Question

A wood drying system consists of boiler in which dried wood (with 25 wt\% moisture content) is burned with ambient air \(\left(25^{\circ} \mathrm{C}\right)\) and hot water is generated. The flue gas available at \(130^{\circ} \mathrm{C}\) (immediately after the boiler) is used to dry the wood from \(55 \mathrm{wt} \%\) moisture content in an integrated manner. The wood enters the dryer at \(25^{\circ} \mathrm{C}\). The initial LHV of the wood is \(9.5 \mathrm{MJ} \cdot \mathrm{kg}^{-1}\) (ar) and the wet wood feed rate is \(250 \mathrm{~kg} \cdot \mathrm{h}^{-1}\). Consider a combustion system in which combustion with \(25 \%\) excess air is applied. Use the wood composition for pellets (daf basis) presented in Table \(2.3\). The boiler and dryer may be assumed to have no heat losses. The \(\mathrm{c}_{\mathrm{p}}\) value of wood can be assumed to be constant at \(1200 \mathrm{~kJ} \cdot \mathrm{kg}^{-1} \cdot \mathrm{K}^{-1}\). a. Which type of dryer do you prefer for this system and why? b. What must be the capacity of the air fan (in \(\mathrm{kg} \cdot \mathrm{h}^{-1}\) )? c. What is the amount of heat transferred per unit of time to the water system in the boiler? d. What is the end temperature of the flue gas after the dryer? e. Is the temperature at the dryer exit above the dew point of the water?

Short answer

As a teacher, create a short-answer question based on the information provided: Question: Explain why it is important for the exit temperature of a wood drying system to be above the dew point of water.

Step-by-step solution

a. Selecting an appropriate dryer

Considering the fact that we are using flue gas with a temperature of 130°C to dry wood, a direct heat rotary dryer is well suited due to its ability to handle high temperatures and provide even drying. Rotary dryers are also versatile, which allows for adjustments in drying parameters for different wood types.

b. Calculating the capacity of the air fan

To determine the capacity of the air fan, we need to first calculate the mass flow rate of the flue gas, which depends on the combustion process and excess air. We will start by finding the mass of dry wood burned, then calculate the mass of air required for combustion, and finally, find the mass flow rate of flue gas. 1. Calculate the mass of dry wood burned per hour: \(250 \text{ kg/h} \times (1 - 0.55) = 112.5 \text{ kg(dry wood)/h}\) 2. The stoichiometric air-to-fuel ratio can be written as \(A/F\). We will use the mass of air (using a representative value of 1.2 kg) required for complete combustion of 1 kg of dry wood. On a wet basis, we get: \( \text{A/F (w)} = \frac{1.2 \text{ kg(air)}}{1 \text{ kg(dry wood)}} \times (1 + 0.25) = 1.5 \text{ kg(air)/kg(dry wood)}\) 3. Calculate the mass of air required for combustion: \(1.5 \text{ kg(air)/kg(dry wood)} \times 112.5 \text{ kg(dry wood)/h} = 168.75 \text{ kg(air)/h}\) Now we can determine the mass flow rate of the flue gas, which is the sum of the mass flow rates of the air and wood input: \(168.75 \text{ kg(air)/h} + 250 \text{ kg(wood)/h} = 418.75 \text{ kg(flue gas)/h}\) The capacity of the air fan would therefore be 418.75 kg/h.

e. Comparing the exit temperature of the dryer to the dew point of water

We are asked to check if the exit temperature of the dryer is above the dew point of water. We have not calculated the exit temperature of the dryer, as it requires the information in Table 2.3 to calculate the flue gas composition and heat balance. Regardless, we would want the exit temperature to be above the dew point of the water to avoid condensation and corrosion issues in the system. This can be achieved by ensuring the dryer is properly insulated and keeping the flue gas flow rate sufficiently high to avoid cooling below the dew point.

Key concepts

Combustion Process

The combustion process is essential in a wood drying system for converting chemical energy into thermal energy. This is achieved by burning dried wood with air, providing the heat necessary for the drying process. During combustion, wood combines with oxygen to produce heat, carbon dioxide, and water vapor. The process is influenced by several factors:

- **Wood Moisture Content**: Different moisture levels in wood affect the combustion efficiency.
- **Air Supply**: An excess air level of 25% compared to stoichiometric conditions is often used to ensure complete combustion. Consideration of excess air is crucial to prevent incomplete combustion, which can result in the emission of pollutants.
- **Temperature**: The combustion should be maintained at optimal levels to ensure efficient energy transfer to the drying system.

Understanding these parameters helps in optimizing the energy output from the boiler system, directly impacting the efficiency of the subsequent drying process.

Rotary Dryer

A rotary dryer is a key component in the wood drying system that utilizes the heat from flue gas. This type of dryer is chosen for its ability to handle varied moisture levels and its effectiveness in providing even drying at high temperatures.

- **Design and Efficiency**: The rotary dryer comprises a rotating drum that gently tumbles the wet wood as hot gases flow through it. This motion ensures even distribution of heat and thorough moisture removal.
- **Benefits**: Rotary dryers are versatile, allowing adjustments in temperature and drying time according to the specifics of the material being dried. They are also capable of direct heat transfer, making them suitable for using flue gases directly from the combustion process.

This system is particularly efficient for wood with a high initial moisture content, like 55%, gradually reducing it to optimal levels needed for further processing or use.

Heat Transfer

Heat transfer in the wood drying system occurs primarily between the flue gas and the wood in the dryer. Efficient heat transfer is crucial for ensuring that the wood is adequately dried using the thermal energy from the combustion system.

- **Convection and Conduction**: As the hot flue gas flows through the rotary dryer, heat is transferred to the wood mainly through convection. Also, direct contact between the drum and the wood facilitates heat transfer via conduction.
- **Thermodynamics**: The heat provided must be carefully controlled to achieve the desired moisture level in the wood while preventing any overheating that could damage the wood structure.
- **Energy Balance**: Ensuring a proper energy balance means that most of the heat generated in the combustion process is utilized for drying, minimizing losses and increasing system efficiency.

Understanding the mechanisms of heat transfer is vital for optimizing dryer performance and energy consumption.

Dew Point

The dew point is the temperature at which air becomes saturated and water vapor begins to condense. In a wood drying system, it is critical to maintain temperatures above the dew point to prevent condensation within the dryer.

- **Implications of Condensation**: Allowing moisture to condense could lead to water corrosion and reduce the efficiency of the drying process. It can also lead to damage caused by rust and other moisture-related issues in the system.
- **System Design**: It is important to ensure that the exit temperature of the flue gas remains above the dew point when it leaves the dryer.
- **Monitoring**: Adequate insulation and keeping the flue gas flow rate high help maintain required temperatures, ensuring that condensation does not occur.

By managing the relationship between temperature and dew point, the system minimizes the risks of corrosion and maintains efficient operation.