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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenues from external customers	$ 7,988	$ 8,017	$ 20,736	$ 19,920
Cost of sales	(4,376)	(4,345)	(12,323)	(11,280)
Gross margin	3,612	3,672	8,413	8,640
Selling, general and administrative	(2,966)	(3,273)	(9,126)	(9,201)
Research and development	(513)	(207)	(1,144)	(716)
Operating income (loss) from operations	133	192	(1,857)	(1,277)
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	(2,966)	(3,273)	(9,126)	(9,201)
Research and development	(513)	(207)	(1,144)	(716)
Operating income (loss) from operations	(3,479)	(3,480)	(10,270)	(9,917)
Air Pollution Control [Member]
Revenues from external customers	3,711	2,728	10,692	7,670
Air Pollution Control [Member] | Operating Segments [Member]
Revenues from external customers	3,711	2,728	10,692	7,670
Cost of sales	(2,214)	(1,801)	(7,155)	(5,032)
Gross margin	1,497	927	3,537	2,638
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	1,497	927	3,537	2,638
FUEL CHEM [Member] | Operating Segments [Member]
Revenues from external customers	4,277	5,289	10,044	12,250
Cost of sales	(2,162)	(2,544)	(5,168)	(6,248)
Gross margin	2,115	2,745	4,876	6,002
Selling, general and administrative	0	0	0	0
Research and development	0	0	0	0
Operating income (loss) from operations	$ 2,115	$ 2,745	$ 4,876	$ 6,002