Note 1 - Organization and Significant Accounting Policies (Details Textual) $ / shares in Units, $ in Thousands		12 Months Ended
	Feb. 11, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 shares
Restricted Cash, Total		$ 1,966
Restricted Cash, Current		1,595
Restricted Cash, Noncurrent		371
Unbilled Contracts Receivable		2,348	$ 1,857
Due from Customer Acceptances		549	43
Accumulated Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Net of Tax, Ending Balance		$ 408
Number of Reporting Units		2
Goodwill, Ending Balance		$ 2,116	2,116
Goodwill, Impairment Loss		0	0
Impairment of Intangible Assets (Excluding Goodwill), Total		197	127
Finite-Lived Patents, Gross		553	906
Amortization of Intangible Assets, Total		185	186
Depreciation, Total		$ 663	$ 810
Number of Units with APC Technology		1,200
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance (in shares) | shares		484,500	747,500	932,500
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares) | shares		547,000	728,000
Treasury Stock, Shares, Acquired (in shares) | shares		152,257	140,784
Treasury Stock, Value, Acquired, Cost Method		$ 570	$ 128
Revenue Benchmark [Member] | Customer Concentration Risk [Member]
Number of Major Customers		2	3
Antidilutive [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance (in shares) | shares		584,505	913,000
Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares) | shares		5,600,676
Common Stock, Capital Shares Reserved for Future Issuance (in shares) | shares		2,533,639
APC Technology [Member] | Revenue Benchmark [Member] | Customer Concentration Risk [Member] | One Customer [Member]
Concentration Risk, Percentage			19.00%
FUEL CHEM Technology Segment [Member] | Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Two Customers [Member]
Concentration Risk, Percentage		28.00%
FUEL CHEM [Member] | Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Two Customers [Member]
Number of Major Customers			2
FUEL CHEM [Member] | Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Customer Two [Member]
Concentration Risk, Percentage			11.00%
FUEL CHEM [Member] | Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Customer Three [Member]
Concentration Risk, Percentage			11.00%
Capitalized Third-party Costs [Member]
Finite-Lived Patents, Gross		$ 0	$ 56
Minimum [Member]
Standard Product Warranty Period (Year)		1 year
Minimum [Member] | APC Technology [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Month)		180 days
Maximum [Member]
Standard Product Warranty Period (Year)		2 years
Maximum [Member] | APC Technology [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Month)		1 year 180 days
Chinese Subsidiary [Member]
Restricted Cash and Cash Equivalents, Total		$ 858
Italian Subsidiary [Member]
Cash, Ending Balance		1,111
Chilean Subsidiary [Member]
Cash, Ending Balance		314
Private Placement [Member] | Subsequent Event [Member]
Stock Issued During Period, Shares, New Issues (in shares) | shares	5,000,000
Shares Issued, Price Per Share (in dollars per share) | $ / shares	$ 5.1625
Proceeds from Issuance of Private Placement	$ 25,800
Private Placement [Member] | Subsequent Event [Member] | Warrants Issued in Connection with Private Placement [Member]
Class of Warrant or Right, Number of Warrants Issued (in shares) | shares	2,500,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares) | shares	2,500,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares	$ 5.10
Non-US [Member]
Revenues, Total		$ 3,928	$ 4,585
Percentage of Revenue		17.00%	15.00%