Pilkington advances gas cleaning, energy recovery at Chinese plant
21 Sep 2010
Changshu, China – Pilkington Group Ltd’s glass plant at Changshu on the Chinese eastern seaboard, operates a chemical vapour deposition (CVD) plant for the production of low-e glass. The unit is operated as a joint venture between China’s Shanghai Yaohua Pilkington Glass and the NSG Group
The Changshu facilitty is the third in a series of contracts awarded to ACWA AIR by NSG Group to prevent pollution from the CVD coating system and recover energy for use within the process.
The facility was 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. The project involved using local labour in China, supported by close co-operation between the UK and Chinese engineers.
The CVD plant emissions are significantly better than the Chinese limits on dust (40mg/Nm3), hydrogen chloride (30mg/Nm3) and hydrogen fluoride (3mg/Nm3), said ACWA AIR. These results, it added, have encouraged the JV partners to consider the installation of a second production line.
At Changshu, the coating process 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 oxidiser where the organic tin salts are broken down at high temperature into oxides of tin and hydrogen chloride gas.
Hot gases are discharged from the thermal oxidiser and 75% of the available heat energy is recovered in a boiler, generating steam, which is used for process heating in the CVD system. Any surplus steam is passed to an air-cooled condenser and the condensate is returned to the boiler.
The cooled flue gases, containing hydrogen chloride and hydrogen fluoride, are reacted with sodium bicarbonate, injected into the gas stream from a storage facility, producing sodium halide salts and carbon dioxide. The reaction rates with sodium bicarbonate are extremely fast and completely transform the corrosive gases into innocuous salts.
Solids suspended in the flue gases - a mixture of sodium halide salts, excess sodium bicarbonate and metal oxides - are filtered from the process by a pulse-jet bag filter and discharged into sealed skips. These solids may be recycled to recover tin.
Cleaned gases are discharged to atmosphere through a free-standing chimney stack by an induced draught fan.
