Drying of Peat

commissioned in Sweden using SHS as a transporting medium heated indirectly by the back-pressure steam from a turbine.

Briquetting industries in Ireland and the former Soviet Union have used the so-called Peco dryer for peat for over eight decades (Fagernas and Wilen, 1988). The Peco dryer is a two-stage system where the latent heat of evaporated water from the second stage is used as the drying energy in

air–steam mixture in the pipes, whereas the energy for drying is sup-(second stage). Peat is separated by cyclones after each stage. The energy consumption for the Peco dryer is 1.7–1.8 MJ/kg water evaporated, which electrical power demand for the compressors used for pneumatic convey-ing 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 of 0.60 MPa. Air is drawn through the pipes to remove evaporated water

CRC_73877_Ch007.indd 108

CRC_73877_Ch007.indd 108 12/29/2008 6:18:10 PM12/29/2008 6:18:10 PM

the first stage. The dryer comprises five columns, each containing about

plied by hot water (first stage) and condensing steam outside the pipes According to Munter (1989, personal communication), flash dryers for

exist, which utilizes vapor-recompression technology for the steam-flash The higher capital costs associated with steam flash dryer units coupled

the flash dryer since large lumps and the like will not be suspended by

Peat is usually dried at power plants and briquette factories with flue gases (300–600°C) from the boiler. More recently, steam flash dryers have been

500 pipes of ∼70 mm diameter. The milled peat is carried by airflow or

is much lower than that for a flue gas dryer (3–4 MJ/kg water). However,

flows in 100 mm pipes, whereas steam flows in the shells at a pressure

© 2009 by Taylor & Francis Group, LLC

Superheated Steam Drying 109

The energy consumption is reported to be 2.9–3.2 MJ/kg water removed.

The exhaust energy is generally not utilized. It is interesting to note

400 kg/m³, such dryers operate well.

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, pressurized system in which the peat is exposed to indirectly heated SHS.

The dryer consists of transport ducts, heat exchangers, a cyclone, and blowers. The SHS recirculates at a pressure of 2–6 bar. The primary heat-ing steam is condensed (usually 8–15 bar) on the shell side.

The dry material is separated from steam in a cyclone, and the basic stream of steam is recirculated, whereas the excess steam is bled off. If the material at initial dryness from 45 to 50% is dried to 85–90% dryness,∼1 t of steam is generated per ton of material processed. This steam is available as process steam at 2–6 bar. Typical drying time is 10–30 s and the trans-port velocities are 20–40 m/s. If excess steam is used (e.g., for district heat-ing), then the net dryer energy consumption is only 0.5–0.7 MJ/kg water removed. This may be compared with 3.5–4.8 MJ/kg, which is typical for 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 power plant. The peat is dried, briquetted (3 million tons/year) at a source∼400 km from the power plant, and then milled for combustion. Peat is dried from 60 to 10% d.b. moisture for briquetting at an output level of 20 t/h per drying unit. Two dryers are used in parallel. The total heat-transfer area of the tubular heat exchanger 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 ∼0.5 MJ/kg water, which is only about drying systems.

At Helsinki University of Technology, successful demonstration of (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.

from the upper zone by a screw conveyor. The miffed peat is ∼1 mm 100 W/m²

Sand may be used as a bed material to increase the drying rate, but

sepa-CRC_73877_Ch007.indd 109

CRC_73877_Ch007.indd 109 12/29/2008 6:18:10 PM12/29/2008 6:18:10 PM

content. If there is low content of fiber and wood with a bulk density of

in average size. The bed-to-tube heat-transfer coefficient is rather low at that the flow properties of peat depend on the presence of wood and fiber

The pressurized steam flash dryer originally developed at the Chalmers

hot air flash dryers. Dust and explosion hazards are eliminated in steam

one-sixth to one-seventh the energy consumption of typical flue gas

fluidized bed drying of peat has been reported at the pilot-plant level

Steam evaporated from peat is used as the fluidizing medium. Wet peat is fed into the lower zone of the fluid bed, whereas dry peat is withdrawn

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).

ration of peat from sand is not very effective. If a fluidized bed combustor

© 2009 by Taylor & Francis Group, LLC

with sand is used, then sand separation is not a problem. Fine size magne-to 350 W/m²K while allowing easy magnetic separation from dry peat for use on new, large power plants. The investment costs are high and a demonstration plant is necessary to study the reliability and

techno-same order.

should apply for drying of lignite, biomass, and similar organic materials sumption should be of the same order for the same production capacity (i.e., in the order of 0.5–0.7 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 is recovered and the condensate sent as a waste stream. Indeed, such a problem exists whenever the material being dried can result in the evolution of organic compounds due to heating or interaction with steam at elevated temperatures. The quality of the condensate is affected by the drying con-ditions (e.g., total time and residence time in dryer), 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 biological oxygen demand (BOD) is 140–150 mg/kg dry peat, the chemical oxygen demand (COD) is 500–850 mg/kg dry peat, and the total organic carbon (TOC) is 90–300 mg/kg dry peat in different dryers. Table 7.4 provides a

yields higher solid content (∼400 mg/L), higher COD (∼2600 mg/L), and

TABLE 7.4

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

NH₃–N (mg/L) 1.2 28.0 11.0

Phosphorus (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

CRC_73877_Ch007.indd 110

CRC_73877_Ch007.indd 110 12/29/2008 6:18:11 PM12/29/2008 6:18:11 PM

tite as a bed material increases the bed-to-tube heat-transfer coefficient (Jahkola et al., 1989). This dryer is claimed to be economically justified

economics of the process. The fluid bed installation is likely to be more compact than the flash dryer but with a net energy consumption of the Aside from differences in the flow characteristics, similar conclusions in steam-flash or steam-fluid bed dryers. The cost and net energy

con-compounds when flue gases are discharged into the stack or if the energy

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

Effluents from Steam Peat Dryers

© 2009 by Taylor & Francis Group, LLC

Superheated Steam Drying 111

ent levels are comparable.

In general, increased bed temperature leads to increased load of organ-ics in the condensates. Increasing the bed temperature from 110 to 130°C can increase the BOD, COD, and TOC values nearly threefold. Finally, a fully in Finland for drying of mined peat, which is then fed continuously ranted for a drying application.