Upgrading biomass by hydrothermal and chemical conditioning
AuthorReza, Mohammad Toufiqur
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There are two widely known thermal pretreatment technologies, known as hydrothermal carbonization and dry torrefaction. In dry torrefaction, also known as torrefaction or mild pyrolysis, dry solid biomass is treated in an inert gas environment in a temperature range of 200-300oC for more than one hour. In hydrothermal carbonization (HTC), also known as wet torrefaction, biomass is treated with hot compressed subcritical water (200-260oC) for 5 min-8 h. The solid product, HTC biochar, contains about 55-90% of the mass and 80-95% of the fuel value of the original feedstock. The HTC process takes any wet waste biomass and converts it to a homogenized, friable, hydrophobic, and, mass and energy densified HTC biochar similar to lignite coal. Subcritical water, the temperature between 180-280 °C has ionic strength much higher than water under ambient condition. Hemicellulose hydrolysis occurs at temperatures as low as 180 °C, while cellulose hydrolysis starts around 220-230 °C. Many monomers, aldehydes, and intermediates are produced as a result of hydrolysis of biomass. Reactive intermediate species promote chemical reactions such as decarboxylation, dehydration, aromatization, and condensation-polymerization in the presence of subcritical water. As a result of these reactions, complete degradation of hemicellulose and partial degradation of cellulose was observed in the solid phase. Some carbon-rich cross-linked hydrophobic polymers are produced from the cellulose hydrolyzed intermediates. Van Soest fiber analysis is incapable of distinguishing those new cellulose-derived polymers from naturally present lignin. Slagging and fouling are two major problems that result from biomass combustion or co-firing with coal, especially for the biomass with high inorganic content. HTC provides biochar with the slagging and fouling indices which will predict slagging and fouling, regardless of the biomass type, primarily due to reduced chlorine content. Hot compressed water leaches both loose soil and a major portion of structural minerals in biomass by degrading its constituents during HTC. Up to 90% of calcium, magnesium, sulfur, phosphorus, and potassium were removed at low temperature HTC (200 °C). All heavy metals were reduced by HTC treatment, which opens the door to use HTC biochar as soil amendment. However, HTC still might not be appropriate for some specific thermochemical and biochemical conversion processes as the remaining ash can inhibit these processes. Many biochemical processes require hemicellulose and cellulose, while HTC degrades them, even at its minimum severity. However, an effective chelating agent like sodium citrate can preserve the organic carbohydrate fractions, while effectively dissolving metals. More than 75% structural and 85% whole ash was reduced by treatment with 0.1 of g sodium citrate per gram of raw dry corn stover at mild conditions. FTIR analysis demonstrated that the main components like lignin, cellulose and hemicellulose were unaffected by sodium citrate chelation. HTC biochar exhibits the glass transition at 140 °C and thus can be used as a binder to make durable pellets from raw biomass or even from torrefied biochar. Although torrefied biochar shows similar energy value and hydrophobicity, it is quite different than HTC biochar. Torrefied biochar pellets have poor durability compared to pellets of HTC biochar or even raw biomass pellets. Engineered pellets, the pellets of a mixture of torrefied and HTC biochar, are denser, more hydrophobic and durable than torrefied biochar pellets. HTC biochar was found very effective in making solid bridges among the torrefied biochar. The engineered pellets' durability is increased with increasing HTC biochar fraction. Water in subcritical condition is also effective in conversion of digested sludge. A liquid bio-oil type fuel product was discovered from the hydrothermal treatment of sludge (HTS). The HTS biosolid has higher energy value and ash than raw sludge. Moreover, the dewaterability can be increased greatly by treating waste water sludge with subcritical water.