ECONOMIC IN ECOLOGY

BIOENERGY

and

ENVIRONMENTAL PROTECTION

Emission reduction and energy recoveryin the production of biomass fuels

Heat energy from wood pelletsIn recent years, wood has increasinglygained importance as an energy source inlarge parts of Europe.

Wood pellets are used more and more as a fuel forprivate use as well as in combined heating andpower stations. Nowadays the combustion plants areof a high technical standard, making their heatingcomfort and emissions comparable to other heatingsystems. The thermal utilization of wood is consid-ered CO2 -neutral, which is a decisive advantageover fossil fuels.

To produce pellets, the wood chips (saw dust,wood shaving etc.) are first dried, before beingcompacted by presses into pellets of varioussizes. The waste gases escaping from the chipdriers contain mainly emissions such as wooddust, organic hydrocarbons, αand βpinenes andaerosols, in addition to water vapour. The dryerexhaust air is subjectively noticed as a bluesmoke plume (Blue Haze) with a relatively strongodour.

These are typical emissions, known alsofrom the production of chipboards, OSBand MDF boards.

Wet electrostatic precipitator systemfor emission reduction in the drier exhaust airAs early as 1991 EWK Umwelttechnik GmbH in-stalled the first vertical wet electrostatic precipita-tor system with honeycomb separators,a so-called “WESP”, in the chipboard industry atthe company Homoplax in Switzerland. Becauseof its high degree of separation of dust, aerosols,Blue Haze, odour and hydrocarbons, its high sys-tem availability and economical investment andoperating costs, the WESP system subsequentlygained acceptance over other systems. To dateour company has installed more than 40 wet elec-trostatic precipitator systems for chip driersthroughout Europe.

In the wood pellet industry the first WESP systemwent into operation in the year 2002 at SBE,Svensk BrikettEnergi, in Sweden. In a specialfeature of this system, thermal energy was ex-tracted to a large extent via specially designedwater/water heat exchangers and fed into the dis-trict heating system. This achieved a return oninvestment for the installation costs within lessthan 12 months.

The Northern and Eastern European countries inparticular, because of their geographic locationand available raw material, have a distinct advan-tage of location with respect to these renewableenergy sources.

Wet Electrostatic Precipitator behind chip drier for pellet productionflow volume 78,500 m3/hdiameter 5,700 mmheight (without stack) 17,500 mm

The procedure:

The wet electrostatic precipitation system is a physical separating procedure combining fiveprocessing steps:

Flow sheet vertical wet electrostatic precipitator

1. Quenching:The waste gases are brought to saturation dew pointtemperature in the crude gas line between drier andWESP by the injection of water (evaporation cooling).The cooling leads to a condensation of hydrocarbonssuch as αand βpinenes to aerosols. At the sametime, a preliminary separation of emissions occurs.

2. Scrubber:A further separation, mainly of the water-solublesubstances, occurs in the scrubber which is config-ured as a gas distributor.

3. Wet electrostatic precipitator:The separation of particles, such as micro-fine dustsand aerosols that form the typical Blue Haze and arelargely responsible for the intense odour of wooddriers, occurs in the electrostatic precipitator. Theprecipitation surfaces are cleaned off in intervals byshort surges of water.

4. Postoxidation:The high-voltage field of approx. 86 – 130 kV createdin the electrostatic precipitator leads to the oxidationof volatile hydrocarbons, such as formaldehyde.

5. Droplet separator:The mist collector reduces the discharge of agglom-erated drops of liquid mainly during the field flushing.

The normally closed absorbent loop is continuously cleaned by means of preliminary coarse substance filtra-tion and fine filtration in self-cleaning reversible flow filters. The pre-thickened sludge is dehydrated through adecanter and can be incinerated or reintegrated directly in the production cycle. The clear phase is fed backinto the absorbent loop, so that normally there is no waste water to be disposed of. In condensation systemswith heat extraction the condensate is cleaned by additional water treatment systems.

BATNEEC (Best Available Technology Non-Excessive Economical Cost)

At the present stage, the WESP system meets the BATNEEC requirement, i.e. best available technology withconsideration of non-excessive economical costs. All current limit values for directly heated driers, whether inCentral, Northern, Eastern, Southern or Western Europe, are thus observed. This means essentially the ob-servance of the following limit values:

Particle < 10 mg/Nm³ wet

Hydrocarbon < 100 mg/Nm³ wet

Formaldehyde < 20 mg/Nm³ wet

VOC < 130 mg/Nm³ wet [UK level PG 4/6 (95)]TOC < 300 mg/Nm³ wet [TA Luft 2002]Odour precipitation > 60 %Blue Haze precipitation > 97 %

Integration of heat recovery

Environmental protection normally entails costs.The targeted application of usually minor addi-tional investments can help achieve high savingsin operating costs. With the full utilization of theenergy potential in such WESP systems, even theinvestment costs can be recovered.

Cleaning the waste gases from chip dryers by aWESP system offers the possibility, for example,of recovering and putting to use a large part of thethermal energy used in the drier. Water/air heatexchangers, for example, can preheat the com-bustion air of the drier or generate warm air forworkshop heating. Water/water heat exchangersallow the direct incorporation into the district heatsystem.

Water/Air heat exchanger for preheating of combustion air:80,000 m3/h 1.4 MW

Environmental protection can be economical

At SBE Svensk BrikettEnergi in Ulricehamn, forexample, more than 5 MW of thermal energy ornearly 50% of the drier output are recovered asuseful heating energy by the multi-step conden-sation cooling. 2 x 2 pairs of water/water heat ex-changer extract the thermal energy in two stepsfrom the waste gas flow and transfer it into the mu-nicipal district heating system. With the selectedconcept it was possible to achieve a supply tempe-rature of more than 72 °C on the secondary side.

The condensate generated by the heat extractionis cleaned in a biological treatment plant and dis-charged.

Because of the possibility to feed energy into thedistrict heating system, the complete investmentcosts for the flue gas cleaning and heat recoverysystem were recovered within a short period oftime, even when taking the operating costs intoaccount.

Simple inspection and maintenance of the heat exchangersdue to pre-chambers

Water/Water heat exchanger for heat extraction and feeding intothe district heat system: 5.0 MW 72 °C

EWK Umwelttechnik GmbHBarbarossastraße 30D-67655 KaiserslauternTelefon: +49 (0)631 / 35 77-0Telefax: +49 (0)631 / 35 77-111Internet: www.ewk.deE-Mail: umwelt@ewk.de