Note 12 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Revenues from external customers	$ 27,081	$ 26,941
Cost of sales	(15,425)	(15,298)
Gross margin	11,656	11,643
Selling, general and administrative	(12,803)	(12,275)
Research and development	(1,511)	(895)
Operating income (loss) from continuing operations	(2,658)	(1,527)
Air Pollution Control [Member]
Revenues from external customers	13,483	10,597
Cost of sales	(8,410)	(6,924)
Gross margin	5,073	3,673
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from continuing operations	5,073	3,673
FUEL CHEM [Member]
Revenues from external customers	13,598	16,344
Cost of sales	(7,015)	(8,374)
Gross margin	6,583	7,970
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from continuing operations	6,583	7,970
Other Segments [Member]
Revenues from external customers	0	0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	(12,803)	(12,275)
Research and development	(1,511)	(895)
Operating income (loss) from continuing operations	$ (14,314)	$ (13,170)