Business Segment and Geographic Financial Data - Reporting Segment Net Sales and Gross Margin (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Revenues from external customers	$ 10,155	$ 12,791
Cost of sales	(6,141)	(7,766)
Gross margin	4,014	5,025
Selling, general and administrative	(4,458)	(4,921)
Restructuring charge	(595)	0
Research and development	(266)	(288)
Operating loss from continuing operations	(1,305)	(184)
Air Pollution Control Segment
Segment Reporting Information [Line Items]
Revenues from external customers	5,789	8,583
Cost of sales	(3,889)	(5,597)
Gross margin	1,900	2,986
Selling, general and administrative	0	0
Restructuring charge	(595)
Research and development	0	0
Operating loss from continuing operations	1,305	2,986
FUEL CHEM Segment
Segment Reporting Information [Line Items]
Revenues from external customers	4,366	4,208
Cost of sales	(2,252)	(2,169)
Gross margin	2,114	2,039
Selling, general and administrative	0	0
Research and development	0	0
Operating loss from continuing operations	2,114	2,039
Other
Segment Reporting Information [Line Items]
Selling, general and administrative	(4,458)	(4,921)
Research and development	(266)	(288)
Operating loss from continuing operations	$ (4,724)	$ (5,209)

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

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The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

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