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, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Revenues from external customers	$ 7,851	$ 7,988	$ 19,850	$ 20,736
Cost of sales	(4,444)	(4,376)	(11,462)	(12,323)
Gross margin	3,407	3,612	8,388	8,413
Selling, general and administrative	(3,225)	(2,966)	(9,815)	(9,126)
Research and development	(361)	(513)	(1,159)	(1,144)
Operating income (loss) from operations	(179)	133	(2,586)	(1,857)
Air Pollution Control [Member]
Revenues from external customers	3,224	3,711	9,491	10,692
Cost of sales	(2,095)	(2,214)	(5,927)	(7,155)
Gross margin	1,129	1,497	3,564	3,537
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	1,129	1,497	3,564	3,537
FUEL CHEM [Member]
Revenues from external customers	4,627	4,277	10,359	10,044
Cost of sales	(2,349)	(2,162)	(5,535)	(5,168)
Gross margin	2,278	2,115	4,824	4,876
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	2,278	2,115	4,824	4,876
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	(3,225)	(2,966)	(9,815)	(9,126)
Research and development	(361)	(513)	(1,159)	(1,144)
Operating income (loss) from operations	$ (3,586)	$ (3,479)	$ (10,974)	$ (10,270)