Jamal, Wyszynski:
Onboard Generation of Hydrogen-Rich Gaseous Fuels
- A Review

International Journal of Hydrogen Energy
Vol.19, No.7, pp. 557-572, 1994,
Received for publication 1 September 1993

ONBOARD GENERATION OF HYDROGEN-RICH GASEOUS FUELS - A REVIEW
By: Y. Jamal and M.L.Wyszynski *
School of Manufacturing and Mechanical Engineering
University of Birmingham, Birmingham B15 2TT, UK.
* Author to whom correspondence should be addressed.

ABSTRACT
Hydrogen has a good potential as an alternative fuel for spark ignition engines. It can extend the lean flammability limit of conventional fuels in order to achieve higher thermal efficiency and lower exhaust emissions.
This paper reviews the use of hydrogen and hydrogen-enriched gasoline as a fuel for SI engines and the techniques used to generate hydrogen from liquid fuels such as gasoline and methanol, on-board the vehicle. The processes of thermal decomposition, steam reforming, partial oxidation and exhaust gas reforming are evaluated.
A considerable amount of both theoretical and experimental work has been done in this field. Predictive and experimental results of the various investigators are reviewed and summarised.

INTRODUCTION
The scarcity of fossil fuels and the associated pollution problems have attracted the attention of researchers towards the search for alternative fuels. With any alternative fuel, its availability of the source, as well as the emissions of pollutants are most important from the aspect of energy preservation and environment, respectively, while its potential effects on the engine performance and the form of storage will be significant issues from the stand point of the engine and vehicle technologies.
An alternative energy carrier that has great environmental advantages is hydrogen. It is a clean fuel, when burned in air, produces non-toxic exhaust emissions except at some equivalence ratios, where its high flame temperature results in significant NOx levels in the exhaust products. The use of gaseous fuels inducted with air does, however, limit the total power output of the engine due to the reduction of volume of air aspirated.
The current approach for the reduction of emissions relies on three-way catalytic converters. An alternate and more basic approach to the emissions problem is to modify the initial combustion process in the engine by using lean mixtures. The primary advantage of lean burn is that it increasingly reduces NOx and CO. The problem with it is that engine power declines rapidly, while unburned hydrocarbon emissions increase because of misfire [1]. Unfortunately, an engine still produces unacceptably high NOx exhaust pollutants near the lean flammability limit of gasoline. In a practical sense, lean-burning engines are limited by the onset of engine misfiring as the lean flammability limit of any fuel is approached. For this reason emission standards for NOx could not be achieved in the engine by operating lean, using only gasoline. Mixtures of hydrogen and gasoline, on the other hand, can burn lean enough to meet this requirement [2].
Hydrogen may be used to extend the lean limit of conventional fuels in order to achieve higher efficiency and lower pollutant emissions. Because of its wide flammability limits and high flame speeds the hydrogen-rich fuel lends itself readily to ultra lean combustion and should allow the use of higher compression ratios. Combining the increase in heating value, the recovery of waste energy from the engine exhaust, lean operation and higher compression ratios provides potentially high increase in thermal efficiency for the hydrogen-enriched fuels over that of the conventional fuels.
However, there are significant problems associated with the use of hydrogen, especially concerning production and storage. Hydrogen is commercially produced by electrolysis of water and by coal gasification. These methods are not widely used because they are more expensive than steam reforming of natural gas or partial oxidation of heavy oils [3]. Most of the worlds hydrogen is currently derived from hydrocarbons such as oil or natural gas, via the catalytic steam reformation [4, 5].
There are generally three ways to store hydrogen in an automobile: as a gas dissolved in a metal (metal hydride), as a cryogenic liquid or as a compressed gas. Hydride storage is the simplest and the safest, but it increases vehicle weight and results in a severe fuel economy penalty. Liquid hydrogen is light, but due to its low energy density occupies three times as much volume as gasoline. Storage as a compressed gas is inexpensive and provides for ease of operation but its weight and bulk is the main problem.
One solution to the storage problem is the onboard hydrogen generation from a suitable high energy density patent fuel such as gasoline or methanol. With this form of storage, the technical problem is how to generate the hydrogen. There are a number of methods to generate hydrogen from such fuels. The most common amongst those are partial oxidation, steam reforming, thermal decomposition and exhaust-gas reforming.
Partial oxidation is not usually considered to be attractive in terms of efficiency because it is an exothermic process, and the resulting hydrogen containing fuel gas has a lower calorific value than that of original feed stock. However, it is an interesting means of generating free hydrogen gas for use as a charge supplement in ultra lean combustion exercises [6]. On the other hand, steam reforming is an endothermic process, the fuel gas thus produced has higher calorific value than the feed stock, and the efficiency of the process is quite favourable, particularly if the heat energy requirement is supplied from a source which would otherwise be wasted, like hot engine exhaust gases [7]. Thermal decomposition of hydrocarbons results in the formation of hydrogen and carbon. The difficulty of gasifying or handling the solid carbon makes hydrocarbon decomposition not suitable for on board hydrogen generation. Methanol can be catalytically decomposed into hydrogen and carbon monoxide at temperatures of the order of 250oC [8]. The reaction is endothermic and requires a heat source to provide energy. This energy can usually be supplied by the engine exhaust gas. In exhaust gas reforming fuels are reformed catalytically by direct contact with a portion of hot products of combustion utilising the fact that exhaust gases contain a certain quantity of steam. The fuel gas thus generated contain quantities of hydrogen, carbon monoxide and nitrogen, thus providing a potential for lean combustion leading to lower emissions and higher engine thermal efficiency than conventional fuel [9].
The objective of this paper is to review the use of hydrogen as a fuel for gasoline engine, in particular as a supplementary fuel, and to discuss the different methods of onboard hydrogen generation.

The main parts of the paper:

USE OF HYDROGEN AS A FUEL
Properties Of Hydrogen
Table.1 Comparative Properties of Hydrogen
Spark Ignition Engine performance of Hydrogen
SI Engine Performance of Hydrogen Enriched Gasoline
FUEL REFORMING TECHNIQUES
Thermal Decomposition
Steam Reforming
Partial Oxidation
Exhaust-Gas Reforming
DISCUSSION
have been omitted in this on-line abstract.

CONCLUSIONS
1. Spark ignition engine can be operated efficiently at light loads using hydrogen fuel alone.
2. Some charge dilution to avoid knock is essential to permit operation with hydrogen at higher power output.
3. Hydrogen supplementation of gasoline combustion has been shown to yield reduction in fuel consumption.
4. Hydrogen-rich gaseous fuels can be burned under ultra lean conditions to yield very low NOx emissions without running into lean flammability limit problems.
5.The lean burning conditions give possibilities for very low CO emissions.
6. Enrichment by pure hydrogen does not appear to be a sufficient means of reducing HC emission as measured by total HC methods.
7. Consideration of the hydrogen/gasoline/air combustion process, coupled with the observation of steady state test-bed performance, suggested the possibility that the hydrogen and gasoline oxidation processes are independent and may result in two flames.
8. Onboard hydrogen generation from liquid fuels, either hydrocarbons or alcohols, is technically feasible.
9. The economy benefit from running lean almost compensates for the energy requirements for making hydrogen from gasoline in an atmospheric reactor.
10. Some of the waste heat from the vehicle exhaust gas can be reclaimed by converting it to chemical energy in the fuel.
11. Optimum performance of a reforming reactor occurs at the lowest possible excess oxidant factor just short of the soot formation limit.
12. The use of a catalyst in the reforming reactor allows a closer approach to equilibrium H2 yields.
13. The use of reformed fuel (compared to liquid raw fuel) may result in higher engine thermal efficiency in the low power range, with the improvement due to the increase in the heating value of the gaseous fuel resulting from the reforming reaction.
14. Use of reformed fuel (compared to liquid fuel) in a spark ignition engine may result in lower exhaust emissions, particularly of the heavier and aromatic hydrocarbons and NOx.
15. In all cases considered, a considerable loss in maximum power capacity of the engine occurs as a result of the use of gaseous fuels and by operating under lean conditions. Effective utilisation of onboard hydrogen generation calls for lightweight high displacement engines to overcome this disadvantage.
16. An automobile could not be operated over the required power range when it was fed exclusively with reformed fuel. A supplementary fuel supply would be required to reach the higher loads.

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