Ammonia as hydrogen carrier for transportation; investigation of the ammonia exhaust gas fuel reforming

Wentao Wang,José M Herreros,Athanasios Tsolakis,Andrew P.E York

doi:10.1016/j.ijhydene.2013.05.144

Abstract

In this paper we show, for the first time, the feasibility of ammonia exhaust gas reforming as a strategy for hydrogen production used in transportation. The application of the reforming process and the impact of the product on diesel combustion and emissions were evaluated. The research was started with an initial study of ammonia autothermal reforming (NH3 – ATR) that combined selective oxidation of ammonia (into nitrogen and water) and ammonia thermal decomposition over a ruthenium catalyst using air as the oxygen source. The air was later replaced by real diesel engine exhaust gas to provide the oxygen needed for the exothermic reactions to raise the temperature and promote the NH3 decomposition. The main parameters varied in the reforming experiments are O2/NH3 ratios, NH3 concentration in feed gas and gas – hourly – space – velocity (GHSV). The O2/NH3 ratio and NH3 concentration were the key factors that dominated both the hydrogen production and the reforming process efficiencies: by applying an O2/NH3 ratio ranged from 0.04 to 0.175, 2.5–3.2 l/min of gaseous H2 production was achieved using a fixed NH3 feed flow of 3 l/min. The reforming reactor products at different concentrations (H2 and unconverted NH3) were then added into a diesel engine intake. The addition of considerably small amount of carbon – free reformate, i.e. represented by 5% of primary diesel replacement, reduced quite effectively the engine carbon emissions including CO2, CO and total hydrocarbons.

Highlights

The use of hydrogen in internal combustion engines has long been believed to be beneficial in terms of emissions reduction such as HCs, CO, CO2 and particulate emissions [1,2]
Because of ammonia’s relatively high auto-ignition temperature (651 C compared to 254 C for diesel), complete in-cylinder combustion of ammonia is difficult, which leads to significant emission of NH3 [17]
It is clear that to achieve NH3 decomposition in the relatively low temperature range applicable to diesel exhaust i.e. 150e400 C, the decomposition needs to be accompanied by an exothermic reaction

Summary

Introduction

The use of hydrogen in internal combustion engines has long been believed to be beneficial in terms of emissions reduction such as HCs, CO, CO2 and particulate emissions [1,2]. Its low volumetric energy density and its high transportation cost make on e board hydrogen storage difficult [5]. Previous studies have shown that H2 can be produced by means of hydrocarbon reforming [6,7] This method can be adopted for the purpose of on-board reforming of hydrocarbon fuel i.e., using recovered heat and oxidant from exhaust gases for. For on-board applications the exhaust gas temperature of a typical diesel engine is only in the range of 150e400 C. If diesel engine exhaust is used to provide the O2, and part of the exhaust heat is recovered as a primary energy source for the reaction the NH3 e ATR is transformed into NH3 exhaust gas reforming. The yielded reformate (H2 and unconverted NH3) was sent back to a diesel engine to examine how a reforming system affects the combustion process and emissions

Catalyst

Reforming process efficiency

NH3 decomposition over Ru e Al2O3 catalyst

Combined NH3 oxidation and decomposition

Application of NH3 exhaust gas reforming in diesel combustion and emission

Conclusions