Domestic Heating: Can Hemp-Hurd Derived Pellet Be an Alternative?

2.1 Hemp-hurd pelletization

The hemp-hurd is generally found in the form of sticks/chunks (Figure 1), due to the shredding process that the hemp faces in the harvesting procedure.

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Figure 1. Hemp-hurd chunks as received from the producer

This form factor is not suitable for most of the domestic stoves which are fueled with wood logs or pellets. For this reason, the hemp residues needed to be pelletized to homogenize the form factor and make them suitable for the selected stove. Literature reports studies regarding the palletization of agricultural residues for energy purposes (i.e., palm fruit skins [9], vine pruning [5], digestate [10]).

As reported in the paper from Pedrazzi et al. [11], the palletization of hemp-hurd is challenging due to its structure composition and a certain amount of fir sawdust need to be added to obtain a stable and compact pellet.

In this work, the hemp-hurd chunks were firstly pulverized by means of an electric grinder and then mixed with a 50% of fir sawdust. A domestic-scale electric pelletizer was used to compress the biomass mix into pellets 6 mm diameter.

2.2 Pellet stove instrumentation

To test the capability of hemp-hurd to be a reliable fuel for domestic heating, a commercial pellet stove with a nominal power of 9 kW_th was used as a test rig. The selected stove use forced air flow to dissipate the combustion heat and no hydronic devices are present. The stove can be subdivided into 4 main blocks (Figure 3a):

• The feeding system: a timed feeding auger regularly feeds the combustion process and a 25 L hopper, placed above it, ensures a minimum operating range of 8 hrs.
• The combustion system: it consists of a stainless-steel fire chamber, with an embedded grate cleaning mechanism, followed by a series of turbolated exhaust-gases/air heat exchangers to cool down the fumes. The stove is kept under vacuum by the fumes blower which is placed at the chimney inlet.
• The cooling system: on the room side, an air blower forces the ambient air through the external surface of the heat exchanger heating it up and so reducing the fumes temperature.
• The electronic control unit: it controls the power of the stove by modifying the auger feed rate, it regulates the speed of the cooling fan according to the fumes temperature and it regulates the vacuum fan speed according to the power.

In the paragraphs below, the test rig setup is shown together with the instrument used to carry out energy and efficiency calculations.

2.2.1 Air flowrate measurements

Three different flows need to be considered to take into account the energy pathways of the stove (Figure 3a): the air flowrate of the air blower for exhaust gas cooling (A), the flowrate of the blower for the combustion air (B) and the flowrate of the biomass (C).

The cooling air flowrate was measured by means of a fan anemometer (TESTO 417) which was adapted to the stove by building a plenum between the fan inlet and the instrument itself. The second measurement is the airflow necessary for the combustion and, in this case, a volumetric meter was connected to the inlet hose of the fire chamber. The exhaust flowrate was then calculated according to the equations reported in the paragraph 2.3.4.

2.2.2 Temperature measurements

A thermocouple datalogger TC-08 from PicoTech was used to log the temperatures at strategic locations in the stove.

In the Table 1 below, a scheme of the thermocouple used is reported. The same nomenclature is also used in the results chapter and the progressive numbers are reported in Figure 3b, c, d showing the position of the probes on the stove.

Table 1. Thermocouples installation scheme

As shown in the thermal image in Figure 2, the temperature distribution at the heat exchanger outlet on the room side is far from be homogeneous. For this reason, 5 thermocouples were placed at the HX outlet to average the result.

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Figure 2. Thermal image of the heat exchanger outlet which shows the temperature inhomogeneity at the heat exchanger outlet

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Figure 3. Scheme (a) and instrumentation of the wood pellet stove (b, c, d)

2.3 Test methodology

2.3.1 Pellet chemical characterization

Pellet characterization follows several steps in order to obtain all the information needed to assess the heating value of the biomass. The moisture content of the pelletized material was measured according to the ASTM E1756-08 standard method (Eq. (1)) both for the hemp-hurd and for the A2 grade commercial pellet.

   $M_{B I O, W B}=\frac{m_{B I O, w e t}-m_{B I O, d r y}}{m_{B I O, w e t}}$       (1)

The chemical composition of the different pellets was characterized through the ultimate analysis, performed with a CHNS-O analyzer, that returns the weigh fraction of carbon, hydrogen, nitrogen, sulfur and oxygen. The ash content was determined through a 6 hours muffle furnace calcination at 600℃. The chemical and moisture analyses were used to calculate the biomass higher heating value according to the Channiwala-Parikh correlation [12] (Eq. (2)).

$\mathrm{HHV}_{\mathrm{BIO}}=349.1 \mathrm{C}+1178.3 \mathrm{H}+100.5 \mathrm{~S}-103.40$

$-15.1 \mathrm{~N}-21.1 \mathrm{ASH}$      (2)

The chemical analysis was then repeated on the combustion residues collected from the ash vessel at the end of each test and the composition and ash content of these residues was used to accurately calculate the amount biomass that was really burned during the test. This data is necessary to calculate the amount of exhaust gases flowrate starting from the measurement of the inlet airflow for combustion. The followed procedure is shown in the paragraph 2.3.3.

2.3.2 Combustion test procedure

The combustion tests were carried out with the stove installed inside a room. The room (ambient) temperature was kept constant during the tests by mixing the room air with the colder external one.

To compare the performance of hemp-hurd derived pellets versus A2 standard pellet, three tests were carried out for each class at three different stove nominal power output: 5, 7 and 9 kW_th which correspond to three different feeding auger speed (5, 7 and 9).

The stove power is selected via ECU digital panel and at each power output the stove regulates the speed of the feeding auger to match the power needed, according to the pellet heating value manually entered in the ECU. Since there is no loop control on what the actual power of the stove is, an ex-post calculation based on the collected data was carried out and presented in the equations below. The testing procedure is resumed in the following steps:

• The stove hopper was emptied until a fixed level is reached (starting level).
• The hopper was filled with the selected amount of biomass.
• The stove was fired up following the constructor procedure passing from ignition phase to steady state power output phase.
• Temperatures were automatically logged every second while air flow rates were manually logged.
• When the biomass in the hopper reached the starting level again, the stove was stopped.
• The fire chamber was cleaned, and the amount of combustion residues weighed and analyzed in terms of chemical composition and ash content.

2.3.3 Biomass flow rate calculation at steady state

During each test the pellet stove passes through several steps from startup to shut down:

• Ignition phase: the feeding auger turns for a fixed amount of time and loads a certain quantity of biomass. The hot bulb, placed on a side of the fire-chamber wall, ignites some pellets while the exhaust fan starts to extract fumes and then drawn combustion air from the room. 
• Stabilization phase: once a portion of biomass is fired up, the exhaust fan increases its speed, drowning more combustion air to the fire chamber.
• Ramp up phase: once the fumes temperature reaches a certain threshold, the auger starts to feed the combustion chamber at the nominal power output of 2 kW_th. Step by step, the stove increases the thermal power output (i.e. the auger rotating speed) to reach the set power.
• Steady state phase: the stove reached the prefixed power output and all the controls (fans and auger) operate at steady state conditions.

The biomass consumption calculation, for the purpose of estimating the chemical power, need to be carried out at steady state, while the stove is at the prefixed power output. For this reason, the ignition load and the biomass consumption in the ramp up period need to be removed from the total.

While the ignition load can be measured by stopping the stove after the initial load phase and weighing the biomass contained by the fire chamber, the ramp up biomass load can only be indirectly calculated from the equations below by knowing the total biomass consumption and the time spent at each power step. The apex ( ' ) indicates nominal results based on the stove setup parameters.

$E_{B I O, t o t}^{\prime}=\sum_{i=2}^{\text {steady }} \dot{P}_{B I O, i}^{\prime} \cdot \Delta t_{i}$   [J]   (3)

$E_{\text {BIO,steady }}^{\prime}=\dot{P^{\prime}}_{\text {BIO,steady }}^{\prime} \cdot \Delta t_{\text {steady }}$   [J]  (4)

$\begin{aligned} m_{\text {BIO,steady }} &=\frac{E_{\text {BIO,steady }}^{\prime}}{E_{\text {BIO,tot }}^{\prime}} \cdot\left(m_{\text {bio,tot }}\right.\\ &\left.-m_{\text {bio,ign }}\right) \end{aligned}$   [kg]  (5)

Hence, the biomass flowrate at steady state condition can be calculated as follows:

$\dot{m}_{B I O}=\dot{m}_{B I O, \text { steady }}=\frac{m_{\text {BIO,steady }}}{\Delta t_{\text {steady }}} \quad\left[\operatorname{kg} h^{-1}\right]$   $\left[\mathrm{kg} h^{-1}\right]$   (6)

According to the biomass HHV the real power output was calculated according to the next equation:

$\dot{P}_{B I O}=\dot{P}_{B I O, \text { steady }}=\dot{m}_{B I O} \cdot H H V_{B I O}$    [W] （7）

2.3.4 Exhaust gases flowrate estimation

The fumes flowrate estimation is based on two information:

• The measurement of the air flowrate deputed to the biomass combustion by means of the gas meter placed at the stove air inlet;
• The calculation of the exhaust gases composition starting from the biomass and the combustion residues composition following the next procedure.

The flowrate of each element composing the biomass was calculated according to the biomass flowrate at steady state conditions and the mass concentration of the element X.

$\dot{m}_{X, B I O}=\dot{m}_{B I O} \cdot X_{\%} \quad\left[\operatorname{kg} h^{-1}\right]$     (8)

The same element may remain in the combustion residues, and contributes to the residues flowrate:

$\dot{m}_{X, R E S}=\dot{m}_{B I O} \cdot X_{A S H, \%} \cdot \frac{A S H_{B I O, \%}}{A S H_{R E S . \%}} \quad\left[\mathrm{~kg} h^{-1}\right]$    (9)

The combustion products were calculated according to the net mass flow of elements that participated to the combustion:

$\dot{m}_{C O_{2}}=\frac{\dot{m}_{C, B I O}-\dot{m}_{C, R E S}}{12} \cdot 44 \quad\left[\mathrm{~kg} h^{-1}\right]$    (10)

$\dot{m}_{H_{2} O}=\frac{\dot{m}_{H_{2} O, B I O}-\dot{m}_{H_{2} O, R E S}}{2} \cdot 18 \quad\left[\mathrm{~kg} h^{-1}\right]$     (11)

The combustion of biomass is usually performed with an excess of air (thus of oxygen) [13] to guarantee a complete burning of the solid material. The calculation of the residual oxygen in the exhaust gases can be obtained by adding the oxygen in the combustion air to the one present in the molecular structure of the biomass and removing the oxygen present in the combustion residues.

$\dot{m}_{O_{2}}=\dot{m}_{O_{2}, A I R}+\dot{m}_{O_{2}, B I O}-\dot{m}_{O_{2}, R E S}$$-\dot{m}_{O_{2}, \text { stoichio }}$    $\left[\mathrm{kg} h^{-1}\right]$   (12)

where,

$\dot{m}_{O_{2}, \text { stoichio }}=\frac{\dot{m}_{C, B I O}-\dot{m}_{C, R E S}}{12} \cdot 32+\frac{\dot{m}_{H, B I O}-\dot{m}_{H, R E S}}{2} \cdot 16 \quad\left[\mathrm{~kg} h^{-1}\right]$     (13)

is the oxygen needed for the stoichiometric combustion of the net biomass flowrate. According to these equations it is possible to define the excess air (e) as in the following:

$e=\frac{\dot{m}_{O_{2}, A I R}}{\dot{m}_{O_{2}, \text { stoichio }}}$     (14)

In Figure 4 a Sankey diagram resumes the biomass and air pathways explained in the above equations.

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Figure 4. Example of the combustion pathways for standard A2 pellet

The exhaust gas flowrate is then calculated as follows:

$\dot{V}_{E X}=\sum_{i}^{N} i_{\%} \cdot \dot{m}_{b i o} \quad\left[\operatorname{kg} h^{-1}\right]$    (15)

where, i stays for the mass fraction of each component of the exhaust: N₂, Ar, H₂O, CO₂ and O₂.

2.3.5 Energy calculation

The heat generated during the combustion is dissipated in several ways: during the startup period, part of the heat is stored as sensible heat in stove structure while the remaining part is dissipated through natural convection and radiation from the hot surfaces of the device, forced convection through the air/fumes HX and part of it is lost at the chimney.

Since the calculation of the convective heat transfer coefficient can be affected by a significant uncertainty, in this paper, the evaluation and comparison of the wood stove efficiency between the different fuels was carried out by comparing:

• The portion of heat transfer due to forced convection (e.g. the heat dissipated by the air/fumes HX);
• The portion of heat lost at the chimney.

The former was calculated according to Eq. (16) which represent the energy balance between the heat exchanger inlet and outlet.

$\dot{Q}_{H X}=\dot{V}_{A I R, H X} \cdot \rho_{A I R} \cdot c_{p, A I R} \cdot\left(T_{A m b i e n t}-\bar{T}_{H X}\right)[W]$    (16)

The latter was calculated in the same way but using the calculated exhaust flowrate (Eq. (17)).

$\dot{Q}_{E X H A U S T}=\dot{V}_{E X} \cdot \rho_{E X} \cdot c_{p, E X} \cdot\left(T_{E x h a u s t}-T_{A m b i e n t}\right) \quad[W]$     (17)

Where the density ($\left(\rho_{A I R}\right)$) and the specific heat capacity $\left(c_{p, A I R}\right)$ were calculated referring to the ambient temperature of the air through the following equations extrapolated from Eq. (16):

 $\rho_{A I R}=-2.673748 \cdot 10^{-14} \cdot T^{5}+4.77340 \cdot 10^{-12} \cdot T^{4}-3.56265 \cdot 10^{-8} \cdot T^{3}+1.53559$

$\cdot 10^{-5} \cdot T^{2}-4.75260 \cdot 10^{-3} \cdot T+1.3$    (18)

$c_{p, A I R}=-2.64444 \cdot 10^{-8} \cdot T^{3}+6.81522 \cdot 10^{-5} \cdot T^{2}+1.40874 \cdot 10^{-1} \cdot T+951.71757$     (19)

and the same properties for flue gases were calculated referring to similar equations, extrapolated from [14], but modified according to the calculated flue gas composition.

2.3.6 Effectiveness and efficiency parameters

According to the above heat power output definitions, two key parameters can be identified to facilitate the comparison between hemp-hurd derived pellets and standard A2 pellets.

The effectiveness of the forced convection heat exchanger:

$\eta_{F C}=\frac{\dot{Q}_{F C}}{\dot{P}_{B I O}}$     (20)

and the overall efficiency of the pellet stove:

$\eta_{t o t}=1-\frac{\dot{Q}_{E x h a u s t}}{\dot{P}_{B I O}}$     (21)

In this work, the capability of hemp-hurd derived pellet to be used as fuel in domestic pellet stove was investigated and compared to standard A2-grade commercial pellet. To increase the pelletability of hemp hurd, a 50% of fir sawdust was added to the hemp chunks and, this mixture, gave the chance to form pellet on average shorter than the commercial ones but with a higher variability. In fact, the average dimension of A2 pellets was 12.2 mm while for the hemp-hurd ones was 8.2 mm. The variability of the dimensions was 4.9 mm for the A2 and 6.2 mm for the hemp pellets which shown pellets even longer than 30 mm that resulted in a lower filling rate of the auger and thus a lower biomass input flowrate if compared to A2.

The ultimate and ash analyses of the fuels shown a very similar composition resulting in an HHV of 19.8 MJ kg^-1 for the A2 and 18.9 MJ kg^-1 for the hemp pellet. Even if the ash content of the hemp pellets resulted in 2.7%, the grate cleaning mechanism of the stove was able to self-clean the fire chamber keeping the performances constant over the test. 

The performance comparison was carried out by testing the fuels combustion at different thermal power output, involving the calculation of the overall stove efficiency and the effectiveness of the air/fumes heat exchanger. Results shown an overall efficiency always over 90% for both the fuels, with a slight advantage for A2 pellet, likely due to the higher flame temperatures which resulted also in a higher heat exchanger effectiveness.

The excess air was noticed to be not aligned with the hemp-hurd pellet combustion, giving values up to 236 %, compared to the 140% of the A2 at the same stove setup. This is mainly due by the fact that there is no feedback loop control con the combustion air flow rate based on the excess air (no oxygen sensor available in this stove model) and it can affect the pollutant emission of the stove. For this reason, further tests should be carried out to investigate the optimal parameters for the hemp fuel.

Concluding, the 50/50 mixture between fir sawdust and hemp-hurd chunks can be considered a pelletizable fuel for domestic pellet stove. Attention should be paid in both investigating the pelletization process, in order to homogenize the pellet dimensions, and to modify the wood stove parameters in order to increase the auger feeding rate and thus to reach the maximum nominal power of 9 kW_th even running under hemp-hurd.