Elsevier

Fuel

Volume 125, 1 June 2014, Pages 137-143
Fuel

Gravity discharge characteristics of biomass-coal blends in a hopper

https://doi.org/10.1016/j.fuel.2014.02.009Get rights and content

Highlights

  • The study exhibits an interesting phenomenon which is named needle particle effect.

  • The needle particle effect has been studied in detail.

  • An improved model has been proposed which agrees well with the measured results.

Abstract

Adding biomass particles into pulverized coal may significantly improve the flowability of the cohesive pulverized coal particles. Experimental study of solids discharge flow from a cylindrical hopper with no aeration revealed an interesting “needle particle effect”, which was believed to be responsible for the smoother solids discharge and significantly increased discharge rates of the coal-biomass blends within the limits of the study. The mechanisms behind the needle particle effect in a gravity discharge system, including adherence, lowering cohesion, arch-breaking and transition of flow pattern, are analyzed and verified by the experimental results. The blends behaved different when different types of biomass particles were added. The observed flowability of the blends was strongly affected by the aspect ratio and surface roughness of the biomass particles. A revised model based on Crewdson’s equation is proposed to predict the gravity discharge rate of biomass-coal blends.

Introduction

The co-gasification of biomass-coal blends is a good approach to use biomass for energy applications. Many papers [1], [2], [3], [4] relating to co-conversion of biomass-coal blends have been published, but few researches focused attention on the flowability of biomass-coal blends. The flowability of biomass-coal blends not only affects the reactor design for co-gasification and the composition of the product gas but also has a significant effect on the continuous, stable, and controllable operation of the gasifier [5], [6], [7], [8]. The work presented in this paper is a part of flow characteristics study of biomass-coal blends.

Gravity discharge rate (W) is a practical parameter in characterizing the flow behavior of granular materials. Several previous studies [9], [10], [11] have been focused on gravity discharge flows in flat-bottomed hoppers and non-cohesive or spherical particles. A widely accepted correlation by Beverloo et al. [9] proposed a power-law dependence of the gravity discharge rate W on the discharge outlet diameter D02.5. At the same time, Brown and Richard [12], [13] proposed the concept of “free-fall arch” which provide the theoretical basis for investigating the granular discharge. Actually, it is interface near the orifice that balances the lower boundary of granular layer. However, according to Crewdson [14], most of the above-mentioned empirical and theoretical correlations may generate large error when applied to fine materials (d < 500 μm) due to the effects of interstitial pressure gradients. Afterwards, a series of studies on gravity discharge of fine particles were investigated by Nedderman et al. [15], [16], [17] who take into account the effect of interstitial pressure gradient on particles gravity discharge. According to them, pressure gradient is a function of particle size instead of orifice diameter.

Adding glidants or secondary particles to a granular base material is very common in industrial processes in order to meet certain process requirements, such as flowability. In co-gasification, the large difference between biomass and coal in physical properties makes it more difficult to characterize the flow behavior of feed blends. In order to make better use of biomass, some researchers [18], [19], [20] have investigated the biomass physical properties. Other studies [21], [22], [23] indicated that the shape and morphology of biomass particles have significant influence on the flowability of the feed. However, the relevant literature on the flow properties of biomass-coal blends is particularly scarce. The flowability of binary granular systems remains a scientific challenge due to the complex forces exerted on individual particles.

Motivated by the researches cited above, we carried out a further study on the influence of biomass on gravity discharge rate for biomass-coal blends. The objectives of the present study are to understand the mechanism behind the “needle particle effect” that improves the flowability of biomass-coal blends, and to create a quantitative model to predict the gravity discharge rate. Furthermore, we expect that this article can provide insights into the underlying phenomena responsible for some problems encountered in handling and processing of biomass-coal blends, which may be applicable to other industrial processes.

Section snippets

Materials

Shenfu bituminous coal and two representative types of biomass, i.e. rice straw and sawdust, were used in this study, their physical properties listed in Table 1. Fig. 1(a) shows the cumulative particle size distribution functions of shenfu bituminous coal, which was measured by a Malvern Mastersizer 2000 particle size analyzer. The cumulative particle size distribution functions of rice straw and sawdust, measured with a BT-2900 particle image analyzer, are illustrated in Fig. 1(b). Since the

Experimental phenomena

Adding glidants is an effective way to modify the behavior of granular materials when subject to aeration and vibration. Glidants fill into the surface crevices of particles or lacuna between particles, often leading to enhance rolling friction and reduce the adhesion forces among particles [26], [27]. Aeration and vibration [28], [29], [30] can loosen the powder structure and lessen the probability of arching near the hopper outlet.

Conventional wisdom would expect that the presence of

Needle particle effect

The addition of biomass particles play a positive role on the flowability of biomass-coal blends when mass fraction of biomass particles (w) is less than 25% for cohesive coal particles. The effect of biomass particles on the discharge rate is demonstrated explicitly in Fig. 3. It shows that the maximum discharge rates with biomass addition were three times greater than without. Owing to biomass substance construction and anisotropic in spatial structure, biomass particles have irregular shape

Conclusions

The experiment is focused on the gravity discharge rate of dry biomass-coal blends from a cylindrical hopper with no aeration. The following conclusions have been developed as follows:

  • (1)

    The experimental results show that needle shape biomass particles can improve the flowability of cohesive pulverized coal in the appropriate mass fraction which is named “needle particle effect”. The promotion mechanisms of needle particle effect mainly includes adherence, lowering cohesion, arch-breaking and

Acknowledgements

The authors are grateful for the National Key Technologies R&D Program (2012BAA09B02), Program for New Century Excellent Talents in University (NCET-12-0854), the National Natural Science Foundation of China (21306050) and the Fundamental Research Funds for the Central Universities.

References (42)

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