Development of a complete bio-NGV production process based on agricultural biogas at farm scale

Bio-NGV (bio- Natural Gas for Vehicles) is a bio-fuel based on the biogas in which components such as CO₂ and H₂S are removed. But the economical feasibility of available and commercial technologies to perform these removals is limited to 80 Nm³/h of treated biogas. Regarding farm scale biogas plant, fuel needs of 1 fill-up per day would represent the treatment of only 1 Nm³/h of raw biogas. The aim of our work was to develop a solution economically adapted to farm scale to produce bio-NGV from biogas. An alternative and low cost material, BOF slag (Basic Oxygen Furnace slag), was then identified to perform CO₂ removal by accelerated carbonation and H₂S removal by iron sulfidation. The accelerated carbonation feasibility was evaluated in batch and dynamic reactors under laboratory conditions. But, removal capacities obtained were insufficient to be used under real conditions. Lime was then tested and CO₂ removal capacities observed were higher than BOF slag ones but still insufficient. Iron sulfidation by H₂S was also tested under laboratory conditions. Batch reactor and dynamic column investigations led to an interesting H₂S removal capacity of 7,0 - 1,0 gH₂S/kg BOF. Based on this result, performances were also validated under real conditions on the biogas produced on site field. According to the whole experimental data, the complete process of bio-NGV production was designed including the H₂S removal performed by BOF slag, CO₂ removal achieved in two membranes and the 200 bars compression. Finally, the overall economical balance has showed that a total investment of 75 000 Euros with a subsidy rates of 30% and a weekly fuel consumption between 6 and 28 fill-up/week will lead to a profitable project. (author)