Use of Improved Densification Conditions for Producing High Fuel Content Products from Biomass Processed by Torrefaction, Hydrothermal Carbonization, and Various Densification Methodologies: Final Report

Abstract

The Natural Resources Research Institute is engaged in work to develop demonstration-level production of solid biofuel densified products that can be stored outside, have high bulk densities for ease of logistical transport, have good handling characteristics that minimize dust generation, possess grindability that is like coal used in power plants, and have fuel contents that match or exceed sub-bituminous coal levels. During the work, two pretreatment technologies have been investigated for concentrating the energy content of raw biomass. These include: torrefaction using an indirectly fired rotary kiln process at the demonstration level and hydrothermal carbonization at the bench and pilot scale. The Institute has also collaborated with Syngas technologies on a pilot-scale moving bed, directly heated steam-based process at the pilot scale and next year will install this technology at the demonstration scale. A key factor in showing the full technical feasibility of using the pretreated materials is to demonstrate that the produced particulate fuel products can be densified to a level that allows good logistical and handling practices to be routinely attained. It has been found that hydrothermally carbonized processed materials can be agglomerated using a variety of densification devices including pelleting and briquetting in a repeatable and practical manner using commercial densification equipment with and without the use of binders. However, torrefied materials have proven to be much more difficult to densify using a variety of densification equipment, especially as the degree of torrefaction increases. Uniformly torrefied materials at high energy level appear to be especially difficult to densify but have the attributes of high fuel value and good grindability, with very little residual fiber content compared to less-torrefied material or steam-exploded biomass. Therefore, the work undertaken and explained in the following discussion has been conducted and shows that highly torrefied materials can be satisfactorily densified to produce high-energy-content products that have good physical properties, possess acceptable moisture resistance, low ash, sulfur and mercury content, and have bulk densities that can lead to improved logistics. The densification practices involve optimizing overall process conditions on an integrated systems basis and include moisture level, densification pressure, mix preparation pressure, and the use of appropriate binders when required. The densification system that seems to show the greatest promise for the highly torrefied materials is briquetting. Work will continue in examining other densification options and in improving the conditions used and discussed in this report.