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Biomass Pyrolysis Comes of Age

By Dr. Lorenz Bauer, Lee Enterprises Consulting

After many years of development biomass pyrolysis is finally maturing as a commercial technology.  Pyrolysis is not incineration.  The goal is to use heat in the absence of oxygen to convert the biomass to a more useful form while preserving as much of the carbon as possible.  The water and oxygen associated with the carbon is reduced.   In concept, pyrolysis is the simplest and lowest cost option for converting biomass to fuels, chemicals, and other useful products.  Markets for the products from biomass pyrolysis are emerging and more technology providers are moving from the demonstration to the commercial scale.   The current and future market sizes and prices for the biogas, biooil, biocoal, biochar and wood vinegar products from commercial plants can be more clearly estimated.

A wide variety of materials can be processed using pyrolysis.   There many sources of low value materials like agricultural waste, forestry by-products, and burned trees.   Pyrolysis is particularly suited for converting high lignin content materials that do not compete with food production.    Recently there has been renewed interest in cofeeding mixed plastic and animal wastes.  Currently available products include syngas, liquid bio oil, char, and wood alcohol.   Power production is the main application but agricultural uses and chemical production are growing in importance.  The interest in pyrolysis is demonstrated by the over 1,000 projects of various sizes that have been reported in the last 15 years.  While many of these are no longer active there is a large number of projects still operating.  Interest in new plants and technologies continues, and there are many groups with opportunity feeds they want to convert via pyrolysis.

Technologies and Economics

Pyrolysis is use in several different processes.   The most common are referred to as fast pyrolysis, slow pyrolysis and gasification.   Products include liquid hydrocarbons, char, biogas and an aqueous phase that contains organic acids.   The product distribution depends on the temperature, residency time, feed pretreatment, and the equipment used.

The economics of pyrolysis technologies are highly variable.  Production costs depend heavily on local feed availability and costs.  This makes smaller scale mobile units particularly attractive.  Areas with reliable sources of feed within a close radius of a potential plant location are preferred.  Biomass pyrolysis can provide an economic stimulus to rural areas. Locations with an existing forestry industry are very attractive.     Waste handling sites near large population centers are considering pyrolysis for their mixed agricultural and other wastes.   The European Union and California have severely restrictive landfill policies that make the adoption of economically marginal processes practical due to reduced tipping costs.   Countries that want to preserve foreign currency and promote the use of local resources in Asia and Africa have also shown interest.   The pyrolysis plants also provide the opportunity for local power generation for areas without the grid infrastructure to support centralized power production.   These factors can make biomass pyrolysis plants viable even at the current low prices for the competitive fossil fuels.  However, the primary source of interest in the technology is related to reduction of the use of fossil carbon.   Establishing regulations requiring the use of renewable carbons and the availability of carbon credits can tip the economics in favor of pyrolysis.

Bio Oil and Fast Pyrolysis

Fast pyrolysis came to the attention of the biofuel community because it produces high yields of liquid product called bio oil.   The process relies on high temperatures and short residency times.  There are close to 100 fast pyrolysis projects in progress, and there have been 10 commercial scale plants either built or under construction.

The goal of the development effort is to use this hydrocarbon rich biooil to produce a fuel that could replace crude oil as starting material for transportation use.   The yields and properties of biooil are highly variable and depend on process conditions. Some of the initially produced materials were very unstable and corrosive with very high organic oxygen contents.  It was difficult to separate the biooil from the aqueous phase produced in the process.    Development efforts have focused on producing biooil with oxygen contents of less than 25 wt% of the oil.  This allows easier separation and improves the the quality of the oil.   Unfortunately this improvement comes at the expense of lower yields of useful carbon.

As produced biooil, without upgrading, is becoming a commodity product with a clear set of specifications.  The ASTM has published a standard method D7544 that includes heat value, density and solids contents for two separate grades.   Commercial sized plants are operating and more are planned to start production in the next two years.   By 2018 the pyrolysis oil production will exceed 500,000 tons per year.  The price of biooil is comparable to that of industrial wood chips on an energy per dollar basis.   However, biooil has a clear advantage in ease of handling and reduced storage costs.  Bio-oil is competitively price with fuel oil in many markets.     The price of Canadian pyrolysis oil delivered to Rotterdam in 2014, ~$13 per GJ, was comparable to that of heating oil in most markets (~$2 per gallon) without any environmental credits.  However, current oil prices would need to rise above $55 barrel for the pyrolysis oil to have price advantage over fuel oil.   The biooil will first be used in industrial applications.  It requires some modifications to equipment to allow its use in smaller generators and combustion engines.  It can be potentially blended with biodiesels and other fuels.

The biooil produced from fast pyrolysis can be stabilized and upgraded by a variety of techniques including separation, derivatization, hydroprocessing, and other techniques to fuels more compatible with current equipment and infrastructure.   The U.S. government’s NREL lab estimates the minimum selling price per gallon of a drop-in fuel made from current fast pyrolysis oil is about $2.53 per gallon.

Adding a catalyst to the pyrolysis process produces a higher quality product that can be more easily upgraded to drop in fuels that contain only molecules found in current hydrocarbon fuels.  This process was demonstrated on a commercial scale by KiOR.  There is a continued effort by many groups to develop more carbon efficient and lower cost catalysts.    NREL estimated the minimum selling price of a gallon of gasoline derived from catalytic pyrolysis could eventually be as low as $1.28 per gallon.  Promising approaches to lower costs, more selective catalysts have been identified and are being pursued by several groups.

Biooil can also be used as a source of useful chemicals.   The biooil contains valuable substituted phenols and aromatics that can potentially be separated and sold at a significant premium over fuel.  There are several groups pursuing this option include Ensyn, UOP,  Anellotech, and others.

Char, Wood Vinegar and Slow Pyrolysis

There are over 380 slow pyrolysis projects at various stages of implementation.   Slow pyrolysis produces two major products, a solid char or biocoal and an aqueous liquid called wood vinegar.  The properties of the products are highly dependent on the feedstocks and process conditions so it is difficult to clearly define the market and potential price.

Humans have been making charcoal by pyrolysis for 1000’s of years.  The charcoal making process was the precursor of continuous slow pyrolysis process.   Depending on the temperature and the residency time the products are primarily solids which are called either biocoal or char depending on the severity of the process.

Biocoal is a direct replacement for coal in power applications.  It can be used in combustion boilers or as a feed for gasifiers.    Making biocoal in a 50 ton per day plant costs about $230 per ton.  Coal prices have been depressed and have been at about $55 per ton in 2017.   Substituting biocoal only makes economic sense in markets where there are economic or regulatory benefits to replacing fossil carbon.

Biochar differs from biocoal in its absorption capacity and moisture content.  It is prepared under more severe conditions.   Currently, a significant amount of biochar that is produced is used as feeds to produce syngas.

A potentially more valuable market for biochar as a soil amendment agent is emerging.  Currently the market is primarily for high value crops like nuts and fruits.  It is considered highly desirable by organic farmers.    According to one source, 280 kilotons of biochar were produced in 2015.   The market was predicted to grow to over 800 kilotons in 2025.

The costs and benefits of biochar are still uncertain.  Current prices are about $1,000 per ton.  The production costs should be similar to the $230 ton cost of biocoal.    Many analyst believe the market price will drop to closer to production costs as supplies increase.   The benefits of biochar to agriculture are still being proved scientifically.   The organic farmers strongly believe in the benefit and are willing to pay a premium price of between $200-400 per acre.   The high cost for use is due to large amounts of biochar that need to be added to the soil to see a significant benefit.  Home gardeners are paying up to $4 per pound.

Part of the reason for this interest is that biochar is marketed as a natural product. The carbon in the biochar is locked in the soil so the char can qualify for environmental credits since carbon is derived from atmospheric carbon dioxide.   It is possible to produce similar products from other “chemical” carbon sources at lower production costs.  However, these would not qualify for environmental credits.

Slow pyrolysis produces a significant amount of an aqueous fraction containing organic acids that has been called wood vinegar.  Wood vinegar has been used in Asia for several decades as an agricultural chemical.  There are numerous reports that it improves plant growth and is a “natural” method for insect control.  It is of interest to organic farmers.

The wood vinegar is decanted from a multiphase mixture of the vinegar, bio-oil, and tar and contains some of these latter materials.   These minor components include phenolic compounds, ester, acetals, ketones, formic acid, and many others. These minor components in the wood vinegar may be critical to many of the applications of the material.  They also can present problems because of their potential toxicity.

Wood vinegar has not reached the status of a commodity chemical anywhere in the world.   Countries that produce wood vinegar commercially include China, Indonesia, Malaysia, Brazil, and Chile.    A recent market study reported that wood vinegar market value would reach $6.7 Million USD by 2022.  However this market is likely understated.   There is a significant effort by agricultural groups in Asia and Australia to promote the use of wood vinegar.  Similarly, a few small companies in the U.S., Europe and Canada are working on market development.  The current wholesale price of wood vinegar on the Asian market is about $4 per gallon.  It is likely this price would drop significantly if the supply of wood vinegar increased, and wood vinegar would find other industrial uses.

Gasification and Biogas

Gasification is the most commonly deployed biomass pyrolysis process with over 450 projects reported; however, a recent study reported that only about 100 are still operating.  The primary product is biogas that can be used for power and heat generation.  The solids and water soluble carbon species generated can be sent to a thermal oxidizer to produce steam for additional power generation.  Small-scale direct biomass to electricity plants have installed costs of $3,000 to $4,000 per kw and a cost of energy of $0.08 to $0.15 per kilowatt hour (kWh).

Gasification is more economical where combined heat and power applications are possible.  However, pyrolysis gasification is significantly more expensive than natural gas plants in the absence of subsidies as long as natural gas is available.  The gasifiers are also more difficult to operate because of the higher production of tar and other by-products.

Gasification is attractive for waste to energy projects because it can be very tolerant of mixed feed sources.  It also produces very low volumes of residue.  Thus, it can capture the value of the tipping fees.  Much of the research involving gasification is aimed at increased it efficiency and operability.  There are a number of projects that are targeting treating mixed agricultural and municipal wastes. Post conversion product cleanup and particular tar removal are a significant amount of the cost of gasification.   Advanced gasification technologies are in the development phase and moving towards commercialization.

Gasification is highly capital intensive and benefits greatly from economy of scale.  Designing economical units that can be located close to agricultural production has been challenging.  The relatively poor track record of initial gasification commercialization has made getting approvals difficult.  Gasification also presents significant barriers in terms of local regulations that were designed to control incineration, electrical generation and chemical producers that have separate permitting processes.

Some of the more active providers of biomass gasification to energy technology in Europe and North America include Babcock & Wilcox Vølund, Agnion Energy Neterra, PRM Energy Systems, PureEnergy Prime Energy, Taylor Energy, and Zeropoint Energy.  Many of these provide pyrolysis systems for plastic, tire and mixed waste conversion.

There are continuing efforts to produce liquid hydrocarbons from the syngas produced by gasification. via small scale Fischer Tropsch. methanation, methanol to olefin and other processes.   As yet, none of these have been convincingly demonstrated at a scale appropriated for biomass processing.  However, there are several companies that are reportedly close to commercial scale demonstration.

Future of Pyrolysis

When the price of oil rises to over $60 per barrel, advanced pyrolysis technologies may make more economic sense and may be more widely adopted.   Cost reduction and higher carbon yields are the main targets of continued research efforts.  However, these are coming at the price of increased complexity that may make operation difficult.  Development for improved methods for upgrading the pyrolysis products to chemicals may also help pyrolysis process economics; however, it will be difficult to justify these costs for smaller plants.

The use of centralized upgrading plants, like refineries, is the obvious solution.  There has been a continuing effort to integrate biooil upgrading into current fossil fuel refineries.  So far these efforts have not been successful for both technical reasons and concerns with risk management.   Recently there has been a successful pilot study coprocessing gas oil and biooil in commercial style FCC reactors that may open a route to processing biooil in a hydrocarbon refinery.

Biomass pyrolysis can replace a significant amount of fossil carbon.  However, at current fossil carbon prices, the pyrolysis products are significantly more expensive.   Some credit for the environmental benefits are needed to justify the expense of the developing and implementing the technologies.   These credits can either be a direct subsidy, a carbon tax, government regulation, or preferably a willingness by end users to pay higher prices for environmentally beneficial products.   Restrictions on landfills and other waste disposal methods can also drive the pyrolysis market.

There are economic benefits to rural industry and agriculture for the implementation of distributed plants that can process biomass.   Markets for biomass pyrolysis products are emerging, particularly in Asia,     Europe, Canada, and California.   However, it is unclear if the size of the markets and the rate of growth will be enough to drive widespread adoption of pyrolysis technology.