Note 1 - Organization and Significant Accounting Policies (Details Textual) $ in Thousands	12 Months Ended
	Dec. 31, 2024 USD ($) shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 shares
Cash	$ 7,879	$ 12,495
Investment Plan, Maximum Amount. Approved	20,000
Unbilled Contracts Receivable	2,075	2,285
Inventory, Net	397	439
Inventory Valuation Reserves	$ 613	613
Number of Reporting Units	2
Goodwill	$ 2,116	2,116
Goodwill, Impairment Loss	0	0
Amortization of Intangible Assets	57	61
Depreciation	403	281
Asset Impairment Charges	0	0
Defined Contribution Plan, Cost	$ 249	$ 233
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Number (in shares) | shares	176,000	270,500	384,500
Class of Warrant or Right, Outstanding (in shares) | shares	2,850,000	2,850,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares) | shares	274,900	246,500
Treasury Stock, Shares, Acquired (in shares) | shares	83,050	0
Treasury Stock, Value, Acquired, Cost Method	$ 95
Employee Retention Credit Receivable	$ 1,677	$ 0
Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Two Customers [Member]
Number of Major Customers	2	2
Concentration Risk, Percentage	27.00%	36.00%
Antidilutive [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Number (in shares) | shares	168,900	390,900
Class of Warrant or Right, Outstanding (in shares) | shares	2,850,000	2,850,000
The 2014 Long-term Incentive Plan [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Available for Grant (in shares) | shares	0
Incentive Plans [Member]
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Available for Grant (in shares) | shares	2,957,150
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized (in shares) | shares	4,215,272
Capitalized Third-party Costs [Member]
Finite-Lived Patents, Gross	$ 26	$ 28
FUEL CHEM Technology Segment [Member]
Goodwill	$ 2,116	$ 2,116
FUEL CHEM Technology Segment [Member] | Revenue Benchmark [Member] | Customer Concentration Risk [Member] | Two Customers [Member]
Number of Major Customers	2	2
Equipment Constructed for Resale [Member]
Inventory, Net	$ 176	$ 207
Spare Parts [Member]
Inventory, Net	221	232
In Process Equipment [Member]
Inventory, Net	$ 44	618
Minimum [Member]
Debt Securities, Held-to-maturity, Maturity Term (Month)	3 months
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]
Debt Securities, Held-to-maturity, Maturity Term (Month)	36 months
Standard Product Warranty Period (Year)	2 years
Maximum [Member] | APC Technology [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period (Month)	18 months
Asset Pledged as Collateral [Member] | Letter of Credit [Member]
Restricted Cash and Investments	$ 2,790
Asset Pledged as Collateral [Member] | Letter of Credit [Member] | Varying Maturity Dates Expiring No Later Than June 30, 2026 [Member]
Restricted Cash and Investments	1,860
Non-US [Member]
Revenues	$ 7,331	$ 5,684
Percentage of Revenue	29.00%	21.00%
CHINA
Cash	$ 548
ITALY
Cash	1,176
CHILE
Cash	$ 159