Note 11 - Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Revenues from external customers	$ 5,461	$ 6,368	$ 12,748	$ 11,903
Cost of sales	(3,465)	(3,690)	(7,947)	(6,935)
Gross margin	1,996	2,678	4,801	4,968
Selling, general and administrative	(2,915)	(2,874)	(6,160)	(5,928)
Research and development	(413)	(289)	(631)	(509)
Operating income (loss) from operations	(1,332)	(485)	(1,990)	(1,469)
Segment Reconciling Items [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	(2,915)	(2,874)	(6,160)	(5,928)
Research and development	(413)	(289)	(631)	(509)
Operating income (loss) from operations	(3,328)	(3,163)	(6,791)	(6,437)
Air Pollution Control [Member]
Revenues from external customers	3,422	2,738	6,981	4,942
Air Pollution Control [Member] | Operating Segments [Member]
Revenues from external customers	3,422	2,738	6,981	4,942
Cost of sales	(2,347)	(1,802)	(4,941)	(3,231)
Gross margin	1,075	936	2,040	1,711
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	1,075	936	2,040	1,711
FUEL CHEM [Member] | Operating Segments [Member]
Revenues from external customers	2,039	3,630	5,767	6,961
Cost of sales	(1,118)	(1,888)	(3,006)	(3,704)
Gross margin	921	1,742	2,761	3,257
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	$ 921	$ 1,742	$ 2,761	$ 3,257