An olefin synthesis plant comprising: a feed pretreatment section configured to pretreat a feed stream; a pyrolysis section comprising one or more pyrolysis reactors configured to crack hydrocarbons in the feed stream in the presence of a diluent to produce a cracked gas stream; a primary fractionation and compression section configured to provide heat recovery from and quenching of the cracked gas stream; remove a component from the cracked gas stream; and compress the cracked gas stream, thus providing a compressed cracked gas stream; and/or a product separation section configured to separate a product olefin stream from the compressed cracked gas stream, wherein the olefin synthesis plant is configured such that, relative to a conventional olefin synthesis plant, more of the energy and/or the net energy required by the olefin synthesis plant and/or one or more sections thereof, is provided by a non-carbon based and/or renewable energy source and/or electricity.