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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Revenues from external customers	$ 4,401	$ 8,948	$ 8,179	$ 19,103
Cost of sales	(3,799)	(5,050)	(6,050)	(11,191)
Gross margin	602	3,898	2,129	7,912
Selling, general and administrative	(2,755)	(4,455)	(6,641)	(8,913)
Restructuring charge	0	(30)	0	(625)
Research and development	(271)	(205)	(595)	(471)
Intangible assets abandonment	0	(51)	0	(51)
Operating loss from continuing operations	(2,424)	(843)	(5,107)	(2,148)
Air Pollution Control Segment
Segment Reporting Information [Line Items]
Revenues from external customers	1,937	4,803	3,133	10,592
Cost of sales	(2,320)	(2,975)	(3,086)	(6,864)
Gross margin	(383)	1,828	47	3,728
Selling, general and administrative	0	0	0	0
Restructuring charge		(30)		(625)
Research and development	0	0	0	0
Intangible assets abandonment		0		0
Operating loss from continuing operations	(383)	1,798	47	3,103
FUEL CHEM Segment
Segment Reporting Information [Line Items]
Revenues from external customers	2,464	4,145	5,046	8,511
Cost of sales	(1,479)	(2,075)	(2,964)	(4,327)
Gross margin	985	2,070	2,082	4,184
Selling, general and administrative	0	0	0	0
Restructuring charge		0		0
Research and development	0	0	0	0
Intangible assets abandonment		0		0
Operating loss from continuing operations	985	2,070	2,082	4,184
Other
Segment Reporting Information [Line Items]
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	(2,755)	(4,455)	(6,641)	(8,913)
Restructuring charge		0		0
Research and development	(271)	(205)	(595)	(471)
Intangible assets abandonment		(51)		(51)
Operating loss from continuing operations	$ (3,026)	$ (4,711)	$ (7,236)	$ (9,435)