Evaluation of the Performance of a Small Biomass Gasifier and Micro-CHP Plant for Agro-industrial Firms

2.1 Gasification process

The gasification process is the thermal conversion of organic materials at elevated temperature and reducing conditions to produce primarily permanent gases, with char, water, and condensable gases as minor products [15, 16].

It is a complex and sensitive process, which depends by the chemical composition and moisture content of the biomass; mass flow and temperature of oxidant; temperature and pressure during the process; heating rate; gasifier design and so on. Between the different technologies of gasifier, the downdraft gasifier (Figure 1) is particularly attractive thanks to its ability for producing a syngas with a very low tar content (1%) in comparison with other gasification technologies.

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Figure 1.  Downdraft gasifier.

The biomass, introduced into the gasifier reactor, is initially subject at the process of drying and pyrolysis. At this stage, the reactor temperature reaches values between 200°C and 500°C so the moisture content of biomass is converted in steam. During this process, char, tar and other volatile species such as CO, CO₂, H₂, and CH₄ are formed. Then, such products flowing in the combustion-reduction zone of the gasifier, where temperatures around 1000°C are reached. At this stage there is the production of ash, char and syngas, which is composed mainly by CO, CO₂, H₂, CH₄, N₂ and H₂O. Thermodynamic balance models allow to predict with sufficient accuracy the chemical composition of the syngas produced through a downdraft gasifier. Usually, thermodynamic models are based on the following assumptions:

•gasifier operates in steady-state conditions at pressure of 101.13 kPa;

•permanence time of the vapors into the gasifier is sufficient to reach chemical balance in adiabatic conditions;

•air is supplied in dry conditions at temperature of 25°C and pressure of 101.13 kPa;

•ash are inert materials and the produced gas is an ideal gas composed by CO, CO₂, H₂, CH₄, H₂O, N₂ and tar.

2.2 Mathematical model​

In accordance with the previously mentioned assumptions, the overall gasification reaction of one mole of biomass is expressed as follows:

$C H_{h} O_{o} N_{n}+w H_{2} O+a\left(O_{2}+3.76 N_{2}\right) \rightarrow x_{1} H_{2}+x_{2} C O+x_{3} C O_{2}+x_{5} C H_{4}+x_{6} N_{2}+x_{t a r} t a r+x_{c h a r} c h a r$   (1)

Where: “h”, “o” and “n” are the number of atoms of hydrogen, oxygen and nitrogen; “w” is the moles number of H₂O; “a” is the moles number of air incoming in the gasifier; the terms “x_1,..6”, on the right of the (Eq. 1), indicate the moles number of the chemical components of the produced gas; “x_tar” and “x_char” indicate respectively the produced moles number of tar and char.

The unknown quantities of the reaction (1) are the numbers of moles “x_i”, from x₁ to x₅, since nitrogen is considered inert and tar and char moles are input parameters of the model. Considering that also the equilibrium temperature (T) is unknown, globally we have six unknown quantities that can be determined solving a system of six equations.  Three mass balance equations; two equations for the equilibrium constants, k₁ and k₂, of the chemical reactions taken in account; one equation of the gasifier energy balance.

A mathematical model has been developed in Matlab that allows the iterative resolution of those six equations.

Figure 2 shows the algorithm scheme of the numerical model developed in Matlab environment.

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Figure 2. Matlab resolution scheme of the developed model

The numerical model of the gasification process was validated comparing the results of the simulation with experimental data [17].

The micro-CHP unit is based on a spark ignition engine fed by the produced syngas. The plant has a circuit of heat recovery on a secondary circuit of heating. Two heat exchangers placed in series with the internal combustion engine constitute the section of heat recovery as depicted in the plant layout in Figure 4.

Moreover, plant layouts that adopt two independent circuits to recover the heat by the exhaust gases and jacket cooling system, can be adopted. In the proposed layout, the engine is cooled with a pipe heat exchanger while the enthalpy content in the exhaust gas is recovered with a plate heat exchanger. The thermal power effectively recoverable from the plant depends on the technology of engine, efficiency and surfaces of the heat exchangers system [26].

The production of electrical energy is achieved connecting the crankshaft to a three-phase generator that converts the DC current in AC current and it is transferred to the distributed electric grid at frequency of 50 Hz by an inverter. The electric generator is switched on angular velocity of the rotor above 1500 (rpm).

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Figure 4. Layout of the micro-CHP plant

4.1 Pollutant emissions

The concentration of combustion products as CO, NO_x, SO_x, and HCl depends by the syngas composition. A syngas with high concentrations of CO and H₂ reduces the probability of CO emissions [27]. One of the three formation modes of NO_x, “Prompt NO_x”, occurs in conditions of rich mixture of fuel, gas at high concentration of CH₄, when the N₂ reacts with active carbons. The other two processes of formation are “thermal NO_x” and “combustible NO_x”, as reported in [28].

Hydrogen and carbon monoxide, contained in the syngas, require a high temperature of burning which facilitates the thermal formation of NO and NO₂. On the contrary, high temperatures favor the complete combustion and reduce the emissions of volatile organics mainly consisting by low fractions of hydrocarbons. The particulate matter, metallic compounds and other undesirable pollutants are usually removed before that the syngas is burned to produce electrical and thermal energy.

4.2 Operative parameters

The performance of the micro CHP plant were evaluated under the operative parameters reported in table 4.

Table 4. Parameters of gasifier

Further, the technical features of a commercial ICE model Cummins G855G [29] were used in the following calculation.  (see table 5) .

Table 5. Data sheet of the engine model

Under stoichiometric combustion, the air-fuel ratio (α_s) is equal to 1.43 (kg_a/kg_b), and it is significantly lower than the one of the natural gas (17.0 kg_a/kg_b). Consequently, the specific power per unit of air-fuel mixture decreases [30].

Therefore, considering a process of complete combustion and adiabatic conditions an increase of excess air (EA) reduces the flame maximum temperature and the NO_x emissions [31].

We have assumed an index of EA equal to 1.90 since it is within the range suggested for the syngas combustion that ranges between 1.6- 2.5 [31, 32]. The adoption of such value of EA (1.90) determines an air-fuel ratio (α) of 2.72 (kg_a/kg_b).

The characteristics of syngas and the operative conditions previously mentioned for the micro-CHP plant do not involve substantial variations for the engine but it is necessary increase the nozzles section of intake-air and the exhaust gases. These changes are necessary to satisfy the flows of syngas and air that assume higher values than the ones of the natural gas during the combustion [33].

The higher value of LHV of natural gas (46÷50 MJ/kg) compared to syngas involves that thermal power produced by syngas is lower than that natural gas for the same fuel flow rate. To achieve the same thermal power, it is necessary to consider the following ratio:

$\frac{\dot{m}_{syn}}{\dot{m}_{C H_{4}}}=\frac{L H V_{C H_{4}}}{L H V_{syn}}=5 \div 10$     (2)

In order to characterize the behavior and the performance of the engine, we have calculated the effective power (P_e), varying the air-fuel ratio (α), ratio between the volume air-intake and the volume of generated air (λ_v) and engine speed (1500 - 2400 rpm). All the calculations were carried out considering the engine runs continuously and fully loaded. The effective power (P_e) was calculated using the following expression:

$P_{e}=\eta_{u} \lambda_{V} \frac{\rho_{a} Z V_{u}}{\alpha} L H V_{s y n} \frac{n_{g}}{\varepsilon}$ (3)

Where η_u is useful efficiency; λ_v is ratio between taken air and generated air; Z is number of cylinder; V_u is cylinder displacement volume; n_s is the number of crank rounds; ρ_a is air density, ε=T/2 is the number of the crank rounds for cycle.

4.3 Energy balances

The thermodynamic model used for the energy analysis of the micro-CHP system is based on the 1^st principle of the thermodynamic. The engine operates according to the Otto cycle and it has a mechanical work expressed as difference between expansion work and compression work. The authors used the equation reported in [34] with the assumption that the inlet temperatures of syngas and combustion air are coincident with the outdoor air temperature.

The input and output power were calculated using the equations of energy balance [35].       

In table 6 are reported data used in the energy model of the micro-CHP with an electric power of 70 kW_el. The cooling water flow rate (ṁ_c) was assumed equal to 16,000 (kg/h) according to the technical characteristics of the engine manufacturer [29].

Table 6. Operative data

The mechanical power (P_m) available at the crankshaft was calculated with the Eq. (4) under the following assumptions: heat losses in combustion chamber (Q_d) and mechanical losses for friction (Q_m) were assumed equal to 7% and 5% of the input power [36]; the losses due to the lubrication oil circuit (Q_o) represent 6% of the input power [36].

$\begin{aligned} P_{m} &=\dot{m}_{s y n} L H V_{s y n}-\dot{m}_{c} c_{p, c} \Delta T_{c}-\dot{m}_{e g} c_{p, e g} \Delta T_{e g} -\dot{Q}_{d}-\dot{Q}_{o}-\dot{Q}_{m} \end{aligned}$   (4)

where:

 ṁ_c is the cooling water mass flow rate

c_p,c is the specific heat of the cooling water

∆T_c is the difference between inlet and outlet temperature of cooling water

ṁ_eg is the exhaust gases mass flow rate

 c_p,eg is the specific heat of exhaust gases

∆T_eg is difference between inlet and outlet temperature of exhaust gases.

The electric power (P_el) was calculated considering an efficiency of electric generator of the 96%. In Table 7 are reported the input and output powers of the internal combustion engine (ICE).

Table 7. Input and output powers

Globally, the engine reaches an electrical efficiency of 20% that is lower of the efficiency of the same engine fueled with natural gas, which has values of 25÷28%. The thermal power recovered by the heat exchangers of exhaust gases and water cooling are reported in the table 8. These thermal powers were calculated adopting an efficiency for the heat exchanger (ε_HX) equal to 85%. The water flow rate to provide to the users (ṁ_w) was established equal to 10,000 (kg/h), in order to guarantee the typical operating conditions of an engine.

Table 8. Recovery thermal power

The micro-CHP plant can recover a thermal power (P_th) of 79.33 and 92.15 (kW_th) respectively from the cooling water and exhaust gases.

The electric characteristic of the micro-CHP system expressed as ratio between electric efficiency (20%) and thermal efficiency (48%) is equal to 0.41. The energy utilization factor (EUF) [37] is calculated as follow:

$E U F=\frac{\dot{P}_{e l}+\dot{P}_{t h}}{\dot{m}_{s y n} L H V_{s y n}}=0.68$      (5)

The Sankey diagram (Figure 5) illustrates the input and the output of energy flux for different subsystems of thermal energy, normalized to 100%, coming from the combustion of the syngas in the micro-CHP system.

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Figure 5. Sankey diagram of the micro-CHP plant

4.4 Gasification efficiency

The gasification efficiency is defined as efficiency of cold gas (η_CG) and it is achieved by the ratio between the energy of the syngas and the energetic potential of the biomass. This ratio depends on the lower heating value (LHV) of both fuels, rate of biomass consumption (ṁ_bio) and flow rate of syngas exiting from the gasifier (ṁ_syn) [19].

$\eta_{C G}=\frac{L H V_{s y n} \dot{m}_{s y n}}{L H V_{b i o} \dot{m}_{b i o}}$     (6)

In Table 9, are reported the flow-rate of biomass and syngas respectively incoming and exiting from the downdraft gasifier, the indicators of production and efficiency of cold gas of the gasifier. It is possible to achieve a performance of gasification of 60% with a flow rate of syngas of 246 (kg/h) incoming to the engine, if it is used a flow rate of biomass of 120 (kg/h) into the gasifier.

Table 9. Production and efficiency of gasification

The aim of this paper is provide information on the production of electrical and thermal energy achievable from a micro-CHP plant fueled with syngas. In particular, it was adopted a syngas produced by short rotation forestry and others agricultural activities related to olive kernel. The chemical composition of the syngas was obtained using a calculation model based on the equations of the thermodynamic balance. The syngas produced has a LHV equal to 5.16 MJ/kg against a LHV of the biomass of 19.31 MJ/kg. The energy productions were calculated considering a commercial internal combustion engine model Cummins G855G that uses an index of excess air equal to 1.90.

Globally, the micro-CHP plant achieves the following coefficients of performance: Electrical Efficiency equal to 20%; Thermal Efficiency equal to 48%; Energy Utilization Factor (EUF) equal to 0.68. The analysis of the greenhouse gas emissions shows that the emissions produced by the syngas combustion are more of two order greater than the ones of natural gas or propane combustion. However, the environmental balance is positive because the CO₂ derived from the engine supplied with natural gas and propane totally contribute at the increase of greenhouse gas in the atmosphere. Otherwise, the emissions of CO₂ derived from the engine supplied with the syngas produced by the processes of thermochemical conversion of the biomasses, have to be cut by the aliquot of CO₂ taken out of the atmosphere during the phase of growth of the biomasses itself.