Note 12 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Revenues from external customers	$ 26,677	$ 25,133
Cost of sales	(14,294)	(14,510)
Gross margin	12,383	10,623
Selling, general and administrative	(14,050)	(13,761)
Research and development	(2,014)	(1,564)
Operating income (loss) from continuing operations	(3,681)	(4,702)
Air Pollution Control [Member]
Revenues from external customers	8,908	11,242
Cost of sales	(5,093)	(7,050)
Gross margin	3,815	4,192
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from continuing operations	3,815	4,192
FUEL CHEM [Member]
Revenues from external customers	17,769	13,891
Cost of sales	(9,201)	(7,460)
Gross margin	8,568	6,431
Selling, general and administrative	0	0
Research and development	0	0
Operating income (loss) from continuing operations	8,568	6,431
Other Operating Segment [Member]
Revenues from external customers	0	0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	(14,050)	(13,761)
Research and development	(2,014)	(1,564)
Operating income (loss) from continuing operations	$ (16,064)	$ (15,325)