UPDATE 1-Thai PTT plans 20-30 bln baht bonds in 2009

* Plans 20-30 bln baht bonds this year

* Seeks buys in overseas downstream sector

* To conclude affiliate mergers in 6 months (Adds downstream buy plan, affiliate mergers)

BANGKOK, April 10 (Reuters) - PTT PCL PTT.BK, Thailand's largest energy firm, said on Friday it planned to sell bonds worth 20-30 billion baht ($565-$847 million) this year to fund investment and for debt refinancing.

"We will look at market conditions to decide timing," Chief Financial Officer Pichai Chunhavajira told reporters.

The bond sale would come after bond issuance by its affiliate PTT Aromatics PTTAR.BK in April and PTT Exploration PTTE.BK in May, Pichai said. The proceeds would help repay 110 billion baht in debt due in the five years to 2013, he said.

Asia-Pacific's third-largest, listed, integrated oil and gas firm by market value was seeking more acquisitions in upstream energy including coal mines, he said.

It said last month it would buy part of Australian miner Straits Resources's (SRL.AX) coal and salt assets.

PTT ranks behind Petrochina (601857.SS) and Sinopec (600028.SS). The Thai firm runs the country's gas pipeline monopoly and controls more than 30 petroleum, gas exploration, petrochemical and refinery businesses.

PTT would conclude a study on the possibility of merging its four main subsidiaries in the next six months, Pichai said.

The companies involved are PTT Aromatics and Refining PTTAR.BK, top refiner Thai Oil TOP.BK, IRPC IRPC.BK and PTT Chemical PTTC.BK.

At 0829 GMT, PTT shares were up 1.86 percent at 164 baht while the broader index .SETI was 1.2 percent higher. ($1=35.39 Baht) (Reporting by Saranya Suksomkij; Writing by Viparat Jantraprap; Editing by Alan Raybould)