"Green biorefinery" (GBR) could be an alternative option for using grassland biomass. GBR involves applying technology to chemically and physically fractionate (split) biomass such as grass and grass silage into marketable products. From the grass fibre fraction insulation materials can be produced. From the grass juice fraction, proteinaceous products for animal feed and lactic acid (LA) for plastic production (polylactic acid - PLA) can be produced. This paper is the first part in the analysis to develop a blueprint for a first generation Irish GBR system. The focus of this paper is on the technical aspects of developing three GBR system models and subsequent scenario analyses. The three GBR system models were a combination of feedstock system and biorefinery technology; Grass/silage basic technology (GS), Silage basic technology (S) and Silage High Tech (AT). The models, which were integrated mass and energy balances, were then assessed at the three different input volumes, to generate nine scenarios. The scenarios which required further economic analysis in a companion paper to determine their overall feasibility (technical and economic) in the development of blueprint for a first generation Irish GBR system were identified. From the technical analysis outlined in this paper six scenarios were identified which require further economic analysis. These were scenarios generated for the GS, S and AT GBR systems, at two input volume rates. (C) 2011 Elsevier Ltd. All rights reserved.