PNNL

Abstract

Hydrogen storage utilizing organic carriers has many benefits and will be an enabler for storing and transporting large quantities of green hydrogen made from renewable resources. Ethanol, with a theoretical capacity of 35 g H2/L (equivalent to ca. 500 bar compressed H2 gas), is a liquid organic hydrogen carrier that is readily available from biological resources. Ethanol can be reformed with steam to mixtures of carbon oxides and hydrogen or release hydrogen maintaining the carbon backbone by conversion to ethyl acetate, allowing utilization in a cyclic process – keeping the carbon in play and avoiding the release of carbon dioxide to the environment. The cyclic process studied here is a bimolecular dehydrogenative coupling (DHC) reaction to yield hydrogen and ethyl acetate. The analysis provided herein contrasts two important and related aspects: thermodynamics of the chemical reaction itself and thermodynamics of the process. Thermodynamics of the DHC reaction of ethanol to hydrogen and ethyl acetate are characterized by a Gibbs energy of reaction at standard conditions close to zero in the gas phase. Taking advantage of this, we explore process conditions which allow shifting the reaction between hydrogenation and dehydrogenation within a moderate window of temperature and pressure conditions. The reaction proceeds in two steps involving an acetaldehyde intermediate; however, accumulation of significant acetaldehyde is unlikely due to its low thermodynamic stability compared to ethanol and ethyl acetate. Thermodynamics of the process is positively influenced by the rather low enthalpy of reaction. Therefore, the energy demand for dehydrogenation is comparatively small, resulting in an overall system efficiency of 78%. Compared to other hydrogen carriers, this is a promising result, making the ethanol / ethyl acetate system an interesting candidate for hydrogen storage applications ranging from emergency backup power to long duration and seasonal energy storage.