First of all, the process of creating SNG still requires the mining of coal. A recent study published in Nature Climate Change has actually noted that this process can wind up yielding up to 82 percent more greenhouse gas emissions than that produced by burning coal for energy.
In addition, the SNG process requires more water than what was needed for coal energy production. A lot more water. In fact, it needs 18 times more water.
This could pose a problem, especially considering where these new SNG plants are going to be built. The World Resources institute (WRI) found that a bulk of the plants are going to be located in areas where water is scarce. The plants may wind up using a whopping 20 percent of the area’s industrial water supply. This could mean outages or a reduced capacity later down the road.
Source: http://www.care2.com/causes/china-shows-us-how-to-not-fight-air-pollution.html#ixzz2kpgiEclA