Organic Waste Torrefaction – A Review: Reactor Systems, and the Biochar Properties

Torrefaction is a thermochemical process in a narrow temperature ranging from 200 to 300°C, where primarily hemicellulose fibers are depolymerized. This process is carried out under atmospheric pressure and in anaerobic conditions; heating ratio is low (<50°C/min) and the residence time is relatively long, up to 1 h. During the process, a biomass is partially decomposed and forms different condensing and noncondensing gases. The final product is a constant substance rich in carbon, which is called a torrefied biomass—biochar and biocarbon. Currently an increase in energy demand is impacting the environment considerably. For this reason, in this chapter the organic waste torrefaction technology will be presented, including the reactor systems review. Torrefaction process may be conducted in different types of reactors, with diverse technologies. From this variety, two main groups of reactors can be distinguished, with direct and indirect heating. Direct heating group consists of reactors with multiple design, such as Multiple Hearth Furnace, microwave reactor, moving bed, vibrating belt, the reactor belt, and auger. Indirect heating reactors are less common and this group consists of rotating drum and auger reactor. All mentioned reactor types will be presented and discussed

Torrefaction of Sewage Sludge: Kinetics and Fuel Properties of Biochars

We propose a ‘Waste to Carbon’ thermal transformation of sewage sludge (SS) via torrefaction to a valuable product (fuel) with a high content of carbon. One important, technological aspect to develop this concept is the determination of activation energy needed for torrefaction. Thus, this research aimed to evaluate the kinetics of SS torrefaction and determine the effects of process temperature on fuel properties of torrefied products (biochars). Torrefaction was performed using high ash content SS at six (200~300 °C) temperatures and 60 min residence (process) time. Mass loss during torrefaction ranged from 10~20%. The resulting activation energy for SS torrefaction was ~12.007 kJ·mol−1. Initial (unprocessed) SS higher heating value (HHV) was 13.5 MJ·kg−1. However, the increase of torrefaction temperature decreased HHV from 13.4 to 3.8 MJ·kg−1. Elemental analysis showed a significant decrease of the H/C ratio that occurred during torrefaction, while the O/C ratio fluctuated with much smaller differences. Although the activation energy was significantly lower compared with lignocellulosic materials, low-temperature SS torrefaction technology could be explored for further SS stabilization and utilization (e.g., dewatering and hygienization)