Note 11 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Revenues from external customers	$ 7,490	$ 7,851	$ 19,430	$ 19,850
Cost of sales	(3,829)	(4,444)	(10,281)	(11,462)
Gross margin	3,661	3,407	9,149	8,388
Selling, general and administrative	(3,207)	(3,225)	(9,895)	(9,815)
Research and development	(450)	(361)	(1,510)	(1,159)
Operating income (loss) from operations	4	(179)	(2,256)	(2,586)
Air Pollution Control [Member]
Revenues from external customers	2,707	3,224	6,515	9,491
Cost of sales	(1,428)	(2,095)	(3,713)	(5,927)
Gross margin	1,279	1,129	2,802	3,564
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	1,279	1,129	2,802	3,564
FUEL CHEM [Member]
Revenues from external customers	4,783	4,627	12,915	10,359
Cost of sales	(2,401)	(2,349)	(6,568)	(5,535)
Gross margin	2,382	2,278	6,347	4,824
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	2,382	2,278	6,347	4,824
Other Operating Segment [Member]
Revenues from external customers	0	0	0	0
Cost of sales	0	0	0	0
Gross margin	0	0	0	0
Selling, general and administrative	(3,207)	(3,225)	(9,895)	(9,815)
Research and development	(450)	(361)	(1,510)	(1,159)
Operating income (loss) from operations	$ (3,657)	$ (3,586)	$ (11,405)	$ (10,974)