United Airlines has signed the world's biggest sustainable aviation fuel deal, agreeing to take 1.5 billion gallons over 20 years. The airline says the agreement is one and a half times the size of the publicly announced commitments to sustainable aviation fuel (SAF) of the rest of the world's airlines and aims to produce fuel that is 100 percent drop-in replacement for traditional jet fuel. It also surpasses the 2015 agreement with Fulcrum BioEenergy, also a record with an option to take up to 900 million gallons of SAF. The multi-million-dollar investment in Alder will see the cleantech company use pioneering technology to produce sustainable aviation fuel at scale from biomass such as forest and crop waste. READ: AirNZ still on track for zero-emissions aircraft by 2030 Alder, founded by biofuels entrepreneur Bryan Sherbacow, is commercializing a process to produce crude oil it says is "carbon negative, scalable and cost-competitive with the petroleum it replaces". Sherbacow built the world's first SAF refinery utilizing Honeywell technology. United pointed to US Department of Energy figures showing US forestry residues and agricultural residues alone could provide enough biomass energy to generate more than 17 billion gallons of jet fuel and displace 75 percent of the nation's aviation fuel consumption. It said a decision to broadly adopt regenerative agricultural practices that capture more carbon in healthier soil could generate an additional seven billion gallons of SAF and completely replace the current US fossil jet fuel consumption. The Alder fuel will be combined with Honeywell's Ecofining process that uses hydropcessing technology to deliver commercial volumes of SAF by 2025. "Since announcing our 100 percent green commitment in 2020, United has stayed focused on decarbonizing without relying on the use of traditional carbon offsets,'' United chief executive Scott Kirby said. "Part of that commitment means increasing SAF usage and availability since it's the fastest way to reduce emissions across our fleet. "However, to scale SAF as quickly as necessary, we need to look beyond existing solutions and invest in research and development for new pathways like the one Alder is developing. "United has come further than any other airline making sustainable travel a reality by using SAF to power flights. Our leadership gives customers confidence that they are flying with an airline that recognizes the responsibility we have to help solve climate change." United has committed to net-zero emissions by 2050 without relying on traditional carbon offsets. It has recently invested in carbon capture and sequestration and is also backing electric vertical takeoff and landing aircraft. Separately, United has warned workers who refuse to be vaccinated they will not be allowed in the workplace from October 2. Workers with exemptions who routinely come into contact with passengers, including flight attendants and gate agents, will be placed on indefinite leave until the pandemic meaningfully recedes.