## 2 Foam Dust Reduction Technology
### Research Status
The foam dust removal system developed by the US Bureau of Mines firstly mix the compressed air, the water and the surface-active substances in a mixer, which is sent to the metal mesh of the foaming device to form small foams targeting the dust source. This method has been tested on the longwall working faces of two coal mines in West Virginia and Utah, respectively. The dust removal efficiency of total dust is much higher than that of high-pressure spray dust reduction, and the water consumption is only from 1/5 to 1/10 of that of spray dust reduction. However, the dust removal efficiency for respirable dust is not much improved. [PERSON] from Japan conducted some research on the foam dust reduction, who analyzed the working parameters of the mesh foam injector, and carried out dust removal tests with different installation methods of the foam injector on the shearer. The content of suspended coal dust in the air is reduced by 50% to 70%. To prevent the development of ignitable coal haze, rock or pulverized particles of limestone, [PERSON] et al. [7] from Pakistan recommended to spray above the \"coal dust\" generated in the course of mining in mine accesses to develop an inert dust combination. Figure 1: Schematic diagram of the structure of the foamer used by Monsanto Company and Dayton Laboratory. Compared with other nations, the studies on the foam dust reduction started late in China. [PERSON] from the Shanghai Research Institute of the Coal Academy of Sciences first started the exploration and research of foam dust reduction. He initially studied the mechanism of foam dust reduction and simulated foam dust reduction in the laboratory. The test results show that foam dust reduction is an effective dust reduction method. However, due to the limited conditions, no industrial tests have been carried out, and a practical foam dust suppression system has not been established in coal mines. Some studies have been conducted in Hubei Provincial Labor Insurance Institute by [PERSON], who began to study foam rock drilling dust removal technology, developed rock drilling foam dust collectors, and conducted field tests on the rough limestone of Wuttai Coal Mine and the granite rock drilling face of Wuhan Iron and Steel Chengchao Iron Mine, and achieved good results. The foaming liquid in the developed equipment is only added intermittently and fails to work continuously. The research results are only applicable to drilling construction sites, without taking the narrow spaces and continuous dust production into consideration, such as excavation work faces and transfer points. Professor [PERSON] from the University of Science and Technology Beijing studied the foam dust reduction, who theoretically analyzed the structure and working parameters of the mesh foamer. Based on the continuous exploration of experiments and research on the flowing characteristics of foam, a technology is used to separate the foaming device from the foam nozzle. As shown in Figure 2, multiple foam nozzles are installed around the dust source, with a foaming device installed in a location that is permitted by the space and condition of the working face; Next, the foam is transported to the nozzle, and good diffusion properties and aim of the nozzle allow the foam to control the dust [20]. This technology is featured with the simple operation, the stability and reliability and the application in the narrow spaces underground [21; 22; 23; 24; 25].