Internal combustion engines such as diesel engines are widely used in a variety of fields for high combustion efficiency and a small amount of carbon dioxide to be produced. Unfortunately, the exhaust gas from those internal combustion engines includes nitrogen oxides (hereinafter, referred to as NOx.). NOx emission has been regulated more severely because of increasing environmental concerns these days. For this reason, development of techniques of removing NOx with high efficiency and at low cost has been demanded, and NOx reduction has been examined in various respects (see Non Patent Documents 1 and 2).
Specific examples of the techniques of removing NOx include a selective catalytic reduction (SCR) technique that selectively reduces NOx in the exhaust gas from a diesel engine by a catalyst. The SCR technique, however, needs chemicals such as urea and ammonia for reduction of NOx, and has problems such as management of the chemicals and expensive cost. Additionally, in the case where the exhaust gas has a high NOx concentration of hundreds to thousands ppm, an apparatus as large as the diesel engine body is necessary.
Another example proposes that the feed air having water sprayed thereto is fed to an internal combustion engine to reduce NOx (see Patent Document 1). Moreover, a method for adding water to a fuel and feeding the resulting fuel as a W/O (Water/Oil) emulsion to an engine or the like is also proposed.
Further, a method for reducing NOx using a membrane is proposed. For example, Patent Document 2 describes reduction of NOx in which using an oxygen selectively permeable membrane, a concentration of nitrogen in the air is enriched, and the nitrogen-enriched air is fed to an internal combustion engine. Patent Document 3 describes reduction of NOx in which using a steam permeable membrane, steam is permeated from the discharge air, and the permeated steam is fed to feed air.