Introduction
Tar
Sampling & analysis
Tar removal
Process
Projects
Links
Contact
Introduction
Tar is a complex mixture of condensable hydrocarbons, but a unique
definition is lacking. Generally, producer gas from biomass gasifiers
contains tar, which forms a serious problem for its use in e.g. engines
and turbines. BTG has developed the catalytic, reverse flow tar cracking
(RFTC) reactor for conversion of tar in producer gas using a commercial
Ni-catalyst. Besides tar also light hydrocarbons and ammonia are nearly
completely removed.
Tar
The Problem
Trouble-free use of producer gas in a gas engine or gas turbine requires an
acceptable low level of contaminants like: tar mist, acids, hydrogen chloride,
sulfur gases, ammonia and nitrogen compounds, solid dust particles, alkali
metals and heavy metals. Tar will impose serious limitations in the use of
producer gas due to fouling of downstream process equipment, engine wear and
high maintenance costs. By far, tar removal is the most problematic. Thus the
successful implementation of gasification technology for gas engine/turbine
based power projects depends much on the effective and efficient removal/conversion
of tar from the producer gas.
There are still many questions related to tar and the problems they may cause. Tar, itself is a complex mixture of condensable hydrocarbons, which still requires to be satisfactorily defined. It is also necessary to understand its composition and formation in order to design systems for its optimum removal or conversion and for minimizing its formation in the gasifier and interactions downstream to the end use device.
Definition
Many definitions of biomass tar have been given by as many institutions working
on biomass gasification like:
"the mixture of chemical compounds which condense on metal surfaces
at room temperature"
"the sum of components with boiling point higher than 150°C"
"all organic contaminants with a molecular weight larger than benzene"
However, one general (uniform) definition does not exists. Apart from the general
definition of tars, definitions have been given for heavy tars, gravimetric
tars and light tars. Uniform procedures for measuring these different types
of tar are underway, see section Sampling and Analysis.
Formation
When biomass is heated the molecular bonds of the biomass break; the smallest
molecules gaseous, the larger molecules are called primary tars. These primary
tars, which are always fragments of the original material, can react to secondary
tars by further reactions at the same temperature and to tertiary tars at
high temperature. This tar formation pathway can be visualised as follows:
| Mixed Ogygenates | -> | Phenolic Ethers | -> | Alkyl Phenolics | -> | Heterocyclic Ethers | -> | PAH | -> | Larger PAH |
| 400 oC | 500 oC | 600 oC | 700 oC | 800 oC | 900 oC |
General properties
The amount of compounds that occur in biomass gasification tars can be as high
as several hundreds or even several thousands for low temperature tars. The
amount and composition depend on:
- Type and properties of the biomass (moisture, size)
- Gasification conditions (P, T, residence time)
- Type of gasifier
Differences between the varying types of tar can also be found in values for the combustion enthalpy, viscosity, density and acidity. Combustion enthalpies vary from 20 - 40 MJ/kg, which makes it a very useful fuel, i.e. conversion is strongly preferred above removal.
Problem-related properties
The properties of tars that cause problems in biomass gasification systems
are:
- Condensation behaviour caused by the heavier tar compounds that condense on cool surfaces or form tar aerosols when the temperature of the gasification fuel gas is decreased. Tar aerosols occur as a mist or fog that is composed of fine droplets that may be less than 1 µm in diameter. combustion behaviour of liquid tars. Partly combustion of tar can lead to PAH and/or soot formation, which can give problems with wear and corrosion.
- Polymerisation of tars. Polymerisation reactions of tar compounds are well known in the gas phase. At high temperatures (900 - 1250°C) these hydrocarbons crack/pyrolyse to form soot. At intermediate temperatures the polymerisation reactions are rather slow. In the liquid phase, tars tend to polymerise at temperatures of 100 – 200oC. This "coking" phenomena also occurs at room temperature although at a lower rate. Most information of polymerisation is known from pyrolysis oil.
- Interaction with other contaminants like adsorption of tara at particles like fine coal dust. This means that tar is removed by cleaning devices aimed for particle removal like fabric filters and the like.
Sampling & analysis
Several institutes have developed methods for the sampling and analysis
of tars, on-line and off-line. The sampling part of the off-line methods is
based on trapping the tar by condensation on cold surfaces or filters, by
absorption in a cold organic solvent or by adsorption on a suitable sorbents.
The analysis of the tars is most often performed by Gas Chromatography (GC)
or gravimetrically (by weighing the collected tars, after careful evaporation
of the solvent and condensed water). The latter method has been used by BTG
for over a decade in the framework of the worldwide UNDP/World Bank monitoring
program
Recently, on-line methods have been developed and improvements of these methods are being further investigated. A frequently used relatively new method is the Solid Phase Absorption (SPA) technique developed at KTH, Sweden.
Over the last years or even decades it has been recognised that data on tar concentration from different biomass gasifiers cannot be properly compared due to the differences in tar definitions and tar measurement methods. This has been the major reason to start the development of the common tar measurement protocol with an underlying definition of tar. Information on this project sponsored by the European Commission can be found at www.tarweb.net
Tar removal
Tar can be removed from producer gas by chemical and physical methods.
Chemical methods destruct the tar; physical methods only removing the tar
(yielding a tar waste stream). Several devices are available for tar conversion
and removal. It must be noted that most removal devices are primarily meant
for particle removal.
| Tar Conversion (Chemical methods) | Tar Removal (Physical methods) |
| Catalytic cracking | (Condi-)cyclone |
| Thermal cracking | Filters (baffle, fabric, ceramic, granular beds, RPS) |
| Plasma reactors (Pyroarc, Corona, Glidarc) | Electable_rowostatic precipitators |
| Scrubbers |
BTG has developed the catalytic, reverse flow tar cracking (RFTC) reactor for conversion of tar in producer gas using a commercial Ni-catalyst. Besides tar also light hydrocarbons and ammonia are almost completely removed. To counterbalance the endothermic tar cracking reactions small part of producer gas is combusted.
Chemistry
| Chemical reactions | delta H | |
| Tar | C10H8+ 10 H2O = 10 CO + 14 H2 | 1165 kJ/mol naphtalene |
| Methane | CH4 + H2O = CO + 3 H2 | 206 kJ/mol methane |
| Light HC | CnHm + n H2O = n CO + (1/2 m+n) H2 | 10 kJ/g HC |
| Benzene | C6H6 + 6 H2O = 6 CO + 9 H2 | 9.06 kJ/g benzene |
| Ammonia | 2NH3 = N2 + 3 H2 | 56 kJ/mol NH3 |
| Combustion | H2 + 1/22 O + 2 H2O CO + 1/2 O2 = CO2 CH4 + 2 O2 = CO2 + 2 H2O |
- 242 kJ/mol H2 - 283 kJ/mol CO - 804 kJ/mol CH4 |
Process
The Reverse-Flow, Catalytic Tar Converter (RFTC)
has been developed to remove tar from producer in an energy efficient way.
(see also reverse-flow operation )
Raw producer gas from a biomass gasifier is fed to the RFTC at a temperature
between 350 and 650 °C. In the entrance section of the RFTC the producer
gas is heated up to the desired reaction temperature of 900 - 950 °C.
In the central section a commercial Ni-catalyst is placed, and steam reforming
reactions will take place. Tar components - and also light hydrocarbons
including methane - are converted into CO and H2. Additionally, nearly
all NH3 is removed (forming N2 and H2, reverse ammonia synthesis). To counterbalance
these endothermic reactions a small amount of air is added to the reactor
(about 5% of the producer gas flow). Effectively, the heating value of
the gas is reduced slightly, but fully compensated by the increased amount
of gas. Experimental test have been performed with the catalytic reactor,
but the reactor principle can also be applied to thermal tar cracking.
Catalyst stability
In general, catalysts are sensitive to impurities in
the gas, which may lead to deactivation. Sufficient catalyst lifetime
is important for an economic feasible process. The catalyst used in the
RFTC has been tested for over 6000 hrs with wood-derived producer gas.
During this period no detectable change in catalyst activity was observed.
Addition of extra sulphur reduced the activity (to a new stable level),
but still all tar was removed. Ammonia and methane conversion was reduced.
After stopping the additional sulphur supply the original catalyst activity
was achieved again.
Typical conversion data
| Component | Conversion [%] |
| Benzene1 | 82 |
| Napthalene | 99 |
| Phenol | 96 |
| Total aromatic | 94 |
| Total Phenols | 98 |
| Total tar | 96 |
| Ammonia | 99 |
The RFTC has been integrated in the DUIS demonstration plant (see gasification ). That unit will treat about 200 Nm3/hr producer gas.
Reverse flow
Principle of reverse-flow operation
Reverse flow operation is characterized by the periodic reversal of the feed
flow direction. Operating a packed bed reactor under such transient is beneficial
because
- a high degree of heat integration is realized, as the chemical reactor and heat exchanger are both included in a single apparatus, resulting in high energy efficiencies
- the construction is simple compared to conventional reactor/heat exchanger systems, which results in reduced investment costs.
A schematic drawing of a reverse
flow reactor, as usually applied in chemical industry for exothermic reactions,
is shown in Fig. 1. During start-up the reactor bed is preheated to the
desired reaction temperature of, for instance, 800 - 1000 °C. Subsequently the contaminated gas is
fed to the reactor at a much lower temperature. The gas flow is then heated
by the hot solid phase and, consequently, the bed material will cool down.
As a result, a heat front travels through the reactor towards the outlet. The
reaction starts in the region of the bed where the temperature is high
enough; at this position the reaction heat is released. Because the ratio
of the heat capacities of the solid phase and the gas phase is large, it will
take a rather long time before the heat front reaches the reactor outlet.
However, if no action is taken the final situation will be a completely cold
catalyst bed in which reaction is impossible. To prevent the occurrence of
this undesirable situation the direction of the feed flow is reversed, and
as a result the heat front will travel in the opposite direction. This process
of flow reversal is repeated continuously. In this way it is possible to keep
the heat inside the reactor, provided that the heat generation by the reaction
is high enough to compensate for the convective heat removal by the gas phase.
Because the in- and outlet sections of the bed act as regenerative heat exchangers
only, they can be filled by inert material.
To guarantee stable operation in a reverse flow reactor,
a net positive heat effect is. Known commercial applications are a.o. purification
of exhaust air polluted with traces of organic compounds, and the Simens-Martin
process (steel production). The tar conversion process considered in this
work is endothermic and energy consuming. To achieve an overall positive heat
effect, a second exothermic reaction is carried out simultaneously to counterbalance
the endothermic tar conversion reaction. This is realized by injecting small
amounts of air into the converter, where it is used for the combustion of
a small part of the biomass product gas. By adjusting the air flow into the
tar converter, the temperature level in the reactor can be controlled accurately.
Projects
Tar and tar measurements
- Behaviour of tar in biomass gasification systems; tar-related problems and their solutions, 1999, for Novem
- Parallel testing of tar and dust, 1998, for Novem
- Tar measurement protocol; Realijst_itemsation of a standard procedure for tar and particulate content determination in producer gas from biomass gasifiers, 1998, for Novem
- Development of a standard method for the measurement of organic contaminants (tars) in biomass producer gas, 2000-2002, for European Commission and Novem
- Supporting R&D on the development of a Guideline for sampling and analysis of tar and dust in biomass producer gas, 2000-2001, for Novem
- gasification - catalytic process in which tar, produced through biomass gasification is converted to combustible gas, 1991, for Ministable_rowy of Economic Affairs & Province of Overijssel.
Tar conversion in the RFTC
- Cleaning of hot producer gas in a catalytic, reverse flow reactor, 1995, for EC and November
- Catalytic upgrading of gas from biofuels and implementation of electricty production, 1999, for EC and Novem
- Lifetime of a Ni catalyst for the removal of tar in producer gas, 2001, for: Novem
- Demonstration of chicken manure gasification plant on farm-scale level, 2001-2002. For Novem
Links
www.gasifiers.org
www.gasnet.uk.net
www.tarweb.net
www.woodgas.com
www.thermonet.co.uk
Contact
Ir. H.A.M. Knoef
Tel +31 53 486 11 90