* Average daily output at 2.46 mln T in Sept * Sept crude steel output at 73.75 mln T vs 83.24 mln T in Aug * Jan-Sept production up 2% y/y (Adds details, bullets) BEIJING, Oct 18 (Reuters) - China’s daily crude steel output in September fell to the lowest since December 2018, according to Reuters calculations based on official data, as a power crunch in much of the country and environmental curbs slowed down factory activity. The world’s top steel producer churned out 73.75 million tonnes of the metal last month, compared with August output of 83.24 million tonnes data from the National Bureau of Statistics showed on Monday. That is down 21.2% from same month a year earlier. China’s average daily crude steel output was 2.46 million tonnes in September, down 8.6% from average output in August, Reuters calculations showed. In the first nine months of the year, China produced 805.89 million tonnes of steel, up 2% on an annual basis, according to the data. Industrial production in the world’s second-biggest economy had declined recently because of power rationing triggered by coal shortages, impacting sectors such as steel and aluminium. Capacity utilisation rates at 71 electric arc furnaces in China have been falling since late August and slipped to 49.22% in the last week of September, down 21.09% from same period year earlier, data compiled by Mysteel showed. Production at long-process steel plants was also restrained by Beijing’s efforts to reduce smog and carbon emissions and the pledge that 2021 production cannot exceed last year’s record 1.065 billion tonnes. The industry ministry and environment ministry issued a joint statement last week urging more mills in northern China to keep to those cuts from Nov. 15 to the end of 2021. Additionally, from Jan. 1, 2022, to March 15 mills must cut output by at least 30% of their 2021 production to reduce air pollution. Reporting by Min Zhang and Shivani Singh; Editing by Christopher Cushing and Christian Schmollinger Our Standards: The Thomson Reuters Trust Principles.