Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 204

Biomass with a high moisture content such as sludge is often used as fuel in power plant without proper drying which reduces the efficiency of the boiler. However, by use of a drying process, moisture can be largely removed from biomass. The result is a reduction in the weight of the biomass. This leads to a reduction of the processing costs, as well as of the costs for storage and transport. The dried end product is frequently used as a plant nutrient or it can be used as a fuel. This, however, depends on the properties of the biomass that needs to be dried.

Using dry fuel in a direct combustion boiler results in (i) improved efficiency, (ii) increased steam production, (iii) reduced ancillary power requirements, (iv) reduced fuel use, (v) lower emissions, and (vi) improved boiler operation. One of the main reasons for these benefits is an increased flame temperature. With wet fuel, some heat of combustion is used to evaporate the water in the fuel. With dry fuel, all the heat of combustion goes into heating the air and products of combustion. As a result, dry fuels have a flame temperature on the order of 1,260 to 1,370°C (2,300 to 2,500°F), while green wood has a combustion temperature of approximately 980°C (1,800°F). In cold climates, the heat of fusion of any ice that may be mixed with the fuel will also have a significant effect on the flame temperature.

Dryers can be broadly divided into two categories based on how heat is provided for drying. In direct dryers, the material gets heat from direct contact with a fluid providing the heat – either hot air or hot steam. With indirect drying, the material being dried is separated from the heat source by a heat exchange surface.

Directly heated dryers can be further divided into two more categories: air and superheated steam dryers (SSDs). In air dryers, hot air is contacted with the material to be dried. The air loses its sensible heat and provides the latent heat of evaporation to dry the material. The air also removes the water vapor that is evaporated. The material can be agitated by some mechanical means or by the flowing air.

In superheated steam dryers, the heating fluid is steam rather than air, but the concept is the same. The superheated steam is contacted with the biomass material and loses some of its sensible heat to provide the latent heat of evaporation to dry the fuel. The steam, however, remains above its saturation temperature, so it doesn’t condense. The water vapor leaving the biomass is heated by the superheated steam, so the net result is a larger amount of steam at a lower temperature than when the steam entered the dryer. The excess steam is removed, and the remainder is reheated and recycled back to the dryer.

In the case of the indirectly heated dryer operated under a vacuum, or with a superheated steam dryer, the latent heat of evaporation of the water vapor is easy to recover because it is not diluted by air. With vacuum drying, this heat is available only at a low temperature, but with superheated steam drying, the dryer can be designed to produce steam at practically any pressure for use in other parts of the plant.

There are several steps to drying. First, the material must be heated from the temperature at which it entered the dryer, up to the wet bulb temperature, to produce a driving force for water to leave the wet material. Next, any surface moisture on the material is evaporated – this occurs fairly quickly. Once all the surface moisture is removed, the material must be heated to drive water from the inside of the biomass to the surface so it can evaporate. This occurs during the “falling rate period” when the rate of drying drops as the material becomes drier. During the falling rate period, the surface temperature of the material remains close to the wet bulb temperature. Once the material is completely dry, it begins to heat up to the surrounding temperature, because water is no longer present to keep the temperature low.

Rotary Dryers

Rotary dryers are the most common type for biomass. There are several variations of rotary dryers, but the most widely used is the directly heated single-pass rotary dryer. In this type of dryer, hot gases are contacted with biomass material inside a rotating drum. The rotation of the drum, with the aid of flights, lifts the solids in the dryer so they tumble through the hot gas, promoting better heat and mass transfer. If contamination is not a concern, hot flue gas can be fed directly into the dryer. Other options include using a burner or a steam heater to raise the temperature of incoming air.

The exhaust gases leaving the dryer may pass through a cyclone, multicyclone, baghouse filter, scrubber, or electrostatic precipitator (ESP) to remove any fine material entrained in the air. An ill fan mayor may not be required depending on the dryer configuration. If one is needed, it is usually placed after the emissions control equipment to reduce erosion of the fan, but may also be placed before the first cyclone to provide the pressure drop through downstream equipment.

The inlet gas temperature to rotary biomass dryers can vary from 232 to 1,095°C (450 to 2,000°F). Outlet temperatures from rotary dryers vary from 70 to 110°C (160 to 230°F with most of the dryers having outlet temperatures higher than 104°C ( 220°F) to prevent condensation of acids and resins. Retention times in the dryer can be less than a minute for small particles and 10 to 30 minutes for larger material.

Flash Dryers

In a flash or pneumatic dryer, the solids are mixed with a high-velocity hot air stream. The intimate contact of the solids with the air results in very rapid drying. The solids and air are separated using a cyclone, and the gases continue through a scrubber to remove any entrained particulate material.

Gas temperatures are slightly lower for flash dryers than for rotary dryers, but they still operate at temperatures above the combustion point. The solids retention time in a flash dryer is typically less than 30 seconds, minimizing the fire hazard.

Disk Dryers

For smaller flows of material, a disk dryer or “porcupine” dryer (Figure 3) is an option. In a disk dryer, solids are heated by condensing steam inside of a central shaft with many hollow disks that increase the area for heat transfer. Fingers or breaker bars mix the material and act to keep the heat transfer surfaces free of buildup. The disk dryer can be operated under a vacuum or under pressure, and the condensate from inside the heating shaft can be recovered and returned to the boiler.

Cascade Dryers

Cascade or spouted dryers (Figure 4) are commonly used for drying grain, but they can be used for other types of biomass. Material is introduced to a flowing stream of hot air as it enters an enclosed chamber. The material is thrown into the air, then falls, or cascades, back to the bottom to be lifted again. Some of the material is drawn out through openings in the side of the chamber that control the residence time and amount of drying. Superheated Steam Dryers

Most superheated steam dryers are similar to flash dryers, except that the fluid suspending the solids and providing heat is steam instead of air. Under normal operation, the wet material is mixed with enough superheated steam to dry the material and still end up with superheated steam. Typically 90% of the steam leaving the dryer is recirculated while 10% of the steam, representing the amount of water evaporated from the biomass, is removed and either condensed or used directly in other parts of the plant.

See also: Drying.