Note 11 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues from external customers	$ 7,559	$ 8,155	$ 17,810	$ 16,334
Cost of sales	(3,840)	(2,249)	(9,150)	(8,299)
Gross margin	3,719	5,906	8,660	8,035
Selling, general and administrative	(2,801)	(3,184)	(8,858)	(9,825)
Research and development	(340)	(285)	(1,070)	(880)
Operating income (loss) from operations	578	2,437	(1,268)	(2,670)
Air Pollution Control [Member]
Revenues from external customers	1,944	2,886	3,837	6,019
Cost of sales	(1,134)	304	(2,172)	(2,782)
Gross margin	810	3,190	1,665	3,237
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	810	3,190	1,665	3,237
FUEL CHEM [Member]
Revenues from external customers	5,615	5,269	13,973	10,315
Cost of sales	(2,706)	(2,553)	(6,978)	(5,517)
Gross margin	2,909	2,716	6,995	4,798
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	2,909	2,716	6,995	4,798
Other Segments [Member]
Revenues from external customers	0	0	0	0
Cost of sales	0	0	0	0
Gross margin	0	0	0	0
Selling, general and administrative	(2,801)	(3,184)	(8,858)	(9,825)
Research and development	(340)	(285)	(1,070)	(880)
Operating income (loss) from operations	$ (3,141)	$ (3,469)	$ (9,928)	$ (10,705)