The flue gas concentration in biomass boilers is consistently difficult to reduce, but the problem may not necessarily lie with the boiler itself.

As the use of biomass boilers continues to grow, fuels such as straw, rice husks, and wood chip briquettes have partially replaced coal, shifting the pressure of environmental compliance inspections to the accompanying dust removal equipment. Many sites have encountered this situation: the boiler itself operates without any abnormalities, and combustion is relatively complete, yet the flue gas emission concentrations simply won’t go down—they fluctuate up and down, causing anxiety whenever inspections are scheduled.

When faced with this problem, many people’s first instinct is to check the dust collector’s parameters—whether the filter bags are clogged or if the cleaning cycle is functioning properly. While these checks are certainly necessary, the characteristics of flue gas from biomass boilers differ from those of coal-fired boilers, and the problem often lies elsewhere. Focusing solely on the dust collector itself sometimes fails to identify the root cause.

First, let’s consider the fuel itself. The composition of biomass fuel fluctuates much more than that of coal; moisture content, ash content, and calorific value can vary significantly between different batches of crop straw and rice husks. When the fuel changes, the combustion conditions change accordingly, and the concentration and particle size distribution of particulate matter in the flue gas will also fluctuate. In such cases, dust collection equipment is selected based on specific design conditions; when fuel variations are significant, the actual processing load often exceeds the design range, naturally leading to unstable emission concentrations. This issue is easily overlooked at many sites, where it is often attributed to equipment problems, when in fact the root cause lies in inadequate fuel management.

The combustion process itself also warrants closer scrutiny. Unlike coal combustion, biomass combustion is not as stable; even slight deviations in furnace temperature or air-fuel ratio can easily lead to incomplete combustion. The resulting flue gas contains not only ordinary dust but also unburned carbon particles and tar-like substances. These substances are highly viscous; once they enter the dust collection equipment, they easily clog filter bags and electrode plates. Standard cleaning or shaking methods struggle to remove them completely, and over time, dust collection efficiency drops significantly. In such cases, relying solely on solutions for the dust collection equipment will yield limited results; it is necessary to revisit whether the boiler’s combustion adjustments are properly configured.

The flue gas dew point is a factor that is often overlooked at biomass boiler sites, yet it has a significant impact. Biomass fuels generally have a high moisture content, resulting in correspondingly higher water vapor levels in the flue gas. If insulation upstream of the dust removal equipment is inadequate, or if frequent system startups and shutdowns cause temperature fluctuations, condensation is likely to form inside the pipes and equipment. This condensation mixes with dust and tar, adhering to the surfaces of filter bags or electrostatic precipitator plates to form a sticky layer that is difficult to clean. This problem compounds the “clogged filter bags” issue mentioned earlier, making it even more troublesome to address.

The nature of the dust itself also differs from that of coal-fired boilers. The ash produced by biomass combustion generally consists of finer particles, many of which are somewhat sticky. This imposes requirements on the selection of dust removal equipment that are not entirely the same as those for coal-fired boilers. Some sites simply replicate dust collection solutions designed for coal-fired boilers without making targeted adjustments to filter media selection or cleaning parameters, which naturally leads to operational issues.

Load fluctuations are common during the operation of biomass boilers, especially at sites with unstable fuel supply, where boiler load may change several times within a single day. If dust removal equipment is designed for a fixed load, a sudden increase in load can cause the treatment capacity to fall short, leading to a rapid spike in emission concentrations; conversely, when the load is too low, the flue gas velocity decreases, which may disrupt the normal airflow pattern within the equipment. Adapting to these fluctuating operating conditions presents a significantly greater challenge than dust removal under stable load conditions.

The pretreatment stage at the front end of dust removal equipment also warrants attention. Some biomass boilers are equipped with cyclone dust collectors for pretreatment, followed by bag filters or other fine-treatment equipment. If pretreatment is ineffective, large dust particles and sparks can enter the downstream equipment directly, not only reducing treatment efficiency but also potentially burning through the filter bags, creating safety hazards. Such issues are relatively rare in coal-fired boilers, but biomass combustion does indeed generate sparks and unburned particles more readily.

Unstable emission concentrations are often caused by a combination of factors—including fuel variability, combustion conditions, dew point corrosion, dust properties, and load fluctuations—and cannot be fully explained by any single factor alone. Over the years working in environmental protection, I have visited numerous sites with dust collection systems for biomass boilers. Initially, I assumed the issues stemmed from undersized equipment or improperly adjusted parameters. However, after thorough investigation, I discovered that many problems actually originated from instability in the fuel and combustion processes; the dust collection equipment merely amplified and highlighted these fluctuations. To truly resolve such issues, it is often necessary to address both boiler combustion adjustments and dust collection equipment maintenance simultaneously; relying on just one of these approaches makes it difficult to achieve a complete solution.