Crews from Tumble and Llandeilo were called to a farm in Maesybont, Llanarthne, at 20:40 GMT on Thursday.
A "Coldcut Cobra" ultra-high pressure water jet, which is on trial with Mid and West Wales Fire and Rescue Service, was used to tackle the blaze.
An environmental protection unit was also sent to reduce the risk of pollution.

The posts will be cut over three years during restructuring after the £11bn merger of Standard Life and Aberdeen Asset Management.
The two companies currently have  a combined worldwide workforce of approximately 9,000 people.
The information is contained in a detailed prospectus published by Standard Life on the deal.
Managers said they expected "natural turnover" to account for some of the reductions, while other steps will be taken to minimise compulsory redundancies.
The merger of Edinburgh-based Standard Life and Aberdeen is aimed at creating cost savings that could add up to £200m per year.
The prospectus for shareholders stated "there will be a need to maximise operational efficiencies and cost synergies" to achieve the expected benefits of the merger.
It continued: "At this time it is estimated that the integration and restructuring will result in a phased reduction of approximately 800 roles from the total global headcount of the combined group as at 31 December 2016 of approximately 9,000 over the three-year integration period.
"Synergies will come in part from employee departures arising from natural turnover.
"Other appropriate steps will be taken to minimise the number of compulsory redundancies, including the active management of Standard Life's and Aberdeen's recruitment and vacancies."
The document reveals that the newly-merged company will be renamed Standard Life Aberdeen plc.
Both companies have agreed on a 16-strong board made up of an equal number of Standard Life and Aberdeen directors.
Standard Life chairman Sir Gerry Grimstone will be the chairman of the newly-merged company while Aberdeen's chairman Simon Troughton will become deputy chairman.
Keith Skeoch, the Standard Life chief executive, and Aberdeen boss Martin Gilbert will become co-chief executives of the new firm.
The two companies agreed the terms of the merger, which will create Britain's biggest asset manager, in March.
Under the terms of the deal, Aberdeen shareholders would own 33.3% and Standard Life shareholders would own 66.7% of the combined group.
A general meeting has been scheduled for June at which shareholders will be asked to approve the merger. If backed, the deal is expected to be closed by mid-August.

Stuart Clancy, a Conservative councillor on Broadland District Council in Norfolk, was arrested by police in September 2014.
The Crown Prosecution Service said there was "insufficient evidence for there to be a realistic prospect of a conviction for any offence".
The council, on which he represents Taverham South ward, welcomed the news.
"We are very pleased that this issue has been resolved and Councillor Clancy can now put this difficult period behind him," said a spokeswoman.
The council said the investigation was not connected to his council work.

Lawrence, who competed for Wales at the 2014 Commonwealth Games, finished her swim, bike and run in a total time of four hours, nine minutes and 11 seconds on the Sunshine Coast.
The 26-year-old was nearly two minutes ahead of Australian two-time winner Melissa Hauschildt.
Fellow Welsh competitor Leanda Cave won the same title in 2012.
A large timber and tyre fire which burned throughout the night in Carmarthenshire has been extinguished.
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About 800 jobs are to be cut in a planned merger of two of Scotland's biggest financial companies.
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No further action is to be taken against a deputy council leader arrested on suspicion of fraud.
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Swansea triathlete Holly Lawrence has won the women's half ironman 70.3 World Championship in Australia.