Catalytic Hydrothermal Reforming of Lignin in Aqueous Alkaline Medium

Abstract

This paper proposes catalytic hydrothermal reforming (CHTR) for producing substitute natural gas (SNG) directly from a lignin in aqueous alkaline media, which can fully dissolve the lignin but stabilize it, lowering the reactivity toward water. Among catalysts preliminarily tested, activated-carbon-supported ruthenium (Ru/AC) catalysts showed the highest activity in terms of reduction of the total organic carbon concentration (TOC) in the aqueous solution. CHTR of a lignin was performed employing a 5000 ppm TOC solution of 0.1 M Na 2 CO 3 in a continuous reactor. The presence of Na 2 CO 3 in the solution enabled delivery of the lignin, which is poorly soluble in water, to the reactor as well as suppression of char formation during CHTR. A Ru/AC showed its ability to maintain 98.6% conversion of the lignin at 350 °C even under the alkaline environment for a duration of at least 10 h, with colorless effluent liquid containing 70 ppm TOC of organic carbon. A low content of Ru in the Ru/AC resulted in an insufficient yield of gas because of the deposition of a portion of the lignin as coke over the catalyst, while 20 wt % Ru was enough for the full conversion into gas composed mainly of CH 4 , with cold gas efficiency (CGE) of 100.4% on a higher heating value (HHV) basis. The resulting aqueous solution of Na 2 CO 3 was ready to be reused for CHTR after removal of carbonate ions derived from the lignin by aeration.

Hydrotreating and hydrothermal treatment of alkaline lignin as technological valorization options for future biorefinery concepts: a review

“…The suitability of nickel catalysts for hydrogen selectivity is also confirmed by Guan et al for their nickel‐based catalyst system of Ni/Al 2 O 3 –SiO 2 . However, Raney nickel, Ru/C and Ru/Al 2 O 3 were found to contribute to higher selectivity for methane by Kudo et al Guan et al also investigated the reuse of the Ni/Al 2 O 3 –SiO 2 system compared to Ru/C and found the former to be more stable because it is less prone to sulfur poisoning. This also offers economic advantages with regards to using nickel‐based catalyst systems for the hydrothermal gasification of lignin …”

“…Nonetheless, the alkaline properties of the Na 2 CO 3 solution were reported to suppress char formation. Furthermore, the solution was found to be almost free from organics after reaction and therefore prone to re‐use …”

“…The main products of hydrothermal gasification of alkaline lignin are hydrogen, methane and carbon dioxide. Carbon monoxide and ethane are also produced, but to a lesser extent . Yields for carbon dioxide are the highest ranging from 48.4% to 49.5% in the work of Guan et al and Azadi et al respectively.…”

“…Although gases are the targeted products from gasification, a liquid product is also obtained. The liquid product may contain various types of phenols such as phenol, cresols, ethylphenols, and numerous aromatic hydrocarbons such as naphthalenes and phenanthrenes …”

“…This also offers economic advantages with regards to using nickel-based catalyst systems for the hydrothermal gasification of lignin. 66 Finally, Kudo et al 68 also investigated the catalytic activity of Na 2 CO 3 as a co-catalyst for hydrogen selectivity through the water-gas shift reaction but its effect was found to be insignificant. Nonetheless, the alkaline properties of the Na 2 CO 3 solution were reported to suppress char formation.…”