Drying of Peat

Peat is usually dried at power plants and briquette factories with flue gases (300 to 600°C) from the boiler. More recently, steam flash dryers have been commissioned in Sweden using back-pressure steam.

Briquetting industries in Ireland and the former Soviet Union have used the so-called Peco dryer for peat for over seven decades (Fagernas and Wilen, 1988). The Peco dryer is a two-stage system where the latent heat of evapo- rated water from the second stage is used as the drying energy in the first stage. The dryer contains five columns, each containing about 500 pipes of about 70-mm diameter. The milled peat is carried by airflow or air–steam mixture in the pipes, while the energy for drying is supplied by hot water (first stage) and condensing steam outside the pipes (second stage). Peat is separated by cyclones after each stage. The energy consumption for the Peco dryer is 1.7 to 1.8 MJ/kg water evaporated, which is much lower than that for a flue gas dryer (3 to 4 MJ/kg water). However, electrical power demand for the compressors used for pneumatic conveying is relatively high.

Another dryer for peat (or lignite) is the tubular steam dryer, mostly used in Germany. It is a rotating inclined cylinder in which the material flows in 100-mm pipes, while steam flows in the shells at a pressure of 0.60 MPa.

of peat depend on the presence of wood and fiber content. If there is low content of fiber and wood with a bulk density of 400 kg/m3_{, such dryers oper-}

ate well.

The pressurized steam flash dryer originally developed at the Chalmers University of Technology (Gothenburg, Sweden) in the early 1970s is ideal for drying peat as well as pulp, bark, and so on. This dryer is a closed, pressur- ized system in which the peat is exposed to indirectly heated superheated steam. The dryer consists of transport ducts, heat exchangers, a cyclone, and fans. The superheated steam recirculates at a pressure of 2 to 6 bar. The pri- mary heating steam is condensed (usually 8 to 15 bar) on the shell side.

Dry steam and material are separated in a cyclone, and the basic stream of steam is recirculated while the excess steam is bled off. If the material is dried from 45% to 50% dryness to 85% to 90% dryness, about one ton of steam is generated per ton of material processed. This steam is available as process steam at 2 to 6 bar. Typical drying time is 10 to 30 seconds. The transport velocities are 20 to 40 m/s. If the excess steam is used (e.g., for district heating), then the net dryer energy consumption is only 0.5 to 0.7 MJ/ kg water removed, which may be compared with 3.5 to 4.8 MJ/kg, which is typical for hot-air flash dryers. Dust and explosion hazards are eliminated in steam drying, thus reducing the insurance costs as well.

A recent installation in Sweden uses peat (replacing coal) as the fuel in a 440 MW thermal powerplant. The peat is dried, briquetted (3 million tons/ year) at source about 400 km from the power plant, and then milled for com- bustion. Peat is dried from 60% to 10% (db) moisture for briquetting at an output level of 20 tons/h per drying unit. Two dryers are used in parallel. The total tubular heat exchanger area is 5500 m2

. The electricity demand of the compressors and blowers is 10 MW. About 3.6 MW district heat is produced, giving a net energy demand of the dryer of about 0.5 MJ/kg water, which is only about one-sixth to one-seventh the energy consumption of typical flue gas drying systems.

At Helsinki University of Technology, successful demonstration of flu- idized bed drying of peat has been reported at the pilot plant level (100 kg/ h of water evaporated). A tubular heat exchanger immersed in the bed provides indirect heat for drying. It uses condensing steam at 0.8 MPa. Steam evapo- rated from peat is used as the fluidizing medium. Wet peat is fed into the lower zone of the fluid bed while dry peat is withdrawn from the upper zone by a screw conveyor. The miffed peat is about 1 mm in average size. The

bed-to-tube heat transfer coefficient is rather low at 100 W/(m2_{K ) (i.e., of the}

same order as that in a flash dryer but much lower than that for a fluidized bed due to the poor thermal properties of peat). Sand may be used as a bed material to increase the drying rate, but separation of peat from sand is not very effective. If a fluidized bed combustor with sand is used, then sand separa- tion is not a problem. Fine-size magnetite as a bed material increases the bed- to-tube heat transfer coefficient to 350 W/(m2_{K ) while allowing easy magnetic}

separation from dry peat (Jahkola et al., 1989). This dryer is claimed to be economically justified for use on new, large powerplants. The investment costs are high and a demonstration plant is necessary to study the reliability and technoeconomics of the process. The fluid bed installation is likely to be more compact than the flash dryer but with net energy consumption of the same order.

Aside from differences in the flow characteristics, similar conclusions should apply for drying of lignite, biomass, and similar organic materials in steam flash or steam–fluid bed dryers. The cost and net energy consumption should be of the same order for the same production capacity (i.e., in the order of 0.5 to 0.7 MJ/kg water evaporated), provided the steam generated by the dryer is used elsewhere.

One important consideration in drying of peat is the evolution of organic compounds when flue gases are discharged into the stack or if the energy is recovered and the condensate sent as a waste stream. Indeed, such a problem exists whenever the material being dried can result in evolution of organic compounds due to heating or due to interaction with steam at elevated temper- atures. The quality of the condensate is affected by drying conditions (e.g., time, residence time in dryer, etc.) and by the amount of moisture removed, type of peat, and so on. Acetic acid, formic acid, and furfurals are the main organic compounds in the condensate. The average biochemical oxygen de- mand (BOD) is 140 to 150 mg/kg dry peat, chemical oxygen demand (COD) is 500 to 850 mg/kg dry peat, and the total organic carbon (TOC) is 90 to 300 mg/kg dry peat in different dryers. Table 7.4 provides a summary of the effluents from steam peat dryers based on the Fagernas and Wilen (1988) data. Note that drying of bark in steam-fluidized beds yields higher solids content (⬇400 mg/l), higher COD (⬇2600 mg/l), and higher TOC (⬇450 mg/l) than corresponding figures for peat; other effluent levels are comparable.

In general, increased bed temperature leads to increased load of organics in the condensates. Increasing bed temperature from 110°C to 120 to 130°C can increase BOD, COD, and TOC values nearly threefold. Finally, a novel high-pressure steam flash dryer (25 bar) has been operated successfully in Finland for drying of mined peat, which is then fed continuously to a high-

Temperature,°C 100–140 140–170 170 Pressure, bar — 2–8 5.7 pH of condensate 4.1 5.7 3.8 Solids, mg/l 80 220–400 70–170 NH3EN, mg/l 1.2 28.0 11.0 Phosphorous, mg/l 0.04 1.60 0.07 BOD, mg/l 520 130–190 — COD, mg/l 880 470–630 440–1300 TOC, mg/l 310 90 310–450

BOD: biological oxygen demand; COD: chemical oxygen demand; TOC: total organic carbon.

pressure gasifier. Clearly, such a sophisticated dryer is not warranted for a drying application.