Hydrothermal carbonization of distiller’s grains

Abstract

Wet distiller’s grains are intermediate byproducts of ethanol manufacture that have high moisture contents and require significant energy for drying and conversion into dry distiller’s grains. Hydrothermal carbonization was investigated as a wet process to provide alternative products, and chars were obtained in moderate yield that possessed high heats of combustion. The mechanism of char formation was also investigated employing constituent materials representative of the chemical composition of distiller’s grains. Char formation was discovered to chiefly involve carbohydrates (other than cellulose) and proteins. A surprising discovery was that triacylglycerides and fatty acids created under the reaction conditions did not contribute to char yield and were adsorbed onto the chars and could be easily extracted.

Introduction

The growth of the fuel ethanol industry in the US has been significant, with 1.6 million gallons produced in 2000 and 10.7 million gallons by the end of the decade in 2009 [1]. The vast majority of ethanol is currently produced by the dry milling of corn, and every bushel of corn creates 2.7 gallons of ethanol and 16 pounds of a fermentation residue product known as distiller’s dry grains with solubles (DDGS) [2]. While the highest value and societally most important application of DDGS is as an animal feed, an issue confronting manufacturers concerns the treatment of products obtained once the ethanol has been removed from the fermentation media in the beer well. The resulting liquid/solid mixture is centrifuged to obtain wet distiller’s grains (WDG) having a moisture content of 65–70% [3] and a thin stillage supernatant having a moisture content of 90–95% [4]. Both of these materials are generally intermediate products and require significant amounts of energy to remove water for conversion into final products. Furthermore, WDG is susceptible to spoilage from microbial contamination unless it is further dried to a moisture content of 10% or less. It is estimated that approximately one-third of the WDG can be distributed within its shelf life period of about one week to local animal feeding locations within about a 60 mile radius of the ethanol plant. The remainder, however, must be dried, and it is estimated that as much as 30% of the plant’s operating budget is consumed in drying the remaining two-thirds of the WDG [5]. Therefore, due to the high energy cost of removing water by evaporation, other applications for WDG may be highly desirable.

Hydrothermal treatment offers significant advantages in the conversion of biomass into value-added products. Substrates are not required to be dry, as the various treatments are actually conducted in water. Furthermore, water is generally not removed from products by evaporation. Rather, hot liquid water or steam is cooled using thermal recovery methods and gaseous products collected by distillation, oily liquid products by phase separation, and solid char products by filtration. As indicated in Table 1 [6], hydrothermal methods vary considerably in terms of severity of reaction conditions and products obtained.

An aim of the present investigation was to not only report our findings with WDG but also to better understand which of the constitutive materials were actually undergoing the chemical changes that contributed to char formation. DDG represent a useful model biomass substrate because their chemical compositions are relatively constant. They basically consist of the following component materials given in weight percent [12]: protein = 30%; neutral detergent fiber (NDF) = 42%; fat = 11%; and ash = 6% [2], with residual starch levels of 4% [3]. On this basis, representative proteins, fats, carbohydrates and NDF were examined under a constant set of reaction conditions, singly and in combinations, to empirically ascertain the nature of reactants that contributed to char formation.