Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Revenues from external customers	$ 3,778	$ 10,155
Cost of sales	(2,251)	(6,141)
Gross margin	1,527	4,014
Selling, general and administrative	(3,886)	(4,458)
Restructuring charge	0	(595)
Research and development	(324)	(266)
Operating loss from continuing operations	(2,683)	(1,305)
Air Pollution Control Segment
Segment Reporting Information [Line Items]
Revenues from external customers	1,196	5,789
Cost of sales	(765)	(3,889)
Gross margin	431	1,900
Selling, general and administrative	0	0
Restructuring charge		(595)
Research and development	0	0
Operating loss from continuing operations	431	1,305
FUEL CHEM Segment
Segment Reporting Information [Line Items]
Revenues from external customers	2,582	4,366
Cost of sales	(1,486)	(2,252)
Gross margin	1,096	2,114
Selling, general and administrative	0	0
Research and development	0	0
Operating loss from continuing operations	1,096	2,114
Other
Segment Reporting Information [Line Items]
Revenues from external customers		0
Cost of sales	0	0
Gross margin	0	0
Selling, general and administrative	(3,886)	(4,458)
Restructuring charge		0
Research and development	(324)	(266)
Operating loss from continuing operations	$ (4,210)	$ (4,724)

The aggregate cost of goods produced and sold and services rendered during the reporting period.

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