Effects of Hydrogen Addition on the Thermal Performance and Emissions of Biomass Syngas Combustion in a Horizontal Boiler

Abstract

Due to its low calorific value, abnormal phenomena such as incomplete combustion and flameout may occur during the combustion process of biomass syngas. The applicability of adding hydrogen can assist in the combustion of biomass syngas in boilers to overcome the above defects, and the effects need to be investigated. In this study, a multi-mechanism model is employed to numerically simulate the flow and combustion of a horizontal boiler burning biomass syngas. The reliability verification of the model is conducted by comparing it with the experimental results of combustion in a domestic boiler with biomass syngas. From the views of multi-fields and synergy, the effects of hydrogen addition on the thermal performance and emissions of biomass syngas are further expounded. Two scenarios are taken into consideration: hydrogen addition at a constant fuel volume flow rate and constant heat input. The result indicates that hydrogen addition significantly affects the multi-field synergy, which is advantageous for improving the heat transfer performance and combustion efficiency of biomass syngas. However, when the hydrogen addition ratio exceeds 20% at a constant fuel volume flow rate and 25% at constant heat input, its impact may be reduced. When the hydrogen content increases, the outlet temperature of the combustion chamber decreases, and pollutant emissions are effectively controlled. The turbulent kinetic energy at the reversal section decreases, and the uniformity of the flow field improves. These results provide certain guidance for the efficient utilization of biomass syngas and the operation of boilers burning biomass syngas.

1. Introduction

Due to the global energy crisis and environmental pollution, developing clean alternative fuels is one of the effective means to address these issues. Biomass energy has good application prospects due to its clean, widely distributed, and renewable characteristics [1]. The United Nations Commission on Environment and Development (UNCED) stated in a research report that utilization of biomass energy will account for 50% of the world’s total energy consumption by 2050 [2].

The US Department of Energy (DOE) defines biomass as the organic matter present in all plants and animals [3]. The main types of biomass energy development and utilization are divided into direct combustion, thermochemical conversion, and biochemical conversion, involving gasification, liquefaction, carbonization, and pyrolysis, among which direct combustion and biomass gasification are the main methods used at present [4].

Direct combustion of biomass is prone to coking, and high-temperature alkali metal elements are easily generated during the combustion process, which can corrode the combustion equipment [5]. Biomass gasification is a technology that produces small molecule combustible gases by reacting solid biomass with gasification agents. Biomass syngas is used as fuel in thermal machinery such as boilers, engines, and gas turbines [6,7]. Biomass syngas has certain differences in composition due to its different preparation methods and raw materials [8]. The non-hydrocarbon gas content of biomass syngas is high, resulting in low volume calorific value, poor flame stability, and abnormal combustion phenomena such as incomplete combustion and misfire in the combustion process. How to improve the stability of biomass syngas combustion and reduce emissions is currently a research focus.

As a multi-component gas fuel, the reaction mechanism of biomass syngas combustion is extremely complex, and there is relatively little research on the numerical simulation of biomass syngas combustion. Zhang et al. [9] simulated biomass syngas combustion using four chemical reaction kinetics models and compared the simulation results with literature data and experimental results. The results showed a fair degree of consistency between the simulation results of FFCM-1 and GRI 3.0 and the experimental results. Wang et al. [10] simulated biomass syngas combustion using three chemical reaction mechanism models and compared the results with experimental results. The results showed that the GRI 3.0 reaction mechanism had the smallest deviation from experimental results under the conditions of lean and rich combustion. Liu et al. [11] simplified the GRI Mesh 3.0 mechanism to a 27-component, 54-reaction mechanism, analyzing the temperature sensitivity and key component concentration sensitivity in the 325 step reactions.

In order to solve the problems of poor combustion stability, easy flameout, and pollutant emission to a certain extent, some scholars have considered using different components of biomass syngas or mixing other gases with biomass syngas to study the impact on the combustion process. Khaleghi et al. [12] investigated the temperature, flame stability, and emissions of biogas vortex combustion under the condition of CO₂ mole fraction volume percentage ranging from 0 to 40%. As the CO₂ content of the biogas increased, the NO_x generation rate monotonically decreased. Ilker et al. [13] conducted experiments to explore flame stability with a nitrogen dilution ratio ranging from 0% to 60%. The combustion stability reached the best state when N₂ content reached 30%, and the stability decreased when it exceeded 30%. The dilution of N₂ led to an increase in CO emission, and the NO_x emission was the least when the N₂ content was 20%. Hu et al. [14] experimented with the flame stability of biomass syngas in a combustion bomb with constant volume by changing the ratio of CO₂ and H₂. The flame stability decreased with the increase in CO₂. Moreover, an increase in the hydrogen ratio led to an increase in both hydrodynamic instability and thermal mass diffusion instability and the appearance time of cellular flame was advanced. Mordaunt et al. [15] designed a combustion device to investigate the impact of increasing CO₂ content on the combustion performance of biomass syngas. With the increase in CO₂, lean oil flameout and combustion pressure oscillation were observed, which greatly limited the actual operation of gas turbines.

Hydrogen exhibits the lowest ignition energy and the highest heat generation per unit mass among all gaseous fuels. As a high calorific value fuel, hydrogen has become the primary choice as a blended fuel to solve the problems caused by the combustion of low calorific value fuel. In terms of combustion performance and emissions, Xie et al. [16] numerically simulated the impact of a hydrogen ratio varying from 0% to 100% on the combustion characteristics of laminar premixed methane in gas turbines. The results indicated that the combustion rate of laminar flames increased with the increase in hydrogen content, which increased the emission of NO and inhibited the generation of CO. Khare et al. [17] studied the impact of hydrogen concentration ranging from 0% to 20% by volume on the soot limits of methane/air laminar counterflow diffusion flames. The results indicated that the amount of hydrocarbons decreased as hydrogen concentration increased. Xu et al. [18] explored the impact of hydrogen addition on the combustion of n-decane/hydrogen/air mixtures. The results revealed a linear relationship between LBV and hydrogen addition ratio. The Markstein length decreased as the hydrogen addition ratio increased. Through chemical kinetic analysis, the molar fractions of H and OH led to a linear relationship between LBV and relative humidity. Antonio et al. [19] burned biomass syngas containing CH₄, H₂, and CO₂ in a controlled auto-ignition engine to study the influence of different components of biomass syngas on NO_x emission. An optimal biomass syngas composition was identified, allowing a reduction in NO_x emission compared to traditional biomass syngas. Zhen et al. [20] conducted an experiment to study the impact of hydrogenation on the diffusion flame stability of biogas containing H₂, CO₂, and CH₄. The results demonstrated that the 5% addition of hydrogen had an enhancing effect on combustion stability to a certain extent in this experiment. The effect of CO₂ on flame instability exceeded that of N₂. In terms of thermodynamic analysis, Liu et al. [21] studied the combustion of methane/hydrogen mixed fuel in engines. The H₂ addition ratio increased from 0% to 70%, resulting in a decrease in thermal conductivity loss. The reactions with H₂, H, and H₂O as reactants were inhibited, decreasing the loss from chemical reaction. Zhao et al. [22] increased H₂ substitution in a DME/H₂/air flame mixture and found that the addition of H₂ reduced the exergy losses from chemical reactions and heat conduction, increased the exergy losses from incomplete combustion, and overall reduced the exergy losses. In terms of energy requirement and cost, taking China as an example, the White Paper on China’s Hydrogen Energy and Fuel Cell Industry states that it is predicted that the total hydrogen demand in China will reach 30 million tons by 2025 and 35 million tons by 2030 [23]. Sun et al. [24] estimated that a 1 million kW gas power plant with 15% hydrogen addition combustion would require over 2000 vehicles (300 kg onboard) of hydrogen per year, with a transportation cost of CNY 9/kg.

As mentioned above, there are a lot of studies on hydrogen as a blended fuel, but most of them have used relatively single syngas components [25,26,27,28], and studies on biomass syngas combustion were mostly focused on combustion in engines [29,30,31] or constant volume combustion bombs [11,15], which are different from the structure of the boiler. Therefore, a multi-component mechanism model is employed to study the hydrogen addition of biomass syngas combustion. Multiple physical fields, turbulent kinetic energy (TKE), and field synergy are utilized to investigate and evaluate the effect of hydrogen addition on the thermal performance and emissions of biomass syngas, which has guiding significance for the efficient utilization of biomass syngas. Two hydrogen addition scenarios are used for a more comprehensive comparison. It provides guidance for the efficient utilization of biomass syngas and the design and operation of boilers.

In the following text, the structure of a practical horizontal boiler burning biomass syngas is first introduced, and the geometric and mathematical models are constructed and verified. Then, numerical calculation and result analysis are conducted under two hydrogen addition scenarios. Ultimately, three conclusions are listed.

2. Modeling and Solution

2.1. Geometric Model

The internal structure of a boiler burning biomass syngas involved in the study is shown in Figure 1. The front of the furnace is equipped with a combustor, which sends biomass syngas into the furnace for combustion. The high-temperature flue gas enters the reversal section from the main combustion section and is then discharged into the second return flue pipe. The flue gas and water exchange heat through the flue pipe and the wall of the combustion chamber. Then, the steam is generated. As shown in Figure 2, the research object is the first return journey of the boiler surrounded by the red line, which includes the inlet section, main combustion chamber, and reversal section. The biomass syngas wrapped by air enters the combustion chamber in a swirling, non-premixed way for combustion. The flue gas turns back from the reversal section to the outlet.

To simplify the numerical calculation, facilitate the division of the calculation grid, and accelerate the convergence speed, the following assumptions are adopted in this study. (1) The study only focuses on the combustion characteristics of the boiler combustion chamber without considering the heat transfer characteristics of flue gas after entering the flue pipe. (2) A set of tangentially arranged blades is installed in the actual inlet section, which can cause the airflow to rotate. In order to simplify the calculation, the blade design is ignored in the model instead of considering the swirl effect caused by the blades in the boundary conditions.

2.2. Mathematic Models

2.2.1. Governing Equations

Whether in a mixed flow field with interaction or a flow field with multiple components, the conservation law must be followed. The governing equations involved in this study are as follows.

The mass conservation equation is as follows:

$$\frac{\partial \rho }{\partial t}+\frac{\partial }{\partial x_i}(\rho u_i)=0$$

(1)

The momentum conservation equation is as follows:

$$\frac{\partial }{\partial t}(\rho u_i)+\frac{\partial }{\partial x_i}(\rho u_i{u}_j)=-\frac{\partial P}{\partial x_i}+\frac{\partial }{\partial x_i}({\tau }_{\mathit{ij}})+\rho f_i$$

(2)

The energy conservation equation is as follows:

$$\frac{\partial }{\partial x_i}(\rho C_{p}T)+\frac{\partial }{\partial x_i}(\rho C_p{u}_{i}T)=\frac{\partial }{\partial x_i}(\frac{\mu }{\mathit{Pr}}\lambda \frac{\partial C_{p}T}{\partial x_i})+w_s{Q}_s$$

(3)

The composition conservation equation is as follows:

$$\frac{\partial }{\partial t}(\rho m_s)+\frac{\partial }{\partial x_i}(\rho u_i{m}_s)=\frac{\partial }{\partial x_i}(\frac{\mu }{\mathit{Sc}}\frac{\partial m_s}{\partial x_i})-w_s$$

(4)

2.2.2. Turbulence Model

The turbulence model used in the study is the Realizable k-ε model, which is an improved k-ε model. It works incredibly well in tackling issues such as uniform shear flow and separated flow of rotation [32].

Equation k in the Realizable k-ε model [33,34]:

$$\frac{\partial (\rho k)}{\partial t}+\frac{\partial (\rho k{u}_i)}{\partial x_i}=\frac{\partial }{\partial x_i}[(\mu +\frac{{\mu }_t}{{\sigma }_k})\frac{\partial k}{\partial x_i}]+G_k+G_b-\rho \epsilon -Y_M+E_k$$

(5)

Equation ε in the Realizable k-ε model:

$$\frac{\partial (\rho \epsilon )}{\partial t}+\frac{\partial (\rho \epsilon u_i)}{\partial x_i}=\frac{\partial }{\partial x_i}[(\mu +\frac{{\mu }_t}{{\sigma }_{\epsilon }})\frac{\partial \epsilon }{\partial x_i}]+\rho C_1{E}_{\epsilon }-\rho C_2\frac{{\epsilon }^2}{k+\sqrt{v\epsilon }}+C_{1\epsilon }\frac{\epsilon }{k}{C}_{3\epsilon }{C}_b+E_{\epsilon }$$

(6)

$$C_1=\mathit{max}(0.43\frac{{\eta }^{\prime }}{{\eta }^{\prime }+5})$$

(7)

$${\eta }^{\prime }={(2E_{\mathit{ij}}·E_{\mathit{ij}})}^{1/2}\frac{k}{\epsilon }$$

(8)

$$E_{\mathit{ij}}=1/2(\frac{\partial u_i}{\partial x_j}+\frac{\partial u_j}{\partial x_i})$$

(9)

$${\mu }_t=\rho C_{\mu }\frac{k^2}{\epsilon };C_{\mu }=\frac{1}{A_0+A_s{U}^{\ast }k/\epsilon }$$

(10)

$$A_0=4.0;A_s=\sqrt{6}\mathit{cos}\phi ;\phi =\frac{1}{3}{\mathit{cos}}^{-1}(\sqrt{6}W);W=\frac{E_{\mathit{ij}}{E}_{\mathit{jk}}{E}_{\mathit{kj}}}{{(E_{\mathit{ij}})}^{1/2}}$$

(11)

$$U^{\ast }=\sqrt{E_{\mathit{ij}}{E}_{\mathit{ij}}+\widetilde{{\Omega }_{\mathit{ij}}}\widetilde{{\Omega }_{\mathit{ij}}}};\widetilde{{\Omega }_{\mathit{ij}}}={\Omega }_{\mathit{ij}}-2{\epsilon }_{\mathit{ijk}}{\omega }_k;{\Omega }_{\mathit{ij}}=\overline{{\Omega }_{\mathit{ij}}}-{\epsilon }_{\mathit{ijk}}{\omega }_k$$

(12)

2.2.3. Combustion Model

In this study, the GRI 3.0 chemical reaction mechanism model involving 53 components and 325 elementary reactions is employed to simulate the combustion of biomass syngas mixed with hydrogen. The key reaction formulas for biomass syngas combustion [11] are shown in

Table 1. The key chemical reactions based on the GRI 3.0 mechanism.

	Reaction		Reaction		Reaction
1	O + H2⇔H + OH	19	2OH(+M)⇔H2O2(+M)	37	CH3 + CH2O⇔HCO + CH4
2	O + CH3⇔H + CH2O	20	2OH⇔O + H2O	38	HCO + H2O⇔H + CO + H2O
3	O + CH4⇔OH + CH3	21	OH + HO2⇔O2 + H2O	39	HCO + M⇔H + CO + M
4	O + CO(+M)⇔CO2(+M)	22	OH + CH4⇔CH3 + H2O	40	HCO + O2⇔HO2 + CO
5	O + CH2O⇔OH + HCO	23	OH + CO⇔H + CO2	41	N + NO⇔N2 + O
6	O2 + CO⇔O + CO2	24	2HO2⇔O2 + H2O2	42	N + O2⇔NO + O
7	O2 + CH2O⇔HO2 + HCO	25	2HO2⇔O2 + H2O2	43	N + OH⇔NO + H
8	H + O2 + M⇔HO2 + M	26	HO2 + CH3⇔O2 + CH4	44	HO2 + NO⇔NO2 + OH
9	H + 2O2⇔HO2 + O2	27	HO2 + CH3⇔OH + CH3O	45	NO + O + M⇔NO2 + M
10	H + O2 + H2O⇔HO2 + H2O	28	HO2 + CO⇔OH + CO2	46	NO2 + O⇔NO + O2
11	H + O2 + N2⇔HO2 + N2	29	HO2 + CH2O⇔HCO + H2O2	47	NO2 + H⇔NO + OH
12	H + O2⇔O + OH	30	CH + CO2⇔HCO + CO	48	NNH + O⇔NH + NO
13	H + HO2⇔O2 + H2	31	CH2 + O2⇔OH + H + CO	49	H + NO + M⇔HNO + M
14	H + H2O2⇔HO2 + H2	32	CH2(S) + CO2⇔CO + CH2O	50	HNO + H⇔H2 + NO
15	H + CH3(+M)⇔CH4(+M)	33	CH3 + O2⇔O + CH3O	51	N + CO2⇔NO + CO
16	H + CH4⇔CH3 + H2	34	CH3 + O2⇔OH + CH2O	52	O + CH3⇔H + H2 + CO
17	H + CH2O⇔HCO + H2	35	CH3 + H2O2⇔HO2 + CH4	53	OH + HO2⇔O2 + H2O
18	OH + H2⇔H + H2O	36	2CH3(+M)⇔C2H6(+M)	54	CH2 + O2⇔2H + CO2

. The simulation results of biomass syngas combustion using the GRI 3.0 mechanism model have less deviation from the experimental results, which are available in references [9,10].

Considering the detailed chemical reaction mechanisms, the combustion model uses the Eddy Dissipation Concept (EDC) model. Magnussen et al. [35] developed the EDC model based on the Eddy Break Up model. Spalding et al. [36] proposed that this model can compensate for the shortcomings of Arrhenius’s law in predicting turbulent flames.

The concept of energy cascade is employed to analyze the mechanical energy in turbulence. The equations demonstrate that the energy dissipation rate $\epsilon$ is associated with the properties of fine structures [37].

$$\epsilon =\frac{4}{3}{C}_{D2}\nu \frac{u^{\ast 2}}{L^{\ast 2}}$$

(13)

$$\epsilon =2C_{D1}\frac{u^{\ast 2}}{L^{\ast }}$$

(14)

The Reynolds number of the fine structure is defined and evaluated by the following formula. It can be seen that the fine structure is indeed in the dissipation range, because the Reynolds number is in order units.

$$\mathit{Re}=\frac{u^{\ast }{L}^{\ast }}{\nu }=\frac{2C_{D2}}{3C_{D1}}\approx 2.5$$

(15)

The characteristic parameters of small-scale turbulent structures can be represented by turbulent characteristic parameters, and the characteristic length ${\xi }^{\ast }$ and characteristic time ${\tau }^{\ast }$ are defined as follows [35].

$${\xi }^{\ast }={(\frac{3C_{D2}}{4C_{D1}^2})}^{1/4}{(\frac{\nu \epsilon }{k^2})}^{1/4}=C_{\xi }{(\frac{\nu \epsilon }{k^2})}^{1/4}=C_{\xi }{({\mathit{Re}}_{\tau })}^{-1/4}$$

(16)

$${\tau }^{\ast }={(\frac{C_{D2}}{3})}^{1/2}{(\frac{\nu }{\epsilon })}^{1/2}=C_{\tau }{(\frac{\nu }{\epsilon })}^{1/2}$$

(17)

$$C_{D1}=\frac{3}{2}\frac{C_{\tau }}{C_{\xi }^2}$$

(18)

$$C_{D2}=3C_{\tau }^2$$

(19)

where $C_{D1}$ = 0.135; $C_{D2}$ = 0.5; $C_{\xi }$ = 2.1377; $C_{\tau }$ = 0.4082.

2.2.4. Radiation Model

The P-1 model is employed in this study, which utilizes curved coordinates to quickly process complex geometric models and takes into account material scattering and radiative heat transfer between gas and particles.

$$-\nabla ·q_r=\alpha G-4\alpha \sigma T^4$$

(20)

$$q_r=-\frac{1}{3(\alpha +{\sigma }_s)-C{\sigma }_s}\nabla G$$

(21)

2.2.5. NOx Generation Model

Thermal NO_x and prompt NO_x are mainly considered in the study for the NO_x generation model. Prompt NO_x is generated at the root of the combustion flame, and the generation time is extremely short. It is mainly caused by the reaction between ${\mathrm{N}}_2$ in the air and hydrocarbon ion groups (CH, etc.) in the fuel to generate CN compounds, which are then oxidized to generate NO_x. The reaction equations mainly include

$$\mathrm{CH}+{\mathrm{N}}_2\to \mathrm{HCN}+\mathrm{N}$$

(22)

$$\mathrm{C}{\mathrm{H}}_2+{\mathrm{N}}_2\to \mathrm{HCN}+\mathrm{NH}$$

(23)

$$\mathrm{C}{\mathrm{H}}_2+{\mathrm{N}}_2\to {\mathrm{H}}_2\mathrm{CN}+\mathrm{N}$$

(24)

$$\mathrm{C}+{\mathrm{N}}_2\to \mathrm{CN}+\mathrm{N}$$

(25)

$${\mathrm{C}}_2+{\mathrm{N}}_2\to 2\mathrm{CN}$$

(26)

The generation of thermal NO_x mainly occurs in a high-temperature combustion environment, and the chemical reaction begins with the dissociation of O atoms from the H₂-O₂ group or from O₂. The specific equations are as follows.

$${\mathrm{O}}_2\to 2\mathrm{O}$$

(27)

$$\mathrm{O}+{\mathrm{N}}_2\to \mathrm{N}+\mathrm{NO}$$

(28)

$${\mathrm{N}+\mathrm{O}}_2\to \mathrm{O}+\mathrm{NO}$$

(29)

$$\mathrm{N}+\mathrm{OH}\to \mathrm{H}+\mathrm{NO}$$

(30)

2.2.6. Field Synergy Principle

The field synergy principle is grounded on coordinating the heat flux field and velocity field. It studies the uniform law of convective heat transfer and unifies the existing single-phase heat transfer enhancement theory. The multi-field synergy in the convective heat transfer region is introduced to evaluate the flow resistance and heat transfer [38,39].

Multiple synergy angles are employed to describe multi-field synergy. The synergy angles $\beta$ and $\gamma$ are given as Equations (31) and (32).

$$\beta =\mathit{arccos}\frac{\left|U·\nabla T\right|}{\left|U\right|\left|\nabla T\right|}=\mathit{arccos}\left|\frac{u\frac{\partial T}{\partial x}+v\frac{\partial T}{\partial y}+w\frac{\partial T}{\partial z}}{\sqrt{u^2+v^2+w^2}\sqrt{{(\frac{\partial T}{\partial x})}^2+{(\frac{\partial T}{\partial y})}^2+{(\frac{\partial T}{\partial z})}^2}}\right|$$

(31)

$$\gamma =\mathit{arccos}\frac{\left|\nabla T·\nabla u\right|}{\left|\nabla T\right|\left|\nabla u\right|}=\mathit{arccos}\left|\frac{\frac{\partial T}{\partial x}·\frac{\partial u}{\partial x}+\frac{\partial T}{\partial y}·\frac{\partial u}{\partial y}+\frac{\partial T}{\partial z}·\frac{\partial u}{\partial z}}{\sqrt{{(\frac{\partial T}{\partial x})}^2+{(\frac{\partial T}{\partial y})}^2+{(\frac{\partial T}{\partial z})}^2}\sqrt{{(\frac{\partial u}{\partial x})}^2+{(\frac{\partial u}{\partial y})}^2+{(\frac{\partial u}{\partial z})}^2}}\right|$$

(32)

where $U$ is the velocity; $\nabla T$ is the temperature gradient; $\nabla u$ is the velocity gradient.

2.3. Boundary Conditions and Parameters

The fuel inlet and air inlet are the velocity inlet boundary, and the velocity is calculated based on the specific flow rate of the case. The wall of the boiler, which is wrapped by saturated water, is set as an isothermal wall, and the heat flux density of the wall that is not wrapped by saturated water is constant. The outlet of the boiler is the pressure outlet boundary. The specific values are listed in

Table 2. Boundary conditions and parameters of the boiler.

Section	Parameter	Symbol	Value	Unit
Fuel-inlet	Fuel temperature	Tf,in	973	K
Air-inlet	Air temperature	Tair,in	343	K
Wall (in contact with water)	Water temperature	Tw	473	K
Wall (not in contact with water)	Heat flux density	qw	1000	W/m2
Outlet	Pressure	Pout	50	Pa

.

2.4. Biogas Parameters

In this study, biomass syngas with components including CH₄, CO, CO₂, H₂, and N₂ is employed. In some literature, solid biomass has been gasified to produce syngas and similar components have been found in the results of the research. There are also studies on the combustion of biomass syngas with similar composition [40,41].

The volume fraction of non-combustible components CO₂ and N₂ remains constant. The volume percentage of hydrogen varies from 5% to 30%, while the volume percentage of other components decreases.

Two different scenarios are discussed in the study: one is hydrogen addition at a constant fuel volumetric flow rate (488 Nm³/h), and the specific parameters of fuels are listed in

Table 3. Parameters of hybrid fuels (at constant fuel volumetric flow rate).

Case	Fuel Composition (vol%)					LHV (kJ/Nm3)	Heat Input (kW)	Fuel Volumetric Flow (Nm3/h)
	CH4	CO	H2	CO2	N2
1	6	24	5	15	50	5734	777.78	488.31
2	5	20	10	15	50	5409	733.68
3	4	16	15	15	50	5083	689.59
4	3	12	20	15	50	4758	645.49
5	2	8	25	15	50	4433	601.39
6	1	4	30	15	50	4108	557.29

. The other is hydrogen addition at constant heat input (778.78 kW), and the specific parameters of fuels are listed in

Table 4. Parameters of hybrid fuels (at constant heat input).

Case	Fuel Composition(vol%)					LHV (kJ/Nm3)	Heat Input (kW)	Fuel Mass Flow (kg/s)
	CH4	CO	H2	CO2	N2
1	6	24	5	15	50	5734	777.78	0.0486
7	5	20	10	15	50	5409		0.0493
8	4	16	15	15	50	5083		0.0502
9	3	12	20	15	50	4758		0.0512
10	2	8	25	15	50	4433		0.0523
11	1	4	30	15	50	4108		0.0536

.

2.5. Grid Independence

The computational domain is meshed using ICEM CFD 18.0 software. The grid at the outlet and swirl air inlet is locally encrypted, and the overall unstructured grid is shown in Figure 3.

In this study, four different numbers of grids are adopted to conduct the grid independence validation, which are 1.5 million, 2.8 million, 3.4 million, and 4.2 million, respectively. The quality of the grid is checked in turn to ensure that it meets the quality requirements. Figure 4 shows the axial velocity distribution corresponding to the four sets of grids at the axial distance of x = 1.0 m. The results indicate that the maximum deviation between the results of 3.4 million grids and 4.2 million grids is 0.57%.

On the premise of meeting the accuracy of the calculation, a relatively small number of grids should be selected to save memory during computer operation, accelerate calculation speed, and reduce the impact of grid numbers on experiments. In this study, the number of 3.4 million is ultimately selected as the subsequent simulation grid number.

2.6. Numerical Validation

Numerical simulation is conducted using ANSYS FLUENT 18.0, which is a commercial CFD software. The SIMPLE algorithm is employed to process the pressure–velocity coupling. The second-order upwind difference scheme is employed for the convection term, and the central difference scheme is employed for the diffusion term.

Ref. [42], which investigated domestic boiler combustion of biomass syngas, is referenced for the comparison. A new design of boiler fired with wood pellets was developed. The numerical calculation results were verified by experiments, and the boiler with the new structure met emission requirements. The gas components (15.1% CO₂, 18.4% CO, 14.2% H₂, 3.3% CH₄, 49.0% N₂₎, the fuel flow rate of 0.002059 Nm³/s, the fuel temperature of 1123 K, the airflow rate of 0.004314 Nm³/s, and the air temperature of 300 K are set. This study sets the same fuel parameters and inlet conditions as the reference to verify the reliability of the geometric and mathematical models used for simulation research.

Because the size of the boiler combustion chamber employed in the research is different from that in the literature, its radial distance is normalized. As shown in Figure 5, R* is the normalized radial distance at the highest temperature position. The maximum relative deviation of temperature is limited to 0.06. The results of the simulation align well with the results in the literature, confirming the model’s reliability.

3. Results and Discussion

The investigation is conducted under two hydrogen addition schemes, focusing on the effect of hydrogen addition on biomass syngas combustion from four perspectives: multi-physical fields, turbulent kinetic energy, field synergy, and emission characteristics.

3.1. Temperature Field and Velocity Field

As shown in Figure 6, the temperature distribution results of six cases along the axial and radial directions (x = 1.0 m) at a constant volumetric flow rate. It is observed that as hydrogen content increases, the average temperature in the combustion chamber shows a decreasing trend, which is similar to the research results in Ref. [9]. In the study of CO/H₂/CH₄/CO₂/N₂ mixtures combustion with different H₂ contents, the combustion temperature of the rich H₂ mixture is lower. With the increase in hydrogen content, the content of other combustible gas components correspondingly decreases. Methane possesses a higher C-H ratio, resulting in a higher lower heating value (LHV). The volume energy density of hydrogen is about one-third that of methane. The LHV of fuel decreases, resulting in a decrease in heat input and temperature inside the combustion chamber.

In Figure 6a, it can also be observed that the low-temperature region in the inlet section significantly reduces with the increase in hydrogen content, the highest temperature is reached at about x = 1.0 m, and the increase in hydrogen concentration shows a certain combustion-supporting effect. As shown in Figure 6b, at the radial section of x = 1.0 m, as the hydrogen content increases, the high-temperature area concentrates towards the center position, and the range of the high-temperature area decreases.

Figure 7 shows the temperature distribution of six cases along the axial and radial directions (x = 1.0 m) at constant heat input. With the increase in hydrogen content, the combustion temperature in the combustion chamber decreases, and the high-temperature region shrinks, which is similar to the results at a constant volumetric flow rate.

When the hydrogen content varies from 5% to 30%, the average outlet temperature also decreases to a certain extent. At constant volumetric flow rate, it decreases from 1298 K to 1212 K, and at constant heat input, it decreases to 1238 K. Figure 8 shows the average outlet temperature result.

Figure 9 shows the velocity distribution along the axial and radial directions (at x = 1.75 m) at a constant volumetric flow rate. As shown in Figure 9a, the areas with high axial velocity are mainly distributed at x = 0.4~0.5 m and x = 1.75 m. When gas fuel enters the main combustion chamber, the gas diffusion rate is fast, and the axial velocity rapidly increases, forming a region with higher velocity. In addition, as the hydrogen content increases, the axial velocity decreases. The diameter of the boiler from the main combustion chamber to the reversal section shrinks, and the airflow is compressed, resulting in an increase in axial velocity. The maximum axial velocity occurs at a position of approximately x = 1.75 m. Figure 9b shows the axial velocity distribution at this radial section. According to the results shown in Figure 8, with the increase in hydrogen content, the temperature decreases to a certain extent. Due to the decrease in temperature and reaction intensity, it may lead to a decrease in axial velocity.

Figure 10 shows the axial and radial distribution (at x = 1.75 m) of velocity at constant heat input. The region with high axial velocity is mainly distributed in the same position. Different from the results at a constant volumetric flow rate, the axial velocity increases with the increase in hydrogen content in the area from the inlet section to the main combustion furnace with an enlarged diameter. The reason is that at constant heat input, the increase in fuel mass flow rate causes an increase in the flow rate of the inlet premixed gas. In the area from the main combustion furnace to the reversal section with shrinking diameter, the axial velocity is smaller in the case of 10% and 15% hydrogen content. As shown in Figure 10b, at this position, the radial distribution of axial velocity has a higher coincidence degree when the hydrogen content is between 25% and 30%.

The effects of hydrogen addition on temperature and velocity fields are significant. Under two hydrogen addition schemes, the temperature inside the combustion chamber decreases because of the lower heating value caused by changes in fuel composition. In the results of the velocity field, higher axial velocity occurs at the positions with sudden changes in the diameter of the combustion chamber. However, there is a significant difference in the effect of hydrogen addition on axial velocity under two hydrogen addition schemes. The reason is that the fuel mass flow rate decreases at a constant volumetric flow rate while it increases at constant heat input.

3.2. Turbulent Kinetic Energy

The distribution of TKE can be used to measure the stability of turbulent motion in the combustion chamber, and its value reflects the magnitude and range of energy loss to some extent. Figure 11 shows the radial TKE distribution at four axial positions: (a) x = 0.5 m boiler inlet section, (b) x = 1.0 m boiler main combustion chamber section, (c) x = 1.75 m furnace shrinkage section, and (d) x = 2.0 m reversal section at constant fuel volumetric flow rate. As shown in Figure 11a, the TKE at the center of the boiler inlet section is relatively low. At a volume percentage of 5% hydrogen, the peak value of TKE appears at the wall position, while in other cases, the peak value of TKE appears at the junction of fuel inlet and air inlet.

Figure 11b,c show the TKE at the positions in the main combustion chamber and the furnace shrinkage section. The average TKE with 30% hydrogen content is lower than that with other hydrogen contents, and the instability is weak.

As shown in Figure 11d, the situation of the TKE in the reversal section is relatively complex, and the average TKE here is higher than that in other positions. When the TKE is high, it can lead to energy loss of the fluid and an increase in the flow resistance. Compared with other positions, the fluid here is more unstable. As the hydrogen content increases, the TKE at the reversal section shows a decreasing trend. Due to the correlation between TKE and factors such as fluid velocity, density, and viscosity, an increase in hydrogen content leads to a decrease in TKE, which is in accordance with the results in Figure 9. The peak value of TKE with 30% hydrogen content is about 52% lower than that with 5% hydrogen content. Hydrogen addition weakens the instability of the fluid in this area.

Figure 12 shows the radial TKE distribution at four axial positions: (a) x = 0.5 m boiler inlet section, (b) x = 1.0 m boiler main combustion chamber section, (c) x = 1.75 m furnace shrinkage section, and (d) x = 2.0 m reversal section at constant heat input. As shown in Figure 12a, the TKE at the inlet section increases with the increase in hydrogen content, which may be related to the velocity here to a certain extent. The results are consistent with Figure 10. As shown in Figure 12b,c, the TKE at the positions of the main combustion chamber and the furnace shrinkage section is relatively similar at 20% to 30% hydrogen content, and it is larger than that at 5% to 15% hydrogen content. Figure 12d shows the radial distribution of TKE at the reversal section, which is numerically larger than the average TKE at other positions. The average TKE is highest at 5% hydrogen content, and the distribution of TKE is similar at 10% and 15% hydrogen content. The average TKE at 15% hydrogen content is 11.7% lower than that at 5% hydrogen content. The distribution of TKE is relatively similar at 20% to 30% hydrogen content, and the average TKE is the lowest at 20% hydrogen content. The average TKE at 20% hydrogen content is 13.8% lower than that at 5% hydrogen content.

The average TKE is relatively high at the position of the reversal section, indicating significant energy loss. The reason is that the flow changes direction in the reversal section, resulting in complex turbulent motion. Under two different hydrogen addition schemes, the average TKE at this position shows a decreasing trend with the increase in hydrogen content to varying degrees, indicating a decrease in flow instability and energy loss.

3.3. Field Synergy

The field synergy principle was proposed to reveal the essence of enhanced convective heat transfer. The synergy of the velocity and temperature fields in the convective heat transfer area proved to be the underlying mechanism. The small synergy angle between the velocity and temperature gradient leads to a good heat transfer performance.

Figure 13 shows the contours of the synergy angle β at a constant volumetric flow rate. The better synergy of velocity and temperature gradient can be seen from the boiler inlet section to the central area of the furnace. As the hydrogen content increases, the region with small synergy angle β is enlarged to some extent. As can be seen from the figure, the area with small synergy angle β also appears in the reversal section, and the overall distribution of synergy angle β in this area is more complex. The reason is that the direction of the outlet is opposite to the direction of the fuel inlet, and the direction of the flue gas flow changes here, resulting in the flow field being more complex.

Figure 14 shows the volume average synergy angle β, γ, and the combustion efficiency η at different hydrogen contents at a constant volumetric flow rate. As shown in the figure, when the hydrogen content increases from 5% to 20%, the volume average synergy angle β shows a downward trend. When the hydrogen content is 5%, the volume average synergy angle β is approximately 72.04°. When the hydrogen content increases to 20%, the volume average synergy angle β reduces to 68.97°. When the hydrogen content is greater than 20%, the volume average β gradually increases. At a hydrogen content of 30%, the volume average synergy angle β rises to 69.32°. The heat transfer performance is better when the hydrogen content is 20%.

In addition, as shown in Figure 14, when the hydrogen content increases from 5% to 20%, the volume average synergy angle γ shows an upward trend. When the hydrogen content is 20%, the synergy angle γ is approximately 39.2°. As the hydrogen content continues to increase, the synergy angle γ gradually decreases. According to the field synergy theory, the comprehensive performance of enhanced heat transfer improves with the synergy angle γ [43]. Among the six different hydrogen addition ratios, the volume average synergy angle β of case 4 is the smallest, and the volume average synergy angle γ is the largest, which indicates the best heat transfer performance under this hydrogen addition ratio. To some extent, hydrogen addition can optimize the heat transfer performance of biomass syngas combustion, but excessive hydrogen addition may reduce the optimization effect. This result is similar to Ref. [26], as hydrogen addition within a certain range can improve the overall performance of flow resistance and heat transfer in the combustion chamber. In addition, the changing trend of combustion efficiency η is similar to synergy angle γ. The calculation formula of combustion efficiency η is given in Ref. [44].

Figure 15 shows the contours of synergy angle β at six different hydrogen addition ratios in the center section of a combustion chamber at constant heat input. The distribution of contour is similar to that at a constant volumetric flow rate, and the area with good synergy is distributed from the inlet section of the boiler to the main combustion section. As the hydrogen content increases, the average synergy angle β at the inlet section significantly decreases. When the hydrogen content is 25%, the area with a smaller synergy angle at the inlet section expands to maximum, and the hydrogen content continues to increase to 30%. The average synergy angle at the inlet section slightly increases. When the hydrogen content increases from 5% to 10%, the average synergy angle β at the reversal section significantly decreases. If the hydrogen content continues to increase, the impact on heat transfer here is relatively small.

Figure 16 shows the volume average synergy angle β, γ, and combustion efficiency η with different hydrogen content at constant heat input. Combined with the distribution of synergy angle β in Figure 15 and Figure 16, it is evident that as the hydrogen addition ratios increase, the area with a small synergy angle β from the boiler inlet section to the center of the furnace expands significantly, indicating a great influence on the heat transfer in this area. When the hydrogen content is 25%, the volume average synergy angle β is the smallest, about 69.4°, and the synergy of velocity and temperature gradient is the best. Furthermore, as shown in Figure 16, the synergy angle γ first increases and then decreases with the increase in hydrogen content. It reaches the maximum value at 25% hydrogen content, about 37.9°, which has the best comprehensive performance for enhancing heat transfer. When hydrogen content increases from 20% to 25%, the synergy angle γ increases the most, about 11.1%. The combustion efficiency η reaches the maximum value at 25% hydrogen content. Therefore, at constant heat input, hydrogen addition improves the synergy and combustion efficiency, achieving the best results at 25% hydrogen content. However, if the hydrogen content continues to increase, the heat transfer effect weakens, and the combustion efficiency decreases.

3.4. Pollutant Emissions

This study mainly considers thermal NO_x and rapid NO_x. Figure 17 shows the emissions of pollutants at the outlet of the combustor. At a constant volumetric flow rate, as the hydrogen content increases, the NO_x emission decreases to a certain extent. When the hydrogen content varies from 5% to 30%, the NO_x emission is reduced by 67%. There is a direct correlation between thermal NO_x and temperature, and the reaction rate of thermal NO_x is particularly important when the temperature is higher than 1800 K. As shown in Figure 6, due to the increase in hydrogen content, the calorific value of fuel and the combustion temperature in the boiler decrease, resulting in a decrease in NO_x emission. Du et al. [45] investigated the combustion of H₂/CO₂/CH₄ syngas in a micro gas turbine, and their results showed that the addition of carbon-free fuel (H₂) effectively reduced CO emissions, and lower NO_x emissions were caused by the decrease in flame temperature, which is consistent with the results of this study.

In addition, an increase in hydrogen content leads to a decrease in carbon content in gas fuels, directly resulting in emissions of carbon compounds. Moreover, the improvement of combustion efficiency and stability in the combustion chamber makes the oxidation reaction efficiency better, reduces the generation of incomplete oxidation product CO, and also reduces the CO₂ emission generated by CO oxidation. Hydrogen addition can effectively control the emissions of CO₂ and CO.

Furthermore, the NO_x, CO, and CO₂ emissions at the outlet of the combustor at constant heat input, and the results are similar to those at a constant volumetric flow rate. The NO_x, CO, and CO₂ at constant volumetric flow rates all show a decreasing trend as the hydrogen content increases. The changes in CO emissions are extremely close. Li et al. [46] also found that NO_x emission decreased effectively with the increase in hydrogen content. The reason is attributed to the decomposition of the hydrogen molecule providing two H atoms, which causes the reaction Equation (30) to proceed in reverse.

4. Conclusions

This study takes a certain type of horizontal biomass syngas boiler as the research object. Combined with the results of multiple physical fields, TKE, and field synergy, the effect of hydrogen addition on biomass syngas combustion and emissions is further expounded. Two scenarios are taken into consideration for fair comparison: hydrogen addition at a constant fuel volume flow rate and constant heat input. The conclusions are as follows:

• Temperature and velocity fields are significantly affected due to the addition of hydrogen. The temperature in the combustion chamber shows a decreasing trend as the hydrogen addition ratios increase and the temperature peak moves towards the inlet direction. The reduction in combustion temperature reduces the emission of thermal NO_x, resulting in a significant reduction in the overall NO_x emission. In addition, hydrogen addition effectively reduces the emissions of pollutants CO and CO₂.
• The variation of hydrogen content significantly affects the velocity field and TKE distribution in the combustion chamber. Compared to other positions, the TKE at the position of the reversal section is greater, and the fluid instability is stronger. At a constant fuel volume flow rate, with the increase in hydrogen content, the TKE at the reversal section shows a decreasing trend, which is positively correlated with the trend of velocity changing with hydrogen content at this point. At constant heat input, the average TKE is highest at 5% hydrogen content.
• As the hydrogen content increases, the volume average synergy angle β initially decreases and subsequently increases, while the synergy angle γ and combustion efficiency first increases and then decreases. Hydrogen addition can optimize the heat transfer performance of biomass syngas combustion and improve the comprehensive heat transfer performance, but excessive hydrogen addition may reduce its positive effect. The addition of hydrogen has a slight impact on combustion efficiency, and the deviation between the maximum and minimum values does not exceed 0.3%.

Hydrogen addition can improve the comprehensive heat transfer performance and combustion efficiency of biomass syngas to a certain extent and reduce the emissions of NO_x, CO, and CO₂. Although this study involves a special type of boiler, the evaluation method and results of its combustion performance provide certain guidance for the efficient utilization of biomass syngas and the operation of boilers burning biomass syngas.