The challenge is extracting the methane from the wood waste so that it meets the existing strict standards of the natural gas grid. Gasification and pyrolysis technologies that produce syngas (an intermediate gas that cannot be directly injected onto the pipeline) are well established. However, the production of natural gas with a sufficient methane content, greater than 95% by volume, requires the additional steps of cleaning impurities and methanation of carbon monoxide and hydrogen. Methanation is an established process, but integrating it with the production of syngas at a commercial scale for a reasonable cost has never been achieved in North America. FortisBC is an energy solutions provider in BC serving approximately 1.2 million customers. The company is the primary natural gas utility in BC, and also offers electricity, propane, and alternative energy solutions to its customers. It has approximately 49 000 km of natural gas infrastructure in place, delivering energy throughout the province – infrastructure that can be used to deliver RNG. FortisBC has recently signed an off-take agreement with REN Energy International, which is working on putting these pieces together at its facility in Fruitvale, BC. This agreement will see FortisBC purchasing approximately 1 million GJ of pipelinequality RNG annually. REN will further develop the applications of this technology first brought forward by GoBiGas Sweden, the world’s first such large-scale plant built. When REN’s facility is operational, it will be the first commercial wood waste-to-RNG facility operating in North America.
The RNG pathway The process being used at REN’s facility will have three high-level stages – gasification, gas cleaning, and methanation. An indirect fluidised bed gasifier will be used to produce syngas in the first stage. Direct combustion is not practical for this application because it would require the stripping of oxygen, nitrogen, and
nitrous oxide later in the process. Instead, unreacted solid material is drawn off into a combustor and burned in the presence of air. This process creates external heat for the reactor. Heat transfer is improved with the continuous circulation of sand, which is heated in the combustor and then circulated into the main gasifier. In the gasifier, heat breaks down the complex organic molecules of the wood into simpler components. Syngas produced from wood waste has a number of impurities, but the most troubling impurities are tar compounds. Tar refers to compounds that usually consist of multiple aromatic rings that arise from molecules, such as lignin, that have not been completely broken down in gasification. They are tricky to work with as they condense at high temperatures and can foul heat exchangers and process piping. Instead, this facility will rely on oil-based stripping. Using oil as a stripping medium allows natural gas cooling and tar stripping to occur at the same time. Using oil for both purposes will protect downstream equipment from tar condensation because the natural gas leaving the oil strippers will already have been cooled below the tar condensation temperature. The saturated oil is recovered using heat to separate the oil from the tars. The oil is then recirculated, while the tars are recycled to the gasifier. The final stage is the generation of methane. This happens in two steps. The first step is increasing the proportion of hydrogen in the gas by using the water-gas shift reaction to produce hydrogen from water and carbon monoxide. After this step, a catalytic Sabatier reactor is used to produce methane. Final polishing steps are required to strip out any remaining carbon dioxide, recycle hydrogen and carbon monoxide, and remove water to meet the RNG specification for pipeline injection. Each of the processes described are not major innovations by themselves. The innovation is connecting these systems together to create a process that is sufficiently responsive to changes in chemical composition and can maintain a robust uptime. The other major change is an order of magnitude increase in plant size compared with typical biogas plants in BC. REN is expected to generate over 1 million GJ/y, with existing facilities ranging from approximately 20 000 - 100 000 GJ/y.
Decarbonisation for utilities and forestry
Figure 1. A biomass crane handling wood waste.
Figure 2. A block flow diagram of the gasification and methanation process.
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A demonstrated process of this kind will go a long way in meeting BC’s ambitious goals for decarbonisation, as conventional sources of RNG in BC are limited. A 2016 study by Hallbar, a biogas consulting company, put the technical potential of biogas-derived RNG in BC on the order of 8 ‑ 12 million GJ/y. The addition of wood waste as a feedstock for RNG production increases the technical potential by between 41 - 83 million GJ/y, which brings the total potential to roughly 25 - 47% of a FortisBC customer’s annual usage. This avenue for RNG production, despite its novelty, is the most important for the long-term future of RNG production. The production of RNG is part of the development of a circular economy, creating a number of positive externalities for both the climate and the economy. In this case, the biggest change is in waste management in the forestry industry and with municipal demolition and land clearing (DLC) waste. In the forestry industry, finding end uses for bark and waste
