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Advances to suppliers - Allowance for doubtful accounts (Details) - USD ($)
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Advances to suppliers [Line Items]    
Balance at beginning of period $ 3,699,890 $ 5,731,281
Change of allowance for doubtful accounts (53,436) (895,043)
Write off (15,631) (1,523,489)
Translation adjustments 86,885 387,141
Balance at end of period 3,717,708 3,699,890
Advances To Suppliers [Member]    
Advances to suppliers [Line Items]    
Balance at beginning of period 179,095 1,517,017
Change of allowance for doubtful accounts (144,549) (400,436)
Write off (5) (1,039,958)
Translation adjustments 4,205 102,472
Balance at end of period $ 38,746 $ 179,095