Pilkington Group Ltd.“s Chinese glass plant at Changshu, a joint venture between the UK group and China“s Shanghai Yaohua Pilkington Glass, operates a chemical vapour deposition (CVD) plant for the …
Pilkington Group Ltd.“s Chinese glass plant at Changshu, a joint venture between the UK group and China“s Shanghai Yaohua Pilkington Glass, operates a chemical vapour deposition (CVD) plant for the production of its Energy Advantage-branded low-E glass. The facility, designed in the UK by ACWA AIR and built as a series of modules in the UK, Europe and China before being assembled under ACWA AIR“s supervision, uses local labour in China, supported by close cooperation between the UK and Chinese engineers. It is the third in a series of contracts awarded to ACWA AIR by NSG“s Pilkington Float Glass Business Group to prevent pollution from the CVD coating system and recover energy for use within the process. Emissions from the CVD plant are significantly better than the Chinese limits on dust (40mg/Nm3), hydrogen chloride (30mg/Nm3) and hydrogen fluoride (3mg/Nm3), said ACWA AIR. According to the company, these results have encouraged the JV partners to consider the installation of a second production line. The coating process in operation at Changshu uses complex organic tin salts and a mixture of other chemicals to generate the required surface on the float glass. The vent from the vapour coating process and associated chemical storage vessels is passed to a thermal oxidizer, in which the organic tin salts are broken down at high temperature into tin oxides and hydrogen chloride gas. Seventy-five per cent of the available heat energy from the hot gases discharged from the thermal oxidizer are recovered in a boiler, generating steam, used for process heating in the CVD system. Surplus steam is passed to an air-cooled condenser and the condensate is returned to the boiler. The cooled flue gases, which contain hydrogen chloride and hydrogen fluoride, are reacted with sodium bicarbonate, injected into the gas stream from a storage facility, to produce sodium halide salts and carbon dioxide. The reaction rates with sodium bicarbonate are extremely fast and completely transform the corrosive gases into innocuous salts. A mixture of sodium halide salts suspended in the flue gases, excess sodium bicarbonate and metal oxides, are filtered from the process by a pulse-jet bag filter and discharged into sealed skips, and may be recycled to recover tin. The cleaned gases are discharged into the atmosphere through a free-standing chimney stack by an induced draught fan.