Owing to the increasing impact of traditional energy usage on global climate patterns and the environment, stakeholders are turning their attention to renewable energy sources. Biomass is a primary type of renewable energy which is expected to be an important energy source in the coming years. Of the different non-conventional energy sources available (e.g., wind, solar, tidal, and nuclear), biomass is widely available and is considered to be carbon neutral, in that the net carbon emissions resulting from the burning of biomass are zero.
Raw biomass is generally characterized by its high moisture content and volatility, and by its lower higher heating value (HHV) and energy density levels compared to fossil fuels [1
]. Biomass thus has some disadvantages when used as fuel, such as its low HHV, high moisture content, hygroscopic nature, smoke emission during combustion, its heterogeneous and uneven composition, and transport difficulties [2
]. As an alternative energy source, biomass is also associated with the controversial issue of depleting food crop harvests and possibly natural forests which can be resolved by collecting farm waste [3
]. Existing technologies to convert biomass to energy include thermochemical and biochemical processes [4
]. The thermochemical conversion process is the most common technique, which include combustion, pyrolysis, gasification, and liquefaction [2
]. The treatment of biomass at low temperatures ranging from 200 °C to 300 °C under an inert atmosphere was known to be effective for improving the energy density and shelf life of the biomass [2
]. This treatment is referred to as “torrefaction”, and it has been widely applied to wood and grass biomass over the past few years. Torrefaction is a technology which allows the moisture and low-weight organic volatile components in biomass to be removed, producing a hydrophobic solid product with an increased energy density (on a mass basis) and greatly increased grindability [6
]. It involves the slow pyrolysis of materials in the absence of oxygen, with the aim of maximizing the solid yield, thus producing a large fraction of bio char (70%–85%) for use in energy, metallurgical fuel, chemical, and fertilization applications [9
]. The main improvements of torrefied biomass include reduced moisture and an increased energy density; a reduced oxygen-to-carbon (O/C) ratio, which increases the HHV; the strong fibers of the biomass becoming brittle, which improves grindability by reducing the cost and energy required for grinding; and the ignitability and reactivity is improved, which enhances the efficiency during gasification or pyrolysis [10
Torrefaction can be incorporated into a combined drying, torrefaction, and pelletization process, with both economic and energy efficiency benefits [11
]. However, this process requires a separate plant, the input of the process energy, and the production of gaseous and volatile streams, entailing capital costs, operating costs, and emission control efforts. A balance between these associated costs for fuels which are more grindable and have higher HHV is therefore critical for the future of torrefaction and requires thorough analysis and extensive, reliable data [11
A considerable amount of corn stalk waste is produced annually, as corn is one of the most common staple crops worldwide [3
]. However, very few studies have been carried out on the handling or reuse of cornstalk waste as biomass. In this research, the torrefaction of readily available cheap feedstock corn stalk waste was carried out in a horizontal tubular reactor in the presence of nitrogen gas. Alternative methods to treat corn stalk waste are conversion to alcohol and conversion to an organic fertilizer by means of a bio-treatment. However, these methods generate a large amount of waste. Very few studies have been published regarding the torrefaction of corn stalks. Most factors regarding the torrefaction of corn stalk waste remain to be studied. To provide deep insight into the technique of torrefaction, this study investigates the effects of the torrefaction temperature and the torrefaction holding time on the processing of corn stalk waste.