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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Revenues from external customers	$ 8,017	$ 7,559	$ 19,920	$ 17,810
Cost of sales	(4,345)	(3,840)	(11,280)	(9,150)
Gross margin	3,672	3,719	8,640	8,660
Selling, general and administrative	(3,273)	(2,801)	(9,201)	(8,858)
Research and development	(207)	(340)	(716)	(1,070)
Operating income (loss) from operations	192	578	(1,277)	(1,268)
Air Pollution Control [Member]
Revenues from external customers	2,728	1,944	7,670	3,837
Cost of sales	(1,801)	(1,134)	(5,032)	(2,172)
Gross margin	927	810	2,638	1,665
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	927	810	2,638	1,665
FUEL CHEM [Member]
Revenues from external customers	5,289	5,615	12,250	13,973
Cost of sales	(2,544)	(2,706)	(6,248)	(6,978)
Gross margin	2,745	2,909	6,002	6,995
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	2,745	2,909	6,002	6,995
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	(3,273)	(2,801)	(9,201)	(8,858)
Research and development	(207)	(340)	(716)	(1,070)
Operating income (loss) from operations	$ (3,480)	$ (3,141)	$ (9,917)	$ (9,928)