The heavy reliance of the livestock industry of the European Union (EU) on feed protein imports has initiated a transition to alternative protein sources such as grass proteins. Green biorefineries (which process grass into protein and other related bio‐products) are gaining interest in the EU as the EU searches for ways to cut its import of feed proteins, to reduce its reliance on fossil fuels, and to combat climate change. However, the vulnerability of green biorefineries to fossil energy constraints has not been studied. We estimated the energy conversion efficiencies (EE) and the energy return on investment (EROI) of bio‐products from standalone (SGBR) and integrated grass refinery (IGBR) systems using scenario and energy analysis. The base scenario assumes an SGBR that processes grass into protein, fiber, and brown juice. The three IGBR scenarios assume that grass is processed into protein, fiber, and biomethane (Scenario 1); into protein, fiber, heat, and electricity (Scenario 2); or into protein, fiber, heat, and biomethane (Scenario 3). We found that the EE of the IGBR (83%–85%) largely exceeded that of the SGBR (77%) in all cases. Energy returns on investment were lower for grass than for clover‐grass because of the high fertilizer needs of the former. The standard EROIs (EROIstd) for grass protein ranged from 1.6 to 5.4 over the various feedstocks and scenarios evaluated. The EROIstd decreased when the system boundary was expanded to the point of use (EROIpou), or when they were adjusted for quality (EROIqly). Other bioproducts from both SGBR and IGBR also had high EROIstd, and showed similar patterns to that of grass protein (i.e., EROIstd > EROIpou > EROIqly). Although Scenario 1 had a high EE relative to the base scenario, its heavy reliance on auxiliary energy inputs reduced the EROIs of its products. Our analysis showed the strong impacts of brown‐juice recycling in the energy performance of green biorefinery. It thus deserves close attention when designing and implementing a green biorefinery in a given region. With favorable economic conditions, green biorefineries could contribute to the reduction of food and energy insecurity in Europe in a sustainable way. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd