In the atmosphere, sulfur oxides can lead to acid rain which hurts crops and forests and causes ocean acidification. In March in Ulsan, 414 kilometers southeast of Seoul, Hyundai Heavy Industries and its holding company Korea Shipbuilding & Offshore Engineering Co. set up a center to construct ships powered by both liquefied natural gas ( LNG) engines and fuel cells by the late 2021. In March 2019 Hyundai Heavy received the green light from DNV-GL, a Norwegian-German quality assurance and risk management firm, for the design of a crude carrier with the LNG-fuel cell-propelled engine system. DNV-GL approval allows Hyundai Heavy to accept an order from purchasers for the ship with the device. Samsung Heavy has signed an agreement with Bloom Energy and for the first time in the world has marketed solid oxide fuel cells (SOFCs). SOFCs are power conversion devices which produce electricity.