Note 11 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenues from external customers	$ 5,033	$ 3,778
Cost of sales	(2,675)	(2,251)
Gross margin	2,358	1,527
Selling, general and administrative	(3,100)	(3,886)
Research and development	(415)	(324)
Operating income (loss) from continuing operations	(1,157)	(2,683)
Air Pollution Control [Member]
Revenues from external customers	907	1,196
Cost of sales	(531)	(765)
Gross margin	376	431
Selling, general and administrative
Research and development
Operating income (loss) from continuing operations	376	431
FUEL CHEM [Member]
Revenues from external customers	4,126	2,582
Cost of sales	(2,144)	(1,486)
Gross margin	1,982	1,096
Selling, general and administrative
Research and development
Operating income (loss) from continuing operations	1,982	1,096
Other Segments [Member]
Revenues from external customers
Cost of sales
Gross margin
Selling, general and administrative	(3,100)	(3,886)
Research and development	(415)	(324)
Operating income (loss) from continuing operations	$ (3,515)	$ (4,210)