Note 11 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Revenues from external customers	$ 5,535	$ 5,033
Cost of sales	(3,245)	(2,675)
Gross margin	2,290	2,358
Selling, general and administrative	(3,054)	(3,100)
Research and development	(220)	(415)
Operating income (loss) from operations	(984)	(1,157)
Air Pollution Control [Member]
Revenues from external customers	2,204	907
Cost of sales	(1,429)	(531)
Gross margin	775	376
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from operations	775	376
FUEL CHEM [Member]
Revenues from external customers	3,331	4,126
Cost of sales	(1,816)	(2,144)
Gross margin	1,515	1,982
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from operations	1,515	1,982
Other Segments [Member]
Revenues from external customers	0	0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	(3,054)	(3,100)
Research and development	(220)	(415)
Operating income (loss) from operations	$ (3,274)	$ (3,515)