NC State
Nelson, L., Park, S., and Hubbe, M. A. (2018). "Thermal depolymerization of biomass with emphasis on gasifier design and best method for catalytic hot gas conditioning," BioRes. 13(2), 4630-4727.


This paper reviews ways that biomass can be converted by thermal depolymerization to make synthetic gas, i.e. syngas. Biomass, being carbon neutral, is considered as a form of solar energy stored during the growing season by photosynthesis. An effective biomass is one with low moisture and ash content, high lignin content, high calorific value, and small particle size. Woody biomass with low ash content (<1%), nut shells with high lignin content (30 to 40%), and municipal solid waste with synthetic polymers are effective at creating value-added synthetic gases. An allothermal downdraft gasifier produces a low tar syngas (99.9% tar conversion) at 850 oC and provides a simple and low-cost process. Integrated gasification combined cycle (IGCC) improves thermodynamic efficiency. To avoid thermal loss, a hot gas filtration system uses trona sorption material for sulfur and halogen compounds. Secondary systems can use multiple cyclones followed by reactors employing calcined dolomite, olivine, and others for adsorption or reaction with residual sulfur, ammonia, metals, and halogens. Reforming of residual tar to syngas can take place within chambers with ceramic tubes doped with nano-nickel particles. Syngas can then be used in boilers, gas turbines for production of electricity or production of chemicals by Fischer-Tropsch conversion.

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Thermal Depolymerization of Biomass with Emphasis on Gasifier Design and Best Method for Catalytic Hot Gas Conditioning

Larry Nelson,* Sunkyu Park, and Martin A. Hubbe

This paper reviews ways that biomass can be converted by thermal depolymerization to make synthetic gas, i.e. syngas. Biomass, being carbon neutral, is considered as a form of solar energy stored during the growing season by photosynthesis. An effective biomass is one with low moisture and ash content, high lignin content, high calorific value, and small particle size. Woody biomass with low ash content (<1%), nut shells with high lignin content (30 to 40%), and municipal solid waste with synthetic polymers are effective at creating value-added synthetic gases. An allothermal downdraft gasifier produces a low tar syngas (99.9% tar conversion) at 850 oC and provides a simple and low-cost process. Integrated gasification combined cycle (IGCC) improves thermodynamic efficiency. To avoid thermal loss, a hot gas filtration system uses trona sorption material for sulfur and halogen compounds. Secondary systems can use multiple cyclones followed by reactors employing calcined dolomite, olivine, and others for adsorption or reaction with residual sulfur, ammonia, metals, and halogens. Reforming of residual tar to syngas can take place within chambers with ceramic tubes doped with nano-nickel particles. Syngas can then be used in boilers, gas turbines for production of electricity or production of chemicals by Fischer-Tropsch conversion.

Keywords: Biomass; Depolymerization; Catalyst; Gasifier; Thermochemical; Syngas; Gasification; Pyrolysis; Tar; Reforming

Contact information: Department of Forest Biomaterials, College of Natural Resources, North Carolina State University, Campus Box 8005, Raleigh, NC 27695-8005, USA;

* Corresponding author:



Overview of the Article Topics

The main focus of this review article is the unit operations needed to obtain fuel value from the thermal decomposition of biomass, while avoiding operational problems and harm to the environment. As shown in the list of contents, there are five main sections: the introduction, an overview of biomass materials, discussion of thermochemical processes, considerations of different gasifier designs, and discussion of gas cleaning methods. Due to the wide-ranging scope of this article, readers are encouraged to use the index and topic headings to locate information of greatest interest and concern to them.

Biomass as an Energy Source

Biomass has been defined as a biological organic matter (plant or animal) that has stored energy (Bain 2004). Wood is considered the largest biomass source, which includes forest residues, wood chips, sawdust, yard clippings, and part of municipal solid waste (MSW). The Energy Policy Act (EPACT) (Sec. 206(a)(6)(B) of 2005 has defined biomass as “…any organic matter that is available on a renewable basis, including agricultural crops and trees, wood and wood wastes and residues, plants (including aquatic plants), grasses, residues, fibers, and animal wastes, municipal wastes, and other waste materials.” It has been stated that biomass feedstock includes trees, agricultural crops, agricultural residues, animal wastes, and municipal solid waste (Lucia 2015); this feedstock could be used to produce ethanol, diesel, heat, electricity, plastics, solvents, chemical intermediates, pharmaceuticals, adhesives, fatty acids, acetic acid, carbon black, dyes, pigments, and detergents. Biomass, whether it comes from MSW, agriculture, or forest operations, is a viable and important source of energy. Its importance has been increasing due to the fact that it has great potential for biofuels and chemicals. The 2011 Billion Ton Study Update (Male 2015) showed that there was enough feedstock to potentially displace 30% of our current petroleum consumption in the US and reduce COemissions by 400 million tons. The advantages of biomass as an energy source include its abundance, renewability, carbon neutrality, suitability as a hydrocarbon source, contribution to energy security (as a domestic resource), contribution to rural jobs, and protection of air quality (low sulfur content, etc.).

Biomass is being looked at as the most important energy source for the future, and it has been increasing in importance. It has a diverse role including providing energy, food, fabrics, building materials, chemicals, and paper products. There is 5 to 8 times more energy stored through photosynthesis in biomass than we currently could consume from all possible sources (Prins 2005). Kumar et al. (2009) noted that photosynthesis by plants captures approximately 4,000 EJ/year in the form of energy in biomass and food. Biomass energy in the United States has gradually increased from 3.88 quads (quad is a unit of energy equal to 1015 Btu) in 2008 to 4.49 quads in 2013 (Park 2014). In the United States in 2010, biomass provided approximately 4% of the energy used, with 46% coming from wood biomass, 43% from mainly ethanol biofuels, and 11% from MSW (Biomass Renewable Energy 2017). The potential of MSW for energy often has been overlooked. Typical MSW components in landfill that could be converted to energy include paper, yard waste, scrap wood, furniture, pallets, processed lumber, packaging material, tree debris, and plastic. Plastic materials are recalcitrant and if left in landfills they will not degrade for several decades. In the United States, the EPA has collected numbers for generation and disposal of MSW for over 30 years. From 1960 up to 2011, MSW generation has increased in volume from 88.1 million tons per year to 250.4 million tons per year (EPA 2012). In the United States during the 1960’s, the contents of landfill sites were often burnt for volume reduction, and even today in some countries this is still going on because of limited regulations. In 2012, there were 68.62 million tons of paper and paperboard generated and only 64.6% recovered. In addition, there were 15.82 million tons of wood generated and only 15.2% recovered. Plastic, which is high in carbon, generated 31.8 million tons, of which only 8.8% were recovered (EPA 2012). By thermally converting MSW (which is an energy rich carbon sink) to energy, we would:

  • Reduce contaminated soils and ground water
  • Reduce amounts of atmospheric gases such as methane, CO2, ammonia, hydrogen sulfite, CO, and non-methane organic compounds such as trichloroethylene
  • Reduce HAP and VOC’s such as benzene, toluene, and vinyl chloride
  • Reduce land usages for handling MSW
  • Reduce transportation costs for MSW that is hauled out of some states
  • Reduce use of fossil fuels by converting energy from waste

Processes for Thermo-chemical Conversion: Overview

Processes for thermo-chemical conversion of biomass to process heat, biopower, or biofuel include combustion, gasification, pyrolysis, and torrefaction (Bridgwater 2003). For centuries, humans have used biomass combustion for heat, and now it is being used to create biopower through steam and expansion over a turbine. Combustion can be defined as a rapid oxidation of biomass, or municipal solid waste (MSW), occurring at extremely high temperatures and producing high concentrations of gas with minor amounts of char and vapor/liquid. Incineration plants, sometimes called “waste-to-energy” plants, consists of the following components: waste handling and storage bunker, one or more combustion units with bottom ash handling systems, boiler with turbine generator, pollution control system (nitrogen oxide, mercury, dioxin, acid gas, and particulate removal), pollution control test, and emissions stack. Typical combustion plants use fabric filters or electrostatic precipitators for particulate removal, wet scrubbers for removal of sulfur, acids and halogenated compounds, activated carbon for removal of dioxins, furans, and mercury, and nitrogen oxides are eliminated by catalytic reduction. The attractive features of combustion of biomass include the following: a well-developed and commercially obtainable technology, reductions in the volume of solid waste destined for landfills, recovery of energy from controlled combustion of waste, decreases in carbon emissions by reduction in energy from fossil fuel, and reductions in methane generation from landfills. Disadvantages include the high cost to build and operate such a plant, the need for skilled personnel for the operation, and inefficient operation of small-scale plants. Currently, China is building the world largest waste-to-energy plant in Shenzhen that will convert 5,000 tons/day of waste to energy. The plant is expected to be running by 2020.

Gasification methods have been in use for decades (McKendry 2002; Alonso et al. 2010). In the early 1800s, gasification of coal and peat was used for illumination and cooking. Due to the shortage of petroleum, wood-gas generators were used during World War II to create producer gas that powered motor vehicles. Gasification is an endothermic process that converts a carbon rich material, at high temperatures, under partial oxidation into large quantities of combustible gases and lower amounts of char, ash and liquid. The external oxidant can include air, oxygen, water, and carbon dioxide. Gasification of biomass goes through four conversion zones, which include drying, pyrolysis, combustion, and reduction. The combustible gases and volatiles include hydrocarbon gases, hydrogen, tar, carbon monoxide, carbon dioxide, and water vapors. The by-products, which are not vaporized, include char and ash. Char can then be reacted with oxidants to release heat that is used for the endothermic reactions. The quantitative and qualitative productivity of gases formed during gasification are strongly governed by biomass type and size, moisture content, reaction temperature, gasifying agent, ash content, catalyst, pressure inside reactor, and gasifier design. Typical gasifiers designs include fixed bed (downdraft, updraft, crossdraft), fluidized bed (bubbling, circulating), and entrained flow. Methods for gas cleaning include: particulate removal by multiple cyclones, barrier filters, or ESP, tar reforming using mineral and synthetic catalyst, and sulfur, nitrogen, and halogen removal can take place using calcined dolomite or nickel and iron based catalyst. Combustible gas can then be used for heat or electricity, or processed into biofuels. Advantages of gasification plants versus waste-to-energy plants include lower capital cost, higher efficiency, small and modular size units, and the fact that syngas can be used for process heat, biofuel, or biopower. The disadvantage of gasification is that some systems such as updraft can produce high volumes of tar, while the downdraft and fluidized bed gasifiers produce large amounts of particulate matter (PM) which require extensive gas cleaning.

Pyrolysis is an endothermic reaction that takes place in an oxygen-depleted atmosphere converting biomass feedstock into gas, oil, and char (Mohan et al. 2006; Bridgwater 2012). Pyrolysis can be categorized as slow, intermediate, fast, and these different processes can determine the yield of gas, oil, and char. Fast pyrolysis, which has reaction times in seconds, produces large amounts of bio-oil, while slow and intermediate pyrolysis produce larger volumes of char. Pyrolysis advantages include a process that enhances energy density and thus reduces transportation and handling costs. There are a number of reactors that are used for pyrolysis including bubbling fluidized bed (BFB), circulating fluidized (CFB), auger reactor, rotating cone, and ablative reactor. Both BFB and CFB reactors have commercial potential for fast pyrolysis due to high oil yields and high heat transfer with fast separation of char and vapors. Typical systems for pyrolysis gas cleanup are similar to gasification due to the fact that the gases can be used in a steam generator for production of electricity via steam turbine. Gas cleanup can include cyclones, tar conversion catalyst, sorbents for S, N, halogen conversion, and fabric filters.

Torrefaction of biomass feedstock is a similar process to slow pyrolysis in that a lower temperature, in the absence of oxygen, removes water with depolymerization and devolatilization of hemicellulose with moderate decomposition of cellulose and lignins (van der Stelt et al. 2011). The final product is a hydrophobic, energy dense solid fuel with lower moisture content and reduced biological presence. Reactors used for torrefaction of biomass include rotating drum, auger screw, and fluidized bed. Gas produced during torrefaction can be sent to a combustor, which is used for torrefaction processes or biomass drying. Gas products are composed mainly of water, acetic acid, aldehydes, alcohols, ketones, and lipids such as terpenes, fatty acid, and waxes.

To improve thermal efficiency, both gasification and pyrolysis systems can use hot gas conditioning methods such as cyclones, guard bed using calcined alkaline earth metals, granular bed filters, and ceramic tubes or fibers doped with nano-nickel based catalyst. Pyrolysis and gasification (the focus of this study) are thermochemical processes that involve thermal depolymerization of biomass. It is worth noting that during thermal treatment, hemicellulose, cellulose, and lignin react differently at different temperatures, which results in a range of products. The conditions of processing can be adjusted in order to target the production of value-added products. Pyrolysis depolymerizes organic material in the absence of air or oxygen at temperatures in the range of 500 to 800 oC, while gasification is relatively higher at 800 to 1,000 oC. Typically, three products are produced: gas, bio-oil, and biochar. Gaseous products are sometimes referred to as syngas or producer gas. Sadaka (2017b) refers to syngas as a mixture of hydrogen and carbon monoxide, which is the product of high temperature reaction between biomass and steam or oxygen. Gandhi et al. (2012) define producer gas as a mixture of combustible gases such as carbon monoxide, hydrogen, and methane and non-combustible gases including nitrogen and carbon dioxide, which are the products of air gasification at low temperatures (1292-1832 oF).

Active Biomass Energy Projects

Lane (2015) in Biofuels Digest has listed the top pyrolysis projects for 2015. They include:

  1. Ensyn Corporation. This corporation, with headquarters in Wilmington Delaware, has produced 37 million gallons of renewable fuel and chemicals over 160,000 hours of operation. Their core business converts non-food biomass from forest and agricultural to yield light liquids. The process they used is called Rapid Thermal Processing (RTP), which produces a renewable fuel oil (RFO) that is used in the heating sector and can also be used as a feedstock for bio-refineries. In 2012, Ensyn entered into a joint venture with Fibria cellulose of Brazil, which is one of the major leaders in pulp production, for production of liquid fuels and chemicals to be used in the United States and Brazil (ENSYN 2017). In May of 2014, Ensyn and Honeywell announced that their pyrolysis procedure was capable of producing fuel at a target price of $45 per barrel. A five-year contract was signed in 2014 with Memorial Hospital in New Hampshire where they would supply 300,000 gallons per year of Ensyn’s renewable fuel oil. This has allowed the hospital to replace their petroleum heating fuels, thereby reducing GHG by 85%. In addition, Ensyn has signed a seven-year renewable contract with Valley Regional Hospital in New Hampshire where they would supply 250,000 gallons of RFO and by this they would eliminate all of their heating oil requirements.
  2. Battelle. Battelle has headquarters in Columbus Ohio and is a nonprofit research and development organization that has over 22,000 employees at more than 60 locations globally. Battelle has partnered with Marathon Petroleum and Pacific Northwest National Lab (PNNL) to produce around 60 gallons of finished hydrocarbon fuel from 1 ton of dry feedstock. They have also succeeded in developing a catalyst that can withstand 1,000 hours of bio-oil hydro-treatment that produced transportation fuel from biomass pyrolysis. Because of its small size, their pyrolysis system can be transported to production sites by a flatbed 18 wheel truck.
  3. Empyro BV. Empyro, located in Henglo, The Netherlands, will produce electricity, steam, and oil from woody biomass and residue. Their technology is based on flash pyrolysis and the experience gain by BTG Technology through a 50 ton/day pyrolysis plant in Malaysia.

Table 1. North Carolina Biomass Facilities

  1. Anellotech. Anellotech, located in Pearl River New York, has partnered with IFP Energies Nouvelles and Axens to commercialize a process for low-cost production of benzene, toluene, and xylene (BTX) from bio-based feedstock including palm wastes, bagasse, corn stover, and woody biomass. This process is based on Catalytic Fast Pyrolysis (CFP) of non-food biomass.
  2. Avello Bioenergy. Located in central Iowa, Avello has partnered with ConTech (EPC), Borrengaard (Product R&D), Cargill (Biofuel oil demo), Virent (R&D), Iowa State (biomass & product R&D), Iowa DOT and USDA as advisors to develop biomass fast pyrolysis, from which products will be in the range of $50-$65/bbl oil equivalent range. Using a rapid heating process (fast pyrolysis), Avello produces Bioasphalt, Chemical Feedstock, Biofuel Oil, and Biochar. The proposed feedstock includes forestry pine residue, mill residue, corn stover, switchgrass, and hybrid poplar.
  3. Proton Power Incorporated (PPI). PPI, located in Lenoir City Tennessee, has developed a renewable system (CHyP-Cellulose to Hydrogen Power) that produces hydrogen from biomass and waste sources. PPI system includes biomass prep, biomass mixer, cellulose to hydrogen power (96% CHyP + 4% biochar), and gas cleanup (65% H2, 30% CO2, 5% CO). The advantages of PPI system include: high yield of H2 (65%) in syngas, very low concentration of tars and particulates (cuts back on expensive syngas clean-up), and a process that can tolerate high moisture (45%) content (which eliminates drying) (Proton Power 2017).

In the 2016 United States Biomass Power Map, there were 227 U.S. Biomass Power Facilities. Table 1 lists the biomass facilities in North Carolina and their capacity, feedstock, and plant status. Of the total 227 facilities, 191 plants were operational, 17 sites were idled, 15 were proposed sites, and four sites were under construction. To qualify for this map, a plant must: 1) supply all or part of their power to the grid, 2) biomass in fuel mix must be greater than 40% by volume, and 3) the plant must have a nameplate capacity equal to or greater than 1 MW. Typical fuel feedstocks that are used in operational units includes corn stover, straw, switchgrass, woody biomass, forest residue, tire-derived fuel, municipal solid waste, logging and sawmill residue, sugarcane, orchard and vineyard prunings, nut shells, stone fruit pits, whole tree chips, rice hulls, bagasse, paper mill sludge, construction and demolition (C & D) material, hogged fuel (grounded up or powdered wood), peanut hulls, paper making residues, biogas, landfill gas, black liquor, softwood, and railroad ties. The four under construction had listed the following fuel feedstocks: sweet sorghum, eucalyptus, albizia, urban wood waste, forest residue, pecan shells, and peanut hulls. The top 10 biomass power producing states include Florida (1,089 MW), California (806 MW), Virginia (523.9 MW), Maine (464.2 MW), New York (450.3 MW), Minnesota (373.5 MW), Washington (288.4 MW), Michigan (282 MW), New Hampshire (266.7 MW), and Connecticut (262.8 MW) (U.S. 2016).

Failed Biomass Energy Projects

Charles Kettering, an American inventor, engineer, businessman, and holder of 186 patents, once said that “99% of success is built on failure”. Below is a list of companies that have had failures in converting biomass to value added products and reasons for failure. The intent here is to highlight the reasons for failures so that they won’t be repeated.

  1. KiOR was founded in 2007 by Khosla Ventures and a group of scientists whose vision was to make renewable fuels from cellulose. The production facility, located in Columbus Mississippi, used a biomass fluid catalytic cracking procedure for turning wood chips into hydrocarbons. These hydrocarbons then would go into vehicles, refineries, or pipelines. Approximately one year after startup, KiOR filed for bankruptcy with losses amounting to $629.3 million. Reasons for failure were: 1) a conveyer system for feeding wood chips frequently jammed, 2) blades for turning wood into chips were the wrong size, 3) tar build-up in part of the plant that was used for treating feedstock, 4) in the lawsuit it was mentioned that the problem was a design issue involving how the equipment was put together too hastily, and 5) the bio-oil conversion to gasoline was very poor, most being converted to CO2 and H2O (Mufson 2014). It was stated in one article that during the initial stage of catalytic fast pyrolysis, the most reactive components were being converted to coke, gas, and water with only a small yield of liquid product (PyroWiki 2017a). KiOR stated in its annual report that “The costs and time involved in operating our Columbus facility have been much higher than we initially anticipated” (Fehrenbacher 2014). KiOR has been very tight about their procedure, but some have suggested that the catalyst system had been destroyed and overwhelmed by the very alkaline ash in the biomass.
  2. Oak Ridge National Laboratory (ORNL) in Tennessee constructed a $60 million plant in 2012 that turned wood chips to gas. The gas was then used as fuel in a boiler. After about a year and a half of operation, the plant was shut down due to problems with thinning of vessels and transfer lines. These problems were caused by weak organic acids. The steam plant was designed by Nexterra, and financing and construction were given to Johnson Controls Inc. (JCI) (Munger 2014).
  3. The University of South Carolina in 2007 was going to take wood byproducts and create steam, which was to be used to supply 85% of the campus energy. There were a few incidents from a steam joint rupturing on December 8th, 2007, an expansion joint rupturing on February 9th, 2008, and on June 28th, 2009 a fuel auger rupturing, sending a metal panel 60 feet toward a control office. Reports stated that in a two-year time frame, the plant provided steam for 98 days. Some USC officials have stated that the plant has been a $20 million disaster that was not properly planned and was built by a company that had never constructed a power plant before. The biomass boiler was built by Nexterra, and Johnson Controls Inc. was in charge of construction of the plant (Washington 2011).
  4. Arbre project, located near Eggborough in the United Kingdom, was built as an 8 MW IGCC plant with low pollution controls based on a circulating fluidized bed gasifier. Production began in 2001 with locally grown wood, and in 2002 the project went into liquidation. The original cost was estimated to be $40 million. Failure was attributed to insufficient control and monitoring, technical problems, and gas cooling and cleaning was described as the major problem (Black &Veatch 2008; Ernsting 2015)
  5. Ebara Corporation commercialized the Bailie Process. The Bailie Process is a dual fluid bed process that permitted the use of air for conversion of biomass to gas. There were three plants in Japan that operated on RDF feedstocks. They included a 36 tpd pilot plant, a 91 tpd, and a 408 tpd commercial plant. All of these plants have been shut down (Klass 1998). No data were found on the reason for shutting down.
  6. Brightstar Environmental is a subsidiary of a company from Australia (Energy Developments Limited-EDL) that develops and operates power generation and waste resource recovery and energy projects. The commercial system was installed in Wollongong, Australia and was based on a two-step gasification unit with a primary pyrolysis reactor followed by secondary steam gasification. Even though their system was successful on a pre-commercial scale, it failed to perform on a full-scale commercial size plant, which resulted in dismantling and financial losses from investors and a loss in value of EDL stock of $120 to 140 million. Problems with this system were: a) design issues with the material handling system, b) removal of hot char (400 to 500 oC) from the primary reactor caused problems due to ignition of char between the primary and secondary gasification reactor, c) a switch to a wet char quenching system, which resulted in more problems due to the wet system not being inert and some deposited with pyrolysis oils preventing carbon conversion and problems with the emission parameters, d) after a lengthy shut down of the secondary reactor, char and pyrolysis oil from primary reactor began to build up, creating a waste and operational problems, e) in 2001, emissions tests showed arsenic exceeded limits, SO3 & NOx levels were high, CO levels were very high, and they also found emissions of dioxin, HCl, HF, and polycyclic aromatic hydrocarbons (PAH). This article also states, “Brightstar’s website admitted to emissions of dioxins, heavy metals, and other chemicals of concern”. The facility was shut down in March of 2004 (Brightstar Environmental 2014; Incinerators 2006).
  7. Thermoselect, a Switzerland-based company, provided the technology for gasification of MSW followed by combustion. The plant was located in Baden-Wurttemberg, Germany and was designed to process 225,000 tons of MSW per year. The plant was temporarily closed in 2000 due to release of toxic gases and operational problems including explosion, cracks in a concrete chamber (due to corrosion and heat), and a leaking basin that held cyanide wastewater. In 2002, the plant was having trouble with high levels of TOC (Total Organic Carbon) and nitrogen oxides and exceeded emission levels for particulates. This article also stated that 120,000 cubic meters of wastewater was disposed of in the Rhine River in 2003. Despite the claims that their technology completely destroys dioxins, furans and other harmful substances found in waste, it was shown that the company’s emissions included dioxins/furans, SO2, CO, HCl, HF, Hg, cadmium, thallium, and other heavy metals. The plant was closed in November 2004, resulting in a loss of approximately $500 M (Incinerators 2006).
  8. Scotgen, waste to energy company, installed a plant at Dargavel in Dumfries, Scotland that was to gasify over 20,000 tons of MSW and hazardous waste per year for production of electricity. The plant began operating in 2009 and was shut down in April of 2011. During that period, they had over 200 breaches on emission limits. Other problems included fires, explosions, pipe burst, steam explosion, and a very low energy recovery. One of the main pollutants was dioxin. The World Health Organization (WHO) listed dioxin as a carcinogen that can cause reproductive and developmental problems and can damage the immune system (The Herald 2003; Waukesha County 2017).
  9. Caithness Heat and Power was started in 2004, in Wick Scotland, as a combined heat and power plant. It was to provide heat to approximately 500 homes in the area and sell power back to the grid. The biomass gasifier was to create gas that had to be cleaned before going to gas engines to create power. After several years of technical and financial problems, the plant was closed with a net lost to tax payers of $11.5 million (Express 2014; McCall 2014; Ernsting 2015).
  10. Biomass Engineering built a 1 MW plant in 2006 on a poultry farm near Calthwaite, UK. The gasifier was to produce gas that proceeded to a gas cleaning area and then to the internal combustion engine. The system did not work and was soon shut down (Ernsting 2015). It is suspected that the gas cleaning system failed because of high ash content from chicken litter (Table 3 Ash content for chicken litter was 19.3%). Elemental analysis of the ash showed high levels for phosphorus, calcium, potassium, silicon, magnesium, and sodium (Bain 2004; Bock 2004).

In summary, failures of systems for biomass gasification to energy have resulted from poorly designed conveyor systems, walls of vessels and transfer lines thinning, steam and expansion joints ruptured, fuel auger ruptured, technical problems, insufficient control and monitoring, major problems with gas cooling and cleaning, material handling system, ignition of hot char removal, emission limits exceeded (in particular dioxins), operational problems including explosions, cracks in concrete chambers due to heat and corrosion, leaks in basins that held waste water, release of toxic gases, fires, pipe burst, energy recovery low, bio-oil conversion to gasoline very poor, and tar build up being major problems.

On the other hand, successful biomass gasification systems have benefited from having excellent technical personnel with companies that have funds, such that they can upgrade and solve problems when they occur. For example, Avello partnered with ConTech, Borrengaard (Product R&D), Cargill (Biofuel), Virent (R&D), Iowa State (biomass and product R&D) and Iowa DOT and USDA.

In light of the mixed record of plant implementation and operation, the purpose of the present study is to:

  • Find the best method for conversion of polycyclic aromatic hydrocarbons (PAH) to lower chain carbons.
  • Learn from past successes and failures to help propose a gasification/pyrolysis system that will convert biomass to chemicals, fuels, or energy.
  • Find the best method for high temperature gas conditioning of particulate matter, haloacids, sulfur compounds, nitrogen derivatives including ammonia, carbon dioxide, heavy metals, dioxins, and furans.
  • Find the most efficient gasifier for biomass conversion to fuels, chemicals or energy.


As will be shown in subsequent sections, the gasification of biomass can be highly sensitive to its chemical composition, particle size, density, moisture, and a variety of other detailed attributes. This section gives an overview of such attributes in the case of biomass types that have been most often considered for gasification.


Lignocellulosic biomass refers to plant materials (mainly softwood and hardwood) whose structures are composed mostly of three polymeric compounds. These include cellulose, hemicellulose, and lignin, with smaller amounts of extractives and inorganics. Lignocellulosic biomass is not an ideal fuel due to its generally low density and heating value, high moisture and ash content, and fibrous nature with low grindability. However, during photosynthesis, biomass stores energy from sunlight. When biomass is combusted during gasification or pyrolysis, this energy can be released in the form of stored chemical energy, or the atoms can rearrange to form syngas or higher molecular compounds such as polycyclic aromatic hydrocarbons (PAH). When considering ways to solve our environmental pollution problems, we need to turn to lignocellulosic biomass, the most abundant material for production of bio-fuels (Lucia 2008).

The chemistry and properties of the main components of lignocellulose have been well described in other sources, so their description here will emphasize issues pertaining to pyrolysis. Briefly stated, cellulose is a polymer of glucose monomers (McKendry 2002) having a typical molecular mass of 100,000 g/mole. Hemicellulose, like cellulose, is a polysaccharide but has a branched polymer structure consisting of C5 and C6 carbon sugars. It is a co-polymer of two or more sugars and sugar acids with monomers that can include glucose, mannose, galactose, arabinose and 4-0-methyglucuronic acid, and it has a low DP of 120-200 (Park 2014). Like cellulose, it is a carbohydrate and thus has the formula Cn(H2O)n. Lignin is a large biopolymer molecule composed of phenylpropanol units; its structure is complex, highly variable, and amorphous, with a branched three-dimensional dendritic network (Campbell and Sederoff 1996; Novaes et al. 2010; Park 2014). Lignin’s energy content is 40% of the total energy content of ligno-cellulosic biomass (Leisola et al. 2012). Percentages of the main components can vary considerably from one biomass to another. Table 2 list cellulose percentages from woody biomass to herbaceous and agriculture biomass to other biomasses. For example, on the low side, leaves and grasses can have very small amounts ranging from 15 to 25% cellulose, while paper and cotton seed hairs can have on the high side 95 to 99% cellulose (Table 2, Dakar 2017; Goyal 2017).

Table 2. Percent Cellulose, Hemicellulose, & Lignin of Various Biomass Materials (Dry Basis)

Extractives from lignocellulosic biomass are low molecular weight compounds such as terpenes, tall oil, fatty acids, esters, triglycerides, waxes, resins, tannins, polyhydric alcohols, alkaloids, starches, pectins, and phenolics. The extractives function to protect trees from insects and fungi and can have a rather high heating value of 35±2 MJ/kg (The Bioenergy System Planners Handbook 2017).


Inorganic mineral, also called ash, in biomass is the residue left after ignition or incineration and is often combined with oxygen. It consists of minerals comprising silicon (SiO2), aluminum (Al2O3), calcium (CaO), magnesium (MgO), phosphorus (P2O5), sulfur (SO3), iron (Fe2O3), potassium (K2O), and sodium (Na2O). As noted in Table 3, the ash content can be quite high, as in agricultural and herbaceous biomass. Herbaceous biomasses contain an order of magnitude more ash than wood biomass (Henrich et al. 2008; Sikarwar et al. 2016). For example, the ash content is very high in rice hulls (18.34%), sorghum stalks (12.50%), wheat straw (11.40%), sugarcane bagasse (9.79%), and corn stover (6.26%) (Table 3). However, the ash content for wood and woody biomass is lower as noted for black locust (0.97%), poplar (1.16%), ponderosa pine (0.30%), white fir (0.20%), and Douglas fir (0.01) (Table 3).

Table 3. Ultimate Analysis of Various Biomass Materials

Ultimate Analysis (% dry weight)

Table 4. Percent Elemental Ash Composition of Different Biomass Materials

Ash content plays a vital role in combustion, gasification, and pyrolysis. Klinghoffer (2013) found that char from gasified poplar wood was acting as a catalyst in depolymerization of tar compounds. He discovered that the char had active minerals on the surface, and when these metals were removed by acid washing, the catalytic activity fell by 19%. Catalytic activity was also attributed to the high surface area of char, which was stated as being higher than most commercial catalysts.

Alkaline earth oxides including calcined dolomite CaMg(CO3)2 have proven to be very effective at thermally depolymerizing polycyclic aromatic hydrocarbons (PAH) (Alden et al. 1988; Orio et al. 1997a,b; Dayton 2002; El-Rub et al. 2004; Balas et al. 2008; Yung et al. 2009; Zwart 2009; Nemanova et al. 2010; Ay et al. 2012; Milhe et al. 2013; Mohammed et al. 2013; Akudo and Theegala 2014). On the negative side, ash under high temperature, i.e. combustion and gasification, can cause corrosion, sintering, slagging, deposition, and agglomeration (sticking of metals to solid surface). Group I metals such as potassium can react with silica and sulfates, forming potassium silicate (K2SiO3) and potassium sulfate (K2SO4). Wang et al. (2008) stated that both compounds are capable of depositing on reactor walls, leading to sintering and defluidization problems. The alkali salts can react with silica to form a low melt eutectic mixture, which can lead to agglomeration. Klein and Themelis (2003) reported that the main elements causing alkali slagging were potassium, sodium, chlorine, and silica. Sahni et al. (2015) observed no slagging for biomass having ash content below 5 to 6%, while severe slagging can be expected for biomass with 12% and above. From Tables 3 and 4, one can see that problems could develop if the feedstock was 100% rice hulls, sorghum stalks, or wheat straw. These problems are due to high levels of ash and the ash containing high percentages of SiO2. Also, syngas needs to be free of alkali metals before being sent to gas turbines due to corrosion and deposition problems.

Thermal Degradation

During thermal destruction, cellulose, hemicellulose, and lignin are the main polymers that undergo depolymerization. Carbohydrates (hemicellulose and cellulose) are the least stable and begin to depolymerize first. Hemicelluloses, which contains heteropolysaccharides, are the first to thermally degrade (Rivilli et al. 2011), and it has been stated that hemicellulose content decreases from approximately 85% to 60% at 330 °C (Park et al. 2013). Lv et al. (2010) noted that hemicellulose thermally degrades within a temperature range of 180 to 340 °C. Compared to cellulose, hemicellulose undergoes rapid decomposition and produces less tar, less char, and more gas, methanol, and acetic acid (Sadaka 2017a; Mohan et al. 2006). Rivilli et al. (2011) reported that cellulose, hemicellulose and lignin react independently during pyrolysis of different biomass, giving a unique pattern of products.

Park et al. (2013) stated that thermal degradation of cellulose takes place at higher temperatures (300 to 400 °C) compared to hemicellulose. Mohan et al. (2006) noted that cellulose degradation occurs at 240 to 350 °C. This thermal destruction of cellulose produces anhydrocellulose and levoglucosan (Mohan et al. 2006; Jin et al. 2013).

Lignin begins to thermally depolymerize at a similar temperature as hemicellulose due to its heterogeneous structure. However, lignin is also more difficult to dehydrate and produces greater amounts of residual char than cellulose or hemicellulose (Mohan et al. 2006; Jin et al. 2013). Other products produced during lignin pyrolysis include:

  1. Liquid part includes menthol, acetic acid, acetone, and water
  2. Gaseous part include methane, ethane, and carbon monoxide

A high percentage of oxygen content is indicative of biomass material (Table 3), and this high oxygen content leads to dilution of calorific values. It is imperative that during gasification and pyrolysis, decreases in oxygen content take place; this occurs during torrefaction. This reduction in oxygen content is accompanied by the loss of volatile compounds including CO and CO2 and by dehydration reactions (Park et al. 2013).

Fig. 1. Corn being harvested with corn stovers left behind (Source: Lori Potter, Hub Staff Writer, Kearney Hub Newspaper; figure used with permission of the copyright holder)

Worldwide there are approximately 140 billion metric tons of waste biomass produced every year including residual stalks, straw, leaves, roots, husk, nut, seed shell, waste wood, and animal waste. This waste biomass is equivalent to approximately 50 billion tons of oil (Converting Waste Agricultural Biomass 2009).

With the increased demand for ethanol and food exports, there will be a continual increase in the generation of corn stover (Fig. 1). Corn stover by definition includes the leaf, husk, and cob that remain after collecting the grain. As noted in Table 3, corn stover has a heating value of 17.65 MJ/kg (dry) with carbon and hydrogen of 43.65% and 5.56%, respectively. The negatives include high contents of the following: 1) oxygen 43.31%, 2) nitrogen 0.61%, 3) chlorine 0.60%, and 4) ash 6.26%.

Syngas fermentation is being researched heavily. In this process, biomass (corn stover and others) is being thermally converted to carbon monoxide and hydrogen. Several metabolic systems including Clostridium ljungdahlii are being used for the synthesis of products such as ethanol and butanol (van Kasteren et al. 2011; Daniell et al. 2012; Devarapalli and Atiyeh 2015). Whitham et al. (2016) published an extensive review on Clostridium ljungdahlii for the development of industrial biocatalyst and found that this ethanologenic acetogen could grow while producing energy-rich ethanol. They concluded that acetogen would be an excellent choice for industrial application because of its unique physiology, known metabolic pathways, and considerable genetic background work. Syngas fermentation takes place by microbial metabolism, producing bio-fuel. Daystar et al. (2013) used a NREL thermochemical model and SimaPro for GHG analysis and life cycle assessments on corn stover, loblolly pine, eucalyptus, miscanthus, and switchgrass. They found that corn stover had the lowest alcohol yield and highest GHG emissions per liter of ethanol, and this was attributed to its high ash content. Daniell et al. (2012) has stated that while there are many challenges with the scale-up of syngas fermentation, this process has many advantages over regular fermentation and thermochemical depolymerization. These advantages include feedstock flexibility and production cost.

Fig. 2. Wheat Straw (Source: Landscape Unlimited; figure used with permission of the copyright holder)

Rathmann and Illerup (1995) found from pyrolysis experiments on pulverized wheat straw (Fig. 2) that when the heating rate of 30 °C/min (150 to 1000 °C) were used, the char yield increased by 15%, 20%, and 22% as pressure increased from 1.5, 20, and 40 bars, respectively. Because of the high ash content (11.40% Table 3), problems could develop in gasification and pyrolysis including sintering, deposition, high temperature corrosion, and slagging. Shao et al. (2012) noted that straw presented high fouling problems because of the highly active alkali and alkaline metals. These metals could form sticky layers on heat exchangers or heat transfer surfaces by forming vapor phase chloride compounds. Table 3 lists the ultimate analysis for chlorine at 0.28%, and Table 4 list the elemental ash content for potassium at 24.9%. This according to Shao et al. (2012) is the most problematic elements during biomass combustion.

Fig. 3. Cotton Residue (Source: “Photo credit Ollivier Girard-CIFOR, Retrieved picture from Feedipedia; figure used with permission of the copyright holder)

Cotton residue (Fig. 3) is the waste that is left in the field after cotton has been harvested, which includes stalks, leaves, seeds, cotton lint, and bolls. Umesh et al. (2015), using a bomb calorimeter, found that cotton stalk has a value of 16.01 MJ/kg, which as he stated is “good characteristics for gasification because higher heat generated during combustion leads to high temperature in reaction zone”. These results were in accordance with Jenkins and Ebeling (1985), who reported 15.83 MJ/kg for cotton stalks. Although cotton stalk has good values for gasification, there are other herbaceous residues such as alfalfa straw and corn stover that has higher calorific values (Engineering Tool Box 2017) Table 3).

Capareda and Parnell’s research work (2007) showed that bio-oil produced from cotton gin trash (CGT) by thermal conversion is 40% less by weight and 20% by volume than No. 6 fuel oil. The bio-oil density (1.3 g/mL) was much higher than No. 6 Fuel Oil (0.98 g/mL) due to polycyclic hydrocarbons and other heavier fractions. They mention that the bio-oil could be hydrogenated to produce numerous fuel feedstocks and with the use of different catalyst, this feedstock could produce dimethyl ether (DME) and aviation fuel (JP-8). Capareda and Parnell (2007) concluded by saying that “electrical power and heat energy production from cotton gin trash viagasification is already a proven technology”.

Fig. 4. Switchgrass (Dennis Pennington, Bioenergy, Michigan State University; figure used with permission of the copyright holder)

Co-gasification of coal and biomass is a way of lowering problems from both sectors, which includes harmful emissions from coal and low calorific value from biomass. For example, by blending biomass with coal, one would reduce harmful emissions from coal such as heavy metals and sulfur, which is a contributor to acid rain. Some of the elements found in coal and coal waste include: arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), mercury (Hg), antimony (Sb), copper (Cu), sulfur (S), molybdenum (Mo), tin (Sn), magnesium (Mg), manganese (Mn), beryllium (Be), aluminum (Al), barium (Ba), cobalt (Co), iron (Fe), strontium (Sr), nickel (Ni), silver (Ag), selenium (Se), vanadium (V), zinc (Zn), potassium (K), and sodium (Na) (Schweinfurth 2009; Nalbandian 2012).

Biomass has low energy density due to high oxygen and moisture content. This disadvantage will be offset by the high energy density of coal. Masnadi et al. (2015) noted in their experiment with coal and switchgrass (bubbling fluidized bed at 800 to 860 °C) that there was a considerable decrease in tar. They attributed this decomposition of tar to alkali and alkaline earth metals (AAEM) in switchgrass. They concluded that switchgrass (Fig. 4) ash can act as an inexpensive catalyst for gasification, which means that one can lower the gasification temperature without an increase in tar percentages. Tchapda and Pisupati (2014) also found that AAEM dispersed in biomass fuels will induce catalytic activity during co-conversion with coal.