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, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 shares
Class of Warrant or Right, Number of Warrants Issued (in shares) | shares		2,850,000
Proceeds from Issuance of Private Placement		$ 25,812	$ 0
Payments of Stock Issuance Costs	$ 1,783	1,783	(0)
Restricted Cash and Cash Equivalents, Total		1,161
Restricted Cash and Cash Equivalents, Current, Total		891	1,595
Restricted Cash and Cash Equivalents, Noncurrent, Total		270	371
Unbilled Contracts Receivable		1,277	2,348
Due from Customer Acceptances		55	549
Inventory, Net, Total		$ 348	97
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
Amortization of Intangible Assets, Total		157	185
Depreciation, Total		$ 584	663
Number of Units with APC Technology		1,200
Defined Contribution Plan, Cost		$ 206	$ 222
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance (in shares) | shares		434,500	484,500	747,500
Class of Warrant or Right, Outstanding (in shares) | shares		2,850,000	0
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares) | shares			547,000
Treasury Stock, Shares, Acquired (in shares) | shares		15,162	152,257
Treasury Stock, Value, Acquired, Cost Method		$ 52	$ 570
Antidilutive [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance (in shares) | shares		214,500	584,505
The 2014 Long-term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized (in shares) | shares		5,600,676
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares) | shares		2,603,801
FUEL CHEM [Member] | Customer Concentration Risk [Member] | Revenue Benchmark [Member] | Four Customers [Member]
Number of Major Customers		4
Concentration Risk, Percentage		60.00%
FUEL CHEM [Member] | Customer Concentration Risk [Member] | Revenue Benchmark [Member] | Two Customers [Member]
Number of Major Customers			2
Concentration Risk, Percentage			28.00%
Capitalized Third-party Costs [Member]
Finite-Lived Patents, Gross		$ 52	$ 0
Minimum [Member]
Standard Product Warranty Period (Year)		1 year
Minimum [Member] | APC Technology [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Month)		6 months
Maximum [Member]
Standard Product Warranty Period (Year)		2 years
Maximum [Member] | APC Technology [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Month)		18 months
Equipment Contstructed for Resale [Member]
Inventory, Net, Total		$ 227
Spare Parts [Member]
Inventory, Net, Total		121
Chinese Subsidiary [Member]
Restricted Cash and Cash Equivalents, Total		886
Italian Subsidiary [Member]
Cash, Ending Balance		1,391
Chilean Subsidiary [Member]
Cash, Ending Balance		$ 244
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	$ 5.10
Warrants and Rights Outstanding, Term (Year)	5 years 6 months
Class of Warrant or Right, Outstanding (in shares) | shares		2,500,000
The Placement Agent Warrants [Member]
Class of Warrant or Right, Number of Securities Called by Warrants or Rights (in shares) | shares	350,000
Class of Warrant or Right, Exercise Price of Warrants or Rights (in dollars per share) | $ / shares	$ 6.45	$ 6.45
Warrants and Rights Outstanding, Term (Year)	5 years 6 months
Class of Warrant or Right, Outstanding (in shares) | shares		350,000
Private Placement [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	$ 25,812
Payments of Stock Issuance Costs	$ 1,783
Private Placement [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		$ 4,746	$ 3,928
Percentage of Revenue		20.00%	17.00%