Biomass carbonization in fixed-bed oxidative reactor: experimental campaign and technological developments

Nowadays charcoal can be considered an important vegetable solid matter product due to its large use in various markets, not only as a fuel in domestic cooking but also used for metal refining or solid amendment, and to produce high-value products, such as activated carbon, silicon and chemicals.Biomass carbonization is the process of converting solid, lignocellulosic biomass into charcoal; it is currently operated at several scales, encompassing manual, rudimental batch methods and industrial, continuous systems, with reported throughput up to several tens of thousands ton per year. Reported yields range from 8- 12% for traditional kilns, 12-17% for brick kilns, 14-20% for standard industrial facilities and 25-33% for advanced industrial processes [1]. A major concern with charcoal production facilities is related to volatiles released in the atmosphere: while larger plant operates either a post-combustion of the volatiles or recovers the organic compounds that are being produced, smaller and more rudimentary systems directly vent the offgases, generating plenty of harmful emissions, e.g. product of incomplete combustion, and greenhouse gases. Along with improper post-harvest land and forest management, these emissions represent the main environmental impact of charcoal manufacturing in traditional systems. The lack of coherent, sustained and secure supporting schemes for power generation from biomass can be regarded as an opportunity to promote the transition from energy to bioproducts in the conversion of biomass, and open new business prospects, independently from subsidies and feed-in tariffs. With this aim, since 2013 RE-CORD has been developing an innovative biomass carbonization process (CarbOn,patent issued) for the small-to-medium sized forestry enterprises and is currently operating the first pilot plant, rated for a capacity of 50 kg/h and built from its proprietary layout. The CarbOn pilot plant is a continuous biomass carbonization system based on open top, downdraft technology, operating in oxidative pyrolysis in the temperature range of 500 – 650°C and equivalence ratio (ER) between 0,1-0,2. The pilot is essentially composed of three sections: (1) loading and conversion of biomass; (2) charcoal discharge and cooling system; (3) extraction and combustion of pyrolysis vapours. The plant, made in stainless steel (AISI 304 and 316) and supported on a self-standing 6x2.5m structure, operates with biomass up to 20 %wt. moisture content. The significance of the present work for the scientific community lies in having shown that high quality charcoal can be produced continuously in a cost-competitive, energy efficient and environmentally friendly pilot unit, even at small scale. The first chapter offers a complete a state of the art on lignocellulosic biomass carbonization process as well as a pyrolysis vapour characterization based on existing literature experimental works. Principle information regarding environmental aspects and sustainability related to charcoal manufacturing, uses and charcoal characteristics and qualities are provided. In the second chapter, it is explained in detail functioning and design of the CarbON pilot unit and are also reported the two most representative tests which validated the process. Some preliminary useful observations are described in order to design an industrial demo plant based on the same technology. In chapter three is reported the experimental campaign were two different woodchips were used and where a full characterization of the input/output unit was carried out as well as vapour pyrolysis were sampled and compared to other experiment operating at similar condition. Conversion efficiency was measured by means of char samples characterizing and taking in to account test reactor temperature profile. Using the data based on experimental tests, in chapter four, is evaluated the mass and energy balance of the technology. Based on test result, it has been investigated the combustion of pyrolysis vapour as well as VOC incineration, in view of design anindustrial demo carbonization plant of 250 kg/h capacity. A complete system solution has been proposed with a layout and process scheme. Finally, an economic scenario of a possible adoption of the demo unit coupled with a SRF plot was studied to estimate the investment profitability.