Loading...
Please wait, while we are loading the content...
Similar Documents
Particle-wall Impact Experiments for Entrained-flow Coal Gasifiers
| Content Provider | Semantic Scholar |
|---|---|
| Author | Troiano, Maurizio Montagnaro, Fabio Solimene, Roberto Salatino, Piero |
| Copyright Year | 2017 |
| Abstract | The present study addresses particle–wall interaction phenomena relevant to entrained-flow coal gasifiers. The dynamics of coal, char and ash particles as they are impacted onto a flat surface in hot conditions has been characterized by means of high speed imaging and tracking. Particle–wall collisions were described in terms of normal, tangential and global restitution coefficients. The influence of carbon conversion, impact velocity and surface properties and structure of the target on the dynamical pattern of rebound has been scrutinized. The results indicate that the restitution coefficients decreased as carbon conversion increased. This feature was more pronounced at large carbon conversion, confirming the criticality of the char/slag transition to particle deposition on the wall. Introduction Gasification is one of the most efficient pathways to thermochemical conversion of carbonaceous fuels, as it includes energy conversion and/or production of chemicals. In entrained-flow gasifiers (EFG), fine particles react with gaseous oxidants within a short residence time (in the order of a few seconds). High operating temperatures are used to ensure the destruction of tars and oils and to warrant high carbon conversion [1]. The ash behaviour plays a key role in the performance of entrained-flow gasifiers. Above the softening point, ash becomes sticky and agglomerates causing blockage of the beds or fouling of the heat exchange equipment. Once above the slagging temperature (about 1300–1500°C), ash has a fully liquid behaviour: hence, it is easily drained from the bottom of the gasifier as coarse and fine slag. Both coarse and fine slag may have a relatively large content of unburned carbon, which describes an incomplete gasification of the coal, and critically affects the gasification efficiency in EF processes. The slag layer results in a molten protective coating and reduces wear and heat loss at the wall. However, uncontrolled build-up of the slag layer can bring about plugging and excessive slag deposition on the membrane walls reduces the overall heattransfer coefficient. The rate of ash deposition under inertial conditions depends on 40 Meeting of the Italian Section of the Combustion Institute ash stickiness and properties of the wall surface. Empirical methods, such as slagging indices, ash sticking temperatures and viscosity models were proposed in the literature to determine particle sticking criteria [1]. The shortcoming of these models is that they do not consider the effect of residual carbon on stickiness of particle and target wall. The performance of slagging EFG may be critically affected by the fate of char/ash particles as they interact with the wall slag layer. Montagnaro and Salatino [2] proposed a phenomenological model which considers the establishment of a particle segregated phase in the near-wall region of the gasifier. This feature is beneficial to enhanced carbon conversion. Further studies confirmed the soundness of this phenomenological framework [3,4]. The recent literature has investigated and described particle–wall interactions in terms of a coefficient of restitution (the ratio between the rebound and the impact velocities) [5,6]. It is an important parameter in the multiphase flow modelling of the gasification chamber, as it critically affects the boundary condition for particlewall collisions. Recently Troiano et al. [7] investigated, by impact experiments, the restitution coefficient and the capture efficiency varying particle stickiness, surface wall properties and impact angle and velocity under nearly ambient conditions. The aim of the present study is to characterize the dynamics of coal particle rebound in terms of coefficient of restitution during non-orthogonal particle–wall impact experiments carried out in hot conditions with coal batches pre-gasified to different degrees of carbon conversion. 2. Methodology The characterization of the rebound behaviour of particles in EF slagging gasifiers is not trivial. As a matter of fact, collisions of non-spherical particles occur onto a refractory material or a slag layer. Furthermore, the particle stickiness deeply affects the interaction behaviour. In this study impact tests were carried out to evaluate the restitution coefficient of coal particles after colliding with a planar surface. The particle–wall collisions are characterized in terms of a restitution coefficient ε, defined as the ratio between the rebound and the impact velocity. The restitution coefficient embodies phenomena like elasto–plastic deformation and viscoelastic behaviour of solid materials, surface contact forces and particle–wall friction. It is useful to define a normal, tangential and global coefficient of restitution, εn, εt, and εg, respectively as: εn = −vn,r vn,i ; εt = vt,r vt,i ; εg = vr vi = √ vn,r 2 |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://www.combustion-institute.it/proceedings/XXXX-ASICI/papers/40proci2017.I2.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
