Adaptive torrefaction of stem biomass in a horizontal moving bed with normalized direct measurement of quality characteristics

Abstract

Torrefaction is a method for thermally treating biomass such that its physical and chemical properties are changed to resemble low-rank coal. Torrefied biomass is brittle, homogeneous, resistant to moisture and decay, and has greater energy density than raw biomass, yet retains 90 % or more of its original energy content. These properties facilitate co-firing with coal or full replacement of coal in existing power generation systems.

This research project was initiated to investigate the torrefaction process and develop more advanced methods for the process, control, and measurement in order to improve its economic viability and expand its use as a precise chemical process. This project began with the design and assembly of a pilot plant, based on a reactor design that adhered to the constraints of the torrefaction process. In conjunction with process development, new methods were examined and validated for assessing torrefaction severity using change in solid carbon concentration, as well as using near-infrared diffuse spectroscopy to directly measure differences in char torrefaction severity.

A gas-solid contactor was designed that combined a screw conveyor reactor with elements of moving bed and fluidized bed systems. A pilot plant was constructed based on this “horizontal moving bed” process. This pilot plant was then evaluated and characterized using several types of stem biomass that had been converted to flowable granules using a prototype biomass segmenting unit developed for the project.

Characterization experiments with the pilot plant were conducted using coppiced willow, wheat straw, and other feedstocks. The results illustrated that the horizontal moving bed pilot plant could achieve greater severity of torrefaction and shorter residence time as compared to similar pilot plants, was more flexible in terms of feedstock, and had reliable and repeatable control of temperature and residence time.

An investigation into how carbon content relates to torrefaction severity compared more than 100 torrefaction experiments including in-house experiments and results from a literature review. The result was a polynomial correlation relating the torrefaction mass yield (Ym) to change in carbon concentration (∆C), or; (Ym = 4.29∆C2 - 3.66∆C + 0.98). This correlation fits larger-scale torrefaction experimental values with a coefficient of determination of 0.935. Using the full set of the same data, a linear correlation was developed relating the loss in mass of carbon to the total mass loss for torrefaction experiments; this correlation illustrates that after the first 3.4 % of mass loss, carbon is consistently lost at a rate of 37 % of the total mass loss.

Wheat straw was torrefied in 15 batch and continuous experiments and was then subject to elemental and diffuse reflectance analysis. A linear correlation was developed that related the average change in absorbance (∆ABS) in the short-wave infrared band from 960 nm to 1060 nm to the change in carbon concentration (∆C) between raw and torrefied wheat straw. The ∆C = 0.231∆ABS - 0.0036 fit the experimental values with a coefficient of determination of 0.95.

This torrefaction research project has demonstrated a very promising new process method, as well as methods for measuring and controlling chemical composition with much greater precision than was previously possible. These accomplishments as well as the potential for developing these technologies further are a significant contribution to the field of torrefaction research and development.