Thermochemical conversion of biomass – single particle and packed bed: Experimental and Numerical Studies

Abstract

The current work uses experimental and numerical techniques to analyze the thermochemical conversion of a single biomass particle exposed to various reactive environments – varying temperatures and O2, CO2 and H2O concentrations. The single-particle analysis is extended to packed beds, wherein a biomass bed is subjected to controlled reactive environments as occurring in practical gasification units: the evolution of temperature, conversion and gas composition through the bed is studied. The work ultimately results in a multi-scale packed bed numerical model, resolved to the individual particle within the bed and further resolved to pore-scale diffusion within the particle. The model provided with fuel properties and oxidizer conditions computes the product gas composition, gas yield and conversion rate.

Identifying char combustion as the limiting step in biomass combustion and recognizing that char combustion is a diffusion-limited process (𝑡𝑐~𝑑0𝛽; 𝛽=1.97), the oxygen concentration in the feed is increased to curtail the diffusion limitation. Very interestingly, it is observed that at higher oxygen concentrations (> 40 %) and higher temperatures (>673 K), β surpasses the theoretical threshold of 2 and goes as high as 2.37. In parallel, it is observed that under cases wherein β>2, a luminous film envelope the particle. The higher flux of CO from the particle at high temperatures and oxygen concentration resists the diffusion of reactant to the particle surface, curtailing the conversion process, increasing the conversion time and thereby β>2. An analytical solution is formulated from the first principles, and it is noted that the conversion time, in addition to being a function of d0, also depends on the film diameter through the factor (1-(2/3)(d0/df)).

Gasification is also studied at a particle level by exposing a single biomass particle to CO2 and H2O environments. The threshold temperature beyond which practically significant reactions

occur is 737 0C and 850 0C for Char-H2O and Char-CO2. An increase in temperature and reactant concentration is found to enhance the gasification rate, albeit depending on the underlying conversion regime. The reactant flux is found to improve the conversion rate by increasing the transport of reactant to the surface. However, beyond a threshold flux, the conversion rate is controlled by an intra-particle gradient and is insensitive to flux. An increase in particle size leads to higher carbon mass and longer conversion time. The porosity and internal surface area decrease with an increase in density; hence, the gasification rate decreases.

The current work notes that the entire range of the conversion regime (from kinetic limit to diffusion limit) can be spanned by controlling the temperature and reactant concentration, irrespective of particle size or reaction. For the first time, the current work presents a conversion regime map, a contour plot to identify the nature of the conversion regime by readily measurable and controllable parameters – temperature and reactant concentration.

In packed bed gasification of char by steam, an intriguing observation pertains to conversion inhibition by H2. The H2 generated upstream of the bed inhibits the conversion process downstream. Char surface analysis indicates that along the bed length, the C-O and O-H bonds decrease, and the C-H bonds increase, confirming the hypothesis. To further characterize the system, H2 of the known fraction was introduced in the feed gas. It was found that the steam-char reaction is completely inhibited beyond a threshold concentration of H2 in the feed gas. For a given carbon surface with a finite number of active sites, H2 quickly saturates and bonds with the char surface curtailing the C+H2O reaction and, thereby, the conversion rate. The experimental findings and numerical analysis are used to arrive at threshold H2 concentration and bed height beyond which inhibition occurs for a given temperature, reactant concentration and residence time.

Having validated the numerical model over a range of experimental conditions discussed above, the model is used to compute the product gas composition and reaction propagation rate for packed bed gasification of char and biomass in air and oxy-steam mixtures. The model estimates agree well with in-house experimental results and experimental data from the literature.