Note 12 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Revenues from external customers	$ 25,133	$ 27,081
Cost of sales	(14,510)	(15,425)
Gross margin	10,623	11,656
Selling, general and administrative	(13,761)	(12,803)
Research and development	(1,564)	(1,511)
Operating income (loss) from continuing operations	(4,702)	(2,658)
Air Pollution Control [Member]
Revenues from external customers	11,242	13,483
Cost of sales	(7,050)	(8,410)
Gross margin	4,192	5,073
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from continuing operations	4,192	5,073
FUEL CHEM [Member]
Revenues from external customers	13,891	13,598
Cost of sales	(7,460)	(7,015)
Gross margin	6,431	6,583
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from continuing operations	6,431	6,583
Other Operating Segment [Member]
Revenues from external customers	0	0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	(13,761)	(12,803)
Research and development	(1,564)	(1,511)
Operating income (loss) from continuing operations	$ (15,325)	$ (14,314)