Biomass gasification refers to the thermochemical process of converting biomass fuel into combustible gas. During this process, water vapor undergoes a thermochemical reaction between free oxygen or combined oxygen and carbon in the fuel, generating combustible gases& nbsp; The biomass gasification process is relatively complex, and the gasification reaction conditions are also different. However, all gasification reactions basically include the drying, pyrolysis, oxidation, and reduction reactions of biomass.

下吸式固定床氣化爐，其特征是氣體和生物質的運動方向相同，所以又稱順氣式氣化爐。下吸式固定床氣化爐的熱解產物必須通過制熱的氧化層，因此揮發分中的焦油可以得到充分的分解，燃氣中的焦油含量大大低于上吸式固定床氣化爐。它適用于相對干燥的塊狀物料（含水率低于30%），以及含有少量粗糙顆粒的混合物料，且運行方便可靠。由于下吸式固定床氣化爐的燃氣中焦油含量較低，特別適用于小型發電系統。

The downdraft fixed bed gasifier is characterized by the same movement direction of gas and biomass, so it is also known as a downstream gasifier. The pyrolysis products of the downdraft fixed bed gasifier must pass through the heated oxide layer, so the tar in the volatile matter can be fully decomposed, and the tar content in the gas is much lower than that of the updraft fixed bed gasifier. It is suitable for relatively dry block materials (with a moisture content below 30%), as well as mixed materials containing a small amount of rough particles, and is easy and reliable to operate. Due to the low tar content in the gas of the downdraft fixed bed gasifier, it is particularly suitable for small power generation systems.

干燥層

Drying layer

氣化爐上層為干燥區，從上面加入的物料直接進入到干燥區，物料在這里同下面三個反應區生成的熱氣體產物進行換熱，使原料中的水分蒸發出去，該層溫度為 200~300℃。干燥層的產物為干物料和水蒸氣，水蒸氣隨著下面的三個反應區的產熱排出氣化爐，而干物料則落入熱解區。

The upper layer of the gasifier is a drying zone, where the materials added directly enter the drying zone. Here, the materials exchange heat with the hot gas products generated in the three reaction zones below, allowing the moisture in the raw materials to evaporate. The temperature of this layer is; 200~300℃。 The products of the drying layer are dry materials and water vapor, which are discharged from the gasifier with the heat generation in the three reaction zones below, while the dry materials fall into the pyrolysis zone.

熱解層

pyrolytic layer

生物干物料向下運行進入熱解層，同時將生物質加熱，當生物質受熱后發生熱解反應。通過熱解反應生物質中大部分的揮發分從固體中分離出去，在500~600℃時基本完成，剩下木炭。熱解區的主要產物為炭氫氣水蒸氣一氧化碳二氧化碳甲烷焦油及其他烴類物質等。

The biological dry material moves downwards into the pyrolysis layer, while heating the biomass. When the biomass is heated, a pyrolysis reaction occurs. Most of the volatile components in biomass are separated from the solid through pyrolysis reaction, and are basically completed at 500-600 ℃, leaving charcoal. The main products of the pyrolysis zone are carbon, hydrogen, water vapor, carbon monoxide, carbon dioxide, methane, tar, and other hydrocarbon substances.

氧化層

oxide layer

熱解的剩余木炭與被的空氣發生劇烈反應，釋放大量熱量。由于是限氧燃燒，氧氣的供給是不充分的，因而不完全燃燒反應同時發生，生成一氧化碳，同時也放熱量。在氧化區，溫度可達1000℃，反應方程式為：

The remaining charcoal from pyrolysis reacts violently with the air, releasing a large amount of heat. Due to limited oxygen combustion, the supply of oxygen is insufficient, resulting in incomplete combustion reactions occurring simultaneously, generating carbon monoxide and also releasing heat. In the oxidation zone, the temperature can reach 1000 ℃, and the reaction equation is:

△H △H=408.8千焦

△H  △ H=408.8 kJ

在氧化區進行的均為燃燒反應，并放出熱量，也正是這部分反應熱為還原區的還原反應物料的裂解和干燥，提供了熱源。在氧化區中生成的熱氣體（一氧化碳和二氧化碳）進入氣化爐的還原區，灰分落入下部的灰室中。

The reactions carried out in the oxidation zone are all combustion reactions and release heat, which provides a heat source for the cracking and drying of the reduction reaction materials in the reduction zone. The hot gases generated in the oxidation zone (carbon monoxide and carbon dioxide) enter the reduction zone of the gasifier, and the ash falls into the lower ash chamber.

還原層

Restoring layer

在還原區已沒有氧氣存在，在氧化反應中生成的二氧化碳在這里同炭及水蒸氣發生還原反應，生成一氧化碳（CO）和氫氣（H2）。由于還原反應是吸熱反應，還原區的溫度也相應降低，約為600~800℃。還原區的主要產物為一氧化碳（CO）二氧化碳（CO2）和氫氣（H2）。

There is no oxygen present in the reduction zone, and the carbon dioxide generated during the oxidation reaction undergoes a reduction reaction with carbon and water vapor, producing carbon monoxide (CO) and hydrogen gas (H2). Since the reduction reaction is a endothermic reaction, the temperature of the reduction zone also decreases correspondingly, about 600~800 ℃. The main products of the reduction zone are carbon monoxide (CO), carbon dioxide (CO2), and hydrogen gas (H2).

氣化實際上總是兼有燃料的干燥裂解過程。在實際的操作過程中，上述四個區域沒有明確的邊界，是相互滲透和交錯的。所以在氣化爐出口，產出氣體成分主要為一氧化碳（CO），二氧化碳（CO2）氫氣（H2）甲烷（CH4）焦油及少量共他烴類，還有水蒸氣及少量灰分。

Gasification actually always involves the drying and cracking process of fuel. In the actual operation process, the four regions mentioned above do not have clear boundaries and are mutually penetrating and intersecting. So at the outlet of the gasifier, the main gas components produced are carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), methane (CH4), tar, and a small amount of co hydrocarbons, as well as water vapor and a small amount of ash.