CNG engine system ? the power plant of the future

Near development offers higher thermal efficiency and reduced emissions from natural gas energy source.

In a unique research project, the Japanese ?Ship and Ocean Foundation? (SOF), has, since 1998, been developing an engine that utilizes reformed fuel from compressed natural gas (CNG) and an accompanying system for increasing thermal efficiency.

Using a catalytic NG reforming unit and a CO2 separator, this revolutionary engine system boasts a thermal efficiency better than 70% and a dramatic emission reduction with CO2 reduced to less than half of their present level while NOx and SOx are reduced to almost zero.

Established in 1975, SOF is actively involved in ship and related maritime technology development projects intended to prevent global warming and contribute towards building a sustainable society. In 2002, the United States Department of Energy (DOE) displayed strong interest in this programme commenting that the use of carbon dioxide for natural gas reformation is a next generation technology. Today, this technology is on the verge of being realized, but the last stages of development will need to be conducted on a wider, more global scale.

The main objective of the programme is to turn natural gas into a viable alternative primary energy resource that can be used in place of oil. Energy from reformed exhaust gas is used to drive a gas turbine in combination with an electric generator and steam turbine. As the exhaust gas passes through a two-part process, i.e. a CNG fuel catalytic converter and a reformation unit, almost all the CNG is reformed into carbon monoxide (CO) and hydrogen (H2). These gases are then heated by the hot exhaust gas in a heat exchanger with elements made of porous metals. The calorific value of reformed fuel increases by about 30% in the converter.

The research is also aimed at developing a complete ?CNG reformation engine system? which integrates a highly efficient heat exchanger, with steam and exhaust turbines.

Achieving 60% thermal efficiency in a diesel engine has long been a goal of engine technologists. A study in 2003, which involved research into the combustion process through experimental engine differential simulation, showed that a thermal efficiency of 57.5% is attainable. The development of auxiliary devices such as compact reformation converter and heat exchanger has also reached the stage for preparing the system for practical operation.

It is planned that the CNG reformation engine will be fitted with a 200 kVA output dynamo. This unit is adaptable to a wide range of applications including electricity generation, and vessel and automobile propulsion.

This ten-year R&D programme was initiated in 1998 under the initiative of Dr. Hideo Kawamura and the SOF and has been conducted with cooperation from industry members and academic institutions comprising Fujiceratech, Mitsubishi Heavy Industries and the universities of Kyushu and Keio. The programme has been recognized as a ?creative technological development for the future? by the Nippon Foundation.

In addition, a steering and development committee was established within the SOF to manage the operational aspects of the programme and provide guidance where necessary. This committee is chaired by Dr. Takemasa Kamimoto, professor emeritus, Tokyo Institute of Technology.

The reformed CNG fuel engine has the potential to reduce CO2 emissions to 50% of existing engines, NOx to below 10ppm and SOx to zero. It is hoped that a decentralized hybrid engine system combining generator and electric motor will encourage further development of electric ship propulsion systems and other environmentally friendly innovations.

On a practical note, a hybrid engine system will allow ship crews to continue operating a vessel even in the case of a single engine failure.

In addition, as the construction of natural gas infrastructure accelerates around the world, its impact on the natural environment will become difficult to ignore.

From this viewpoint, the development of a reformed CNG fuel engine is well timed. Along with the possibility of a convenient and low-priced supply of electric energy, this

system may also benefit developing countries and possibly become an alternative to nuclear power.

Research results

The reformed CNG fuel engine system consists of a gas engine, a heat exchanger, a turbo generator that operates using exhaust gas and steam, and a fuel reformation unit. The level of development for each component is as follows:

Heat exchanger:

l By taking advantage of porous metals, the heat exchanger has been reduced in size from a cylinder (diameter 800mm x height 1500mm) to a block (400mm x 420mm x 810mm), which is significantly more compact than conventional units. Experimental trials indicate that this size reduction makes heat transfer smoother, and that thermal efficiency was five times higher than for a conventional unit. The tests have been concluded, and development work is approximately 90% complete with only minor modifications and fine-tuning remaining.

Turbo generator & steam turbine:

l These units are still in an early stage of development. Although existing prototypes are functioning, research is continuing into further improving their effectiveness.

Fuel reformation unit:

l Modules for the absorption and separation of CO2 have been completed, and development work is approximately 90% accomplished.

Natural gas engine:

l Design and experimental trials of a single-cylinder engine have been completed. Development work on an adiabatic engine, a pre-combustion chamber engine and CNG combustion system, which together form the system core, is approximately 90% complete.

Each of the major system components can be applied independently, in a wide range of uses. The rapid progress of the programme means the completion of a 6-cylinder prototype engine can be expected within the next two years. It is hoped this will lead to the construction of a 200 kW engine of 3 x3 x 2m size based on the same engineering principles that will offer similar levels of thermal efficiency. Being much more compact, the thermal efficiency of the reformed CNG fuel engine system compares favourably with that promised by a fuel cell (SOFC) system.