of the bubble and enter into molten slag when the velocity of the coal particle was higher than the critical velocity at the surface of the bubble. On the basis of the previous experiments20,22 and numerical simulation results, the initial velocity of the coal particle and gasification agent was 4.20 m s−1 when they started into molten BFS. The relationship between the coal particle velocity and critical velocity at different motion distances could be seen from Figure 2. The coal particle could move across the bubble (motion distance of
