A Study on the Hydrodynamics of a Bench-Scale Top-Fed Bubbling Fluidized Bed Gasifier using Biomass and Coal as Feedstocks

Abstract

The production of synthetic gas (syngas) from renewable or carbon-neutral sources can significantly reduce greenhouse and other emissions associated with conventional fuels. One of the most promising technologies to efficiently convert carbonaceous feedstocks such as biomass, coal, or municipal waste into syngas for transportation, power, heat, electricity generation, and or production of added-value chemicals is the bubbling fluidized-bed gasifier (BFBG). However, the gasification process inside a BFBG is a very complex high-temperature multiphase flow phenomena still not well understood, particularly when binary mixtures are investigated. As a result, despite the numerous correlations in the literature developed to predict the hydrodynamics inside a BFBG, the results are inconsistent, particularly for the minimum fluidization velocity, Umf.As predicting the fluidization hydrodynamics is paramount for optimum gasification, this investigation observed the effect of some of the most important fluidization parameters such as the particle size and shape, fluidizing gas properties, moisture content, bed aspect ratio etc. This study designed and built two separate experimental platforms: a bench-scale BFBG with automated feeding and a cold flow model with the same geometry and dimensions as the BFBG. The experiments used well-characterized (i.e., known size and shape distribution, density, moisture content, initial mixing condition) inert material (sand, glass beads) and feedstock (biomass (sawdust) and coal). The cold flow investigation results showed that the initial mixing conditions for binary mixtures with biomass had a significant effect on the measured Umf. For example, the relative error in predicting Umf using the available correlation in the literature increased for segregated mixtures. Moreover, lower relative errors in Umf suggested that the fluidization quality was better if the mixture was initially well-mixed (premixed). In addition, a larger biomass moisture content decreased Umf of premixed binary mixtures but increased the relative error between the predicted and the experimental Reynolds number, Re. After reaching the minimum fluidization condition, the fluidization behavior and mixing at various flow rates were also recorded with a high-speed camera. The processed images were used to determine the interval for the fluidizing-gas superficial velocity that produced the best mixing for a particular mixture composition and initial conditions. The images showed that while segregated biomass mixtures did not mix if the bed aspect ratio was larger than five, coal mixtures did mix homogeneously along the reactor bed. Finally, experiments performed at temperatures up to 800 C showed a large increase in the bed pressure drop at minimum fluidization velocity with the bed temperature due to the large effect on the fluidizing gas density and viscosity. On the contrary, Umf decreased when the process temperature increased. Finally, preliminary biomass and coal gasification experiments in the BFBG setup produced...

Fluidization Behavior of Binary Mixtures of Coal in a Top-Fed Bubbling Fluidized Bed Gasifier

“…Tests were conducted at the same time interval for the 200 and 300-g unary sand mixtures. The study conducted by Sivri [3] contains a detailed description and information about the design of the test stands.…”

“…There are many parameters affecting the fluidization hydrodynamics inside the BFBG. Particle characteristics such as size, sphericity, density [2,3], bed aspect ratio (the ratio of the bed height to the bed diameter) [3,4], fluidizing gas and feedstock moisture content [5], temperature, and feeding location have strong effects on fluidization hydrodynamics. Besides, fluidizing gas velocity is a crucial parameter that mainly regulates and determines the fluidizing regime according to the particle characteristics and other factors.…”

“…Larger and denser particles tend to mix slower than smaller and lighter particles. Si and Guo [3] studied the fluidization behavior of binary mixtures of quartz sand with sawdust or wheat stalk in an acoustic bubbling fluidized bed. They found that the addition of sand improved the fluidization quality of the biomass particles.…”