BLENDING OF HYDROGEN WITH NATURAL GAS (HCNG)

Author:- Mohammed Vaseemuddin

This review offers a summary of the recent research progress on hydrogen-enriched compressed natural gas (HCNG) engines. Spark ignition IC engines fueled by HCNG have been shown to be advantageous over traditional gasoline engines in terms of fuelefficiencies and pollutant emissions. Further taking into account the vast availability of natural gas and the renewability of hydrogen, HCNG is considered to be a promising alternative fuel for large-scale applications in spark ignition engines. The past decade has witnessed significant progress in the research and development of HCNG engines. In this work, we will provide a comprehensive survey of recent studies on the power, efficiency, and combustion & emission characteristics of HCNG engines. Additionally, we also focus on correlating fuel properties with experimentally observed HCNG engine performances, providing fundamental insights in the effects of hydrogen addition on engine working processes. To reach this goal, the physicochemical properties of hydrogen and its mixture with natural gas were first analyzed, followed by a detailed presentation and analysis of notable experimental results on HCNG engine performances. Numerical models for HCNG engines, which serve as an efficient way for engine parameter optimization, were also discussed. Apart from HCNG, the usage of other hydrogen-enriched fuels, such as hydrogen gasoline, hydrogen diesel, biogas, and hydrogen ethanol/methanol in 1C engines was also briefly discussed. Being the major component of NG, methane has the highest hydrogen-to-carbon ratio among all hydrocarbons, resulting in NG engines having low specific CO2. UHCs and CO emissions. By switching gasoline to NG, the UHCs and CO emissions could be reduced by 30-35% and 20-30%, respectively. Co2 emission was also reported to be lower than in gasoline engines [8]. In addition, the fact that NG has a better anti-knock property makes it possible for spark ignition (SI) NG engines to have a higher compression ratio and thus higher thermal efficiencies, as compared to their gasoline counterparts. However, if operated at stoichiometric conditions, the high NG flame temperature could lead to elevated engine-out NOx emissions such that a three-way catalyst is required for NOx emission reduction to reach the regulation limits. An alternative way is to use lean burn technology which can inhibit in-cylinder NOx formation by combusting fuel at lower temperatures. Indeed, the EURO-V emission standard can be met satisfactorily by lean burn NG engines without the use of exhaust gas after-treatment devices. To meet the stricter NOx emission limit set by the EURO-VI standard, even leaner NG-air mixtures need to be used for a further reduction of flame temperature if the expensive De-NOx devices
are to be avoided. However, due to NG’s relatively slow flame speed, it is typically impossible for traditional SING engines to run at such lean conditions without significantly compromising engine efficiencies. The enrichment of NG with a fast-burning fuel, i.e. hydrogen, which has a laminar burning velocity seven times higher than NG, is an effective method to extend the lean operation limit (LOL) of NG engines. Hydrogen, characterized by its fast flame speed, wide flammability limits, and low minimum ignition energy (MIE), is an ideal fuel additive to conventional hydrocarbon fuels (including NG) to enhance their lean burn capabilities. These hydrogen features have been recognized to have important implications in improving both the emission characteristics and thermal efficiencies of NG engines. In addition, hydrogen is characterized as a clean and renewable energy carrier. Unlike hydrocarbon fuels, the only harmful product of hydrogen combustion is NOx. Although not present in large quantities in nature, hydrogen can be produced by a variety of methods including fossil or renewable fuel reforming, water electrolysis, etc. The application of neat hydrogen in engines has recently received some interest however, its widespread use is still likely to be hindered by many practical difficulties which include but are not limited to large-scale hydrogen production, storage, fueling infrastructures as well as engine abnormal combustion. At the current stage, being an additive to hydrocarbon fuels seems to be a more reasonable approach to promote the application of hydrogen as a transport fuel. Further catalyzed by the facts that hydrogen can be produced on-board by NG reforming D, engines running on the mixtures of hydrogen and NG have attracted great attention from its birth to the present.

The hydrogen fractions need to be optimized and corresponding adjustments are required for various other engine operating and design parameters. Hydrogen fraction has significant impacts on engine thermal efficiency and emission characteristics. In general, increasing the level of hydrogen addition could result in an increase in cylinder temperature and a decrease in combustion duration. These can contribute to more complete combustion and therefore higher thermal efficiency, and lower UHCs, and CO emissions. On the other hand, however, a higher level of hydrogen could also lead to elevated NOx emissions due to the high combustion temperature. In addition, heat loss
tends to increase with the increase of hydrogen fraction, having a negative impact on engine thermal efficiency. As a result, the balance between thermal efficiency and pollutant emissions needs to be determined. Many researchers have investigated this issue and have reached somewhat different conclusions. For instance, Moreno et al. [18] suggested the best balance could be achieved by using the HCNG30 fuel, while HCNG20 fuel blend was recommended by Ma et al. (19). depending on the operating conditions.

When it comes to determining the optimal HCNG fuel compositions for applications in engines, it is essential to understand the effects of hydrogen addition on the blend’s fundamental physicochemical properties. These properties, including density, diffusivity, adiabatic flame temperature, laminar flame speed, ignition delay, etc., play important roles in the combustion process and thus significantly affect engine power output, fuel economy as well as emission performance. Furthermore, engine design parameters and operating conditions such as compression ratio, idling speed, ignition timing, and equivalence ratio also exert important impacts on engine performance and therefore should be adjusted and optimized, considering their interplay with fuel properties. Many experimental and numerical studies focusing on the influences of these parameters on HCNG engine performance have been conducted and based on these results, researchers tried to understand the various effects of hydrogen addition and determine the optimum hydrogen fraction and engine design/operating parameters. This paper aims to provide a comprehensive summary of recent research progress in the fundamental combustion properties of HCNG mixtures and the effects of its application on the engine’s combustion and emission performance. Our focus will be the application of HCNG in SI reciprocating engines. The influence of hydrogen addition on NG engine power output, fuel economy, and emission performances is analyzed in detail. Engine lean burn characteristics are given special attention since many benefits of HCNG engines, as compared to traditional NG engines, come from the fact that hydrogen addition can significantly extend the lean burn capacity of NG engines. Although this study focuses more on hydrogen-enriched natural gas, the effects of hydrogen addition on other practical transport fuels are also briefly discussed.