Monday, November 8, 2010: 2:35 PM
251 A Room (Salt Palace Convention Center)
Anaerobic digesters convert a variety of biomass feeds into biogas containing methane and carbon dioxide. It is costly to upgrade the product gas for pipeline applications and combustion is relatively inefficient. Using bromine to activate the methane in the presence of carbon dioxide allows subsequent chemical conversion of the methyl bromide without the need of significant processing of the biogas. The overall methane conversion occurs in a three basic steps, i) bromination under bromine limiting concentrations, ii) reaction of methylbromide to form hydrocarbon products, and iii) recovery of the bromine with a catalyst/metal oxide solid reactant. The process does not utilize a synthesis gas intermediate and is tolerant to common methane contaminants including the high concentrations of carbon dioxide present in biomethane. Process optimization and reaction engineering allows us to use relatively non-selective thermal bromination at modest temperatures ~ 400 C for methane activation with conversions in excess of 75%. Methyl-Br has many reactivity features in common with Methyl-OH and thus the wide variety of products available with methanol feeds (e.g. MTO, MTP, MTG) can also be made with methyl bromide and the hydrocarbon products include aromatics suitable for bio-jet fuel. The high molecular weight of the alkylbromides provides a chemical handle for ease of separation. Use of adiabatic reactors and other process options allow minimization of the requirements for costly unit operations with expensive metallurgy though proper choice of the materials of construction is critical. A potential feature to reduce the process cost is the use of a solid-reactant for recovery of the bromine that minimizes bromine or hydrobromic acid inventories and facilitates efficient recovery of the halogen. Bromine has fundamental advantages over chlorine and advanced reactor designs utilizing moving or switched bed zone reactors offer further process cost reduction options. Preliminary engineering-economic models and pilot plant operation show the process to have particular promise as a commercial route to production of hydrocarbon transportation fuels from biomethane derived from byproducts of existing commercial processes and marginal land crops.