INTRODUCTION

The boiler is considered the energy heart of a manufacturing plant. Therefore, choosing the appropriate boiler technology for a unit utilizing heat for production is a key factor in determining production stability, economic efficiency, and energy utilization efficiency.

Regarding the selection of appropriate capacity:

Regarding the selection of appropriate combustion technology:

The selection of appropriate combustion technology is crucial in determining the fuel utilization for the plant:

• If the plant operates in an area with complex fuel sources, where the local fuel typically has high moisture content, large particle size, and the pre-processing (pre-treatment) of fuel is limited, or if the plant generates by-products from its production process (such as raw wood, large bark, other combustible by-products, and waste), the step grate boiler technology is an effective choice.
• If the plant operates in an area where fuel transportation is convenient, and the local fuel supply is diverse and sufficient for various biomass fuels that can be pre-processed to meet size, calorific value, and type standards (which are commonly available and easy to find on the market), then fluidized bed boilers are the optimal choice for the business.

The basic differences between step grate combustion technology and fluidized bed combustion technology are analyzed in detail below:

I. DIFFERENCE IN FUEL PREPARATION

  The step grate boiler system burns fuel of all sizes that passes through the fuel feed chute. The only preparation required is the removal of extremely large objects and blending using specialized equipment (such as cranes, loaders, etc.) in the fuel storage area between fuel regions to stabilize the calorific value.

However, when fuel is used in a fluidized bed combustion boiler, components with high moisture content and large sizes that do not ensure adequate combustion retention time will negatively affect the combustion process in the chamber. Therefore, fluidized bed boiler systems that receive fuel must pre-treat it to reduce its size and moisture content. Otherwise, these components will accumulate at the bottom of the fluidized bed (just above the grate surface and air injection nozzles), obstructing the fluidization process. Thus, biomass, industrial, and agricultural waste fuels need to be shredded or cut into standard sizes (typically a few centimeters) before they can be used in fluidized bed boilers.

Fuel preparation is a crucial difference between step grate boilers and fluidized bed boilers. It is both a complexity and an advantage for fluidized bed boilers because the fuel becomes more uniform, leading to better thermal stability in the boiler compared to step grate boilers. Another important difference lies in the local primary air supply. In a step grate boiler, the conversion process from fuel input to complete combustion occurs sequentially (similar to a one-way flow), with primary air supply varying along the length of the grate to match the specific air demand of each combustion stage. In contrast, a fluidized bed boiler is a single-phase reaction where the air supply is introduced uniformly across the cross-sectional area beneath the grate. The fuel is mixed within the bed, ensuring even distribution and combustion.

II. COMBUSTION PROCESS

In the case of a fluidized bed boiler, the fuel is introduced from the sides or rear of the boiler. The fuel particles dry out and release moisture before they disperse evenly in the fluidized bed. The fluidized bed then transfers heat to the fuel, initiating the combustion reaction. Once the fuel is burned, the lighter fly ash is carried away by the air flow, while any slag particles (which are usually minimal with certain types of biomass) remain in the bed, contributing to the formation of the fluidized layer. The combustion reaction of the fuel continuously maintains the temperature field for the fluidized bed, and the process repeats in a continuous, cyclical manner as new fuel is supplied.

In a step grate combustion boiler, the fuel is transported through sequential combustion zones by the reciprocating motion of alternating moving and stationary grate bars. This setup is suitable for fuels with high moisture content and large particle sizes (which require time to release moisture before self-ignition) due to the presence of a separate drying zone. Before entering the main combustion zone, the fuel first passes through the drying zone, where hot air is supplied from beneath the fuel bed and intense radiant heat from the main combustion zone, along with periodic agitation, effectively dries the fuel. After drying, the fuel moves into the main combustion zone, followed by the burnout zone, and finally into the ash collection zone, completing the combustion process.

Figure 1: Fuel combustion sequence in a step grate boiler.

III. RISKS OF USING INAPPROPRIATE FUEL

Both in step grate boilers and fluidized bed boilers, there are disadvantages if the conditions for proper operation of the boiler are not met.

In the case of a step grate boiler, air enters through a limited number of air supply slots, with each air supply section covering a few meters along the length of the grate. If the fuel characteristics (moisture content, calorific value, combustion rate, etc.) vary too greatly, the combustion process transition zones within the fuel bed will shift. (For example, if the fuel has low moisture and ignites quickly, it may cause reverse combustion at the fuel hopper entrance. Conversely, if the fuel has high moisture content and unusually large size, it may cause improper combustion or clogging at the end of the grate in the ash discharge area). In such cases, the air supply must be adjusted in each zone to accommodate the changing combustion process. This adjustment is difficult to achieve with high accuracy if the boiler is not equipped with control systems that help the operator detect anomalies in a timely manner, which could lead to defects during the transition process. Additionally, there are potential defects caused by the need to cool certain parts of the grate with enhanced airflow due to variations in fuel moisture content and ash content. If the cooling air is insufficient for high-calorific value fuels in the combustion zone, the grate or certain parts of the grate could be damaged.

In a fluidized bed boiler, fuel is supplied almost continuously in sufficient quantities. At the same time, the fuel requires stability in moisture content, size, and calorific value for the combustion process to proceed smoothly and continuously. When the fuel fluctuates (e.g., moisture content increases or the size exceeds the standard), it causes an increase in the combustion retention time within the combustion chamber, interrupting the stable continuous combustion process of the boiler. Oversized fuel particles may accumulate at the bottom of the fluidized bed because the air pressure cannot overcome the weight of the fuel, gradually forming a mass that disrupts the fluidized bed and creates slag, further interrupting the continuous combustion process. High moisture content in the fuel can cause a sudden expansion of the combustion chamber’s volume. The fuel, experiencing slow combustion, leads to localized expansion of the chamber and can cause fluctuations in the chamber's vacuum. If the operator is unable to adjust in time (by reducing the fuel supply and increasing the fan speed), this could result in positive pressure in the furnace, and in more severe cases, lead to an explosion in the combustion chamber. The fluidized bed may lose heat and require re-ignition if the situation is not addressed promptly, leading to significant heat loss from the bed.

IV. DIFFERENCE IN ASH SLAG RATIO

In the sequential conversion process on the step grate of a step grate boiler, the final ash formed at the end of the grate accounts for about 90% of the total ash, with only 10% being fly ash.

In contrast, in a fluidized bed boiler, about 50% is bottom ash (or slag) and 50% is fly ash. This ratio can vary depending on the specific design details, the fluidization regime of the bed, and the fuel composition (during simultaneous combustion).

BOILER, THERMAL OIL BOILER WITH FLUIDIZED BED COMBUSTION CHAMBER – STEP GRATE COMBUSTION CHAMBER OF PHUC TRUONG HAI CO., LTD.

Understanding the advantages and disadvantages of each type of boiler is crucial. Phuc Truong Hai Boiler constantly researches and develops solutions to overcome the shortcomings of each boiler type, while enhancing its advantages to improve the technology's application efficiency, ultimately providing the highest equipment performance to customers!

Figure 2: Fluidized Bed Boiler of Phuc Truong Hai Co., Ltd.

Figure 3: Step Grate Boiler of Phuc Truong Hai Co., Ltd.