The aim of this study is to clarify the appropriate reaction condition for the thermal decomposition of glycerol to a syngas in order to utilize wasted glycerol derived from biodiesel production. The effect of reaction temperature on gaseous products were examined by the experiment and the chemical equilibrium simulation. The experimental results show that a glycerol is decomposed to H₂, CO, and C1–C2 hydrocarbons under the reaction temperature beyond 800 K. In addition, the effects of water addition on decomposition temperature is also investigated. The results show that the water addition is effective to improve the gasification efficiency with relatively low reaction temperature below 1000 K. Furthermore, it is found that the composition of gaseous product obtained from an actual wasted glycerol is similar to that from a reagent glycerol.

In recent years, a global shift has occurred with societies turning toward recycling. The looming problem of global warming has led to a significant increase in the use of biodiesel fuels made from vegetable fat and oil. In this regard, improvement of the waste recycling processes of the highly alkaline crude glycerol, which is by-product of biodiesel fuels, has received significant attention. Crude glycerol damages the incinerator during the incineration process and its purification via neutralization, desalination, and washing steps is very expensive. In our group, it was found that, without preprocessing, crude glycerol can be converted to biogas and other chemical materials via anaerobic digestion. In particular, it has been found that by using sewage sludge as seed materials and depending on the volume of crude glycerol, it is possible to switch between the methane and hydrogen fermentation processes. When the hydrogen fermentation process is predominant, 1,3-propanediol (1,3-PDO), a chemical raw material for the synthesis of polymers, is produced. Furthermore, in the research for fermentation promoters in glycerol digestion, it has been found that a small amount of glucose (less than 1% of the input amount of glycerol) promotes the growth of the hydrogen fermenting bacteria that are resistant to the highly alkaline conditions. Under high loads of glycerol, other mixed bacterial cultures are difficult to digest crude glycerol. However, the alkali-resistant bacteria promoted by glucose were able to successfully and efficiently convert glycerol to hydrogen. The co-digestion of food waste and crude glycerol has also been tested on a pilot scale. It was found that crude glycerol is a good input material as it acts as a pH adjuster and is also a source of carbon. This review summarizes the results of previous work performed our group and outlines the future prospects of the developed technology.

Bio-diesel fuel, BDF, is manufactured using waste food oil in Japan, consequently leaving a glycerol waste fluid as a by-product. BDF is prevailed as ecological fuel, however, the glycerol waste fluid is considered as difficult in recycling in that it contains not only organic glycerol, unreacted methanol and waste food oil but an inorganic alkaline catalyst like KOH, contributing to high viscosity and strong alkalinity. In this research, the glycerol waste fluid was treated by dilution and neutralization to yield oil-liquid layers of which top was separated oil and bottom was a degreased glycerol solution. As the separated oil had a high heating value, it recycled as fuel oil and used for a raw material of bio-reclaimed oil which was made of reclaimed oil and waste food oil. On the other hand, as the degreased glycerol solution was rich in organic carbon while poor in nitrogen content, it recycled as a denitrification agent and used for a substitute for 50% methanol solution ordinary utilized in an excretion treatment facility. The glycerol waste liquid could be completely recycled and used for the purposes through the treatment method.

Development of effective usage for glycerol produced as a by-product during biodiesel production is desired. In glycerol conversion using iron-oxide based catalyst, useful chemicals such as allyl alcohol, propylene (Pathway I) and acetol, carboxylic acid and ketones (Pathway II) were obtained. Propylene and ketones were successfully obtained during the glycerol conversion in accordance with these reaction pathways. Moreover, we found that the dehydration reaction in Pathway II was suppressed by loading potassium on iron oxide catalyst and that the addition of formic acid into the reaction system enhanced allyl alcohol production. In addition, it was revealed that the potassium loading decreased the Lewis acidity and the Brønsted acid sites were newly formed during the reaction, where the hematite structure (α-Fe₂O₃) was changed into the magnetite structure (Fe₃O₄). Therefore, it was concluded that the production of allyl alcohol from glycerol was promoted by the Brønsted acid sites newly formed on the iron oxide catalyst during the structure change to magnetite.