Products Distribution During Sewage Sludge Pyrolysis in a Sand and Olivine Fluidized Bed Reactor: Comparison with Woody Waste

Abstract

The purpose of this article is to improve the understanding on sewage sludge (SS) pyrolysis behavior in a fluidized bed reactor between 700 and 890 °C. The experiments were carried out under a pure nitrogen atmosphere using sand and olivine as bed materials. The results gathered in this paper highlight the impact of temperature and bed materials on various performance criteria. It was observed that the use of olivine as a bed material affects the composition of the syngas, especially the fraction of the light hydrocarbons CH₄ and C₂H_X. It was also noticed that increasing the pyrolysis temperature yielded more syngas and reduced the condensable species and char quantities. Furthermore, an increase in temperature led to an increase in the fraction of benzene in tars. Comparing different kinds of fuels revealed that SS pyrolysis yields less syngas than pinewood and more syngas than municipal green waste. SS pyrolysis led to significantly higher CH₄ and C₂H_X yields than woody waste, as well as a higher tar quantity. It was also noticed that BTEX compounds account for 70% of the tars produced during the pyrolysis of each of the fuels that were studied at 850 °C. Findings show that the thermal decomposition of SS generates a syngas rich in H₂ (25–37 mol%), CO (23–40%), CO₂ (6–19%), CH₄ (9–21%) and C₂H_X (4–17%). The syngas lower heating value is varying between 14,000 and 22,000 kJ/Nm³.

Graphic Abstract

Effect of temperature (700 and 850 °C) on sewage sludge (SS) pyrolysis products distribution in olivine fluidized bed reactor.

Appendices

Appendices

Appendix 1

See Table 8.

Table 8 Fluidized media properties

Appendix 2

Inert nitrogen was chosen as the reference gas (tracer) for the analysis. It is introduced into the reactor with a known flow rate.

The gasification reactor performance is characterized by:

• The composition of the incondensable gas (syngas) yi, determined by micro-GC,

• The volumetric flow rate of the incondensable gases, considered at normal conditions, leaving the reactor, $\dot{V}_{G} \left( t \right)$.

• Knowing the nitrogen flow rate introduced into the reactor $\dot{V}_{N2}$ and its molar (volume) fraction $y_{N2} \left( t \right)$ determined by the chromatographic analysis, we can calculate this quantity:

$$\dot{V}_{G} \left( t \right) = \frac{{\dot{V}_{N2} }}{{(1 - y_{N2} \left( t \right))}}$$

(1)

• The volume flow rate of different incondensable gaseous species. It is calculated by:

$$\dot{V}_{i} \left( t \right) = y_{i} \left( t \right) \cdot \dot{V}_{G} \left( t \right)$$

(2)

where ${\dot{\text{V}}}_{\text{G}} \left( {\text{t}} \right)$ and ${\text{y}}_{\text{i}} \left( {\text{t}} \right)$ are the volume flow rate of syngas and the molar fraction of the compound in syngas (excluding N₂), respectively.

• The production yield of each incondensable gaseous compound, P_Gi, defined as the volume of each gaseous constituent (considered at normal conditions) produced per kg of dry and ash free fuel (daf, _B):

$${\text{P}}_{\text{Gi}} = \frac{{{\dot{\text{V}}}_{\text{i}} \left( t \right)}}{{{\text{F}}_{{{\text{daf}},{\text{B}}}} }}$$

(3)

where P_Gi is production yield of i component (Nm³/kg _daf,B), ${\dot{\text{V}}}_{\text{i}} \left( {\text{t}} \right)$ is the flow rate of noncondensable gas (Nm³/h) and ${\text{F}}_{{{\text{daf}},{\text{B}}}}$ is the mass flow rate of the fuel (daf, _B) (kg/h);

• The production yield of syngas, P_G, is defined as the syngas volume produced per kg of fuel daf, _B:

$${\text{P}}_{\text{G}} = \frac{{\mathop \sum \nolimits_{\text{i}} {\dot{\text{V}}}_{\text{i}} \left( t \right)}}{{{\text{F}}_{{{\text{daf}},{\text{B}}}} }}$$

(4)

with P_G the syngas production yield (Nm³/kg_daf,B), ${\dot{\text{V}}}_{\text{i}} \left( t \right)$ the volume flow rate of the incondensable gases (Nm³/h) and ${\text{F}}_{{{\text{daf}},{\text{B}}}}$ the mass flow rate of the fuel daf, _B (kg/h)

In order to quantify the carbon distribution into different phases, we have defined the following quantities:

• The carbon conversion rate into non-condensable gases, ${\text{X}}_{\text{cG}}$, in steady state is defined as the ratio between the carbon moles amount in the gaseous products and the carbon number of moles contained in the fuel:

$${\text{X}}_{\text{cG}} = \frac{{\mathop \sum \nolimits_{\text{i}} ({\text{N}}_{\text{i}} \cdot \aleph_{\text{i}}^{\text{C}} )}}{{{\raise0.7ex\hbox{${{\text{F}}_{{{\text{daf}},{\text{B}}}} }$} \!\mathord{\left/ {\vphantom {{{\text{F}}_{{{\text{daf}},{\text{B}}}} } {{\text{M}}_{\text{B}} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${{\text{M}}_{\text{B}} }$}}}}$$

(5)

• N_i is the molar flow rate of i constituent of syngas: $N_{i} = \frac{{{\dot{\text{V}}}_{\text{i}} \left( t \right) }}{{V_{m} }}$ and ${\dot{\text{V}}}_{\text{i}} \left( t \right)$ is the volume flow rate of incondensable gases (Nm³/h) and $V_{m}$ is the molar volume in temperature and pressure at normal conditions (= 0.0224 Nm³/mol).

• $\aleph_{\text{i}}^{\text{C}}$ is the number of carbon atoms in i component (i = CO, CO₂, CH₄, C₂H₂, C₂H₄ and C₂H₆) and M_B is the molar mass of the waste (daf, _B) (kg/mol).

• The carbon conversion rate into char ${\text{X}}_{\text{cC}}$, defined as the ratio between the number of moles of carbon in the solid residue (char) and the number of moles of carbon introduced as fuel. The char production yield obtained during the pyrolysis of fuels was determined by combustion after each experiment. One the fuel feeding is interrupted, the fluidizing gas is switched to air, and the evolution of the fumes composition versus time is recorded (CO, CO₂, N₂, O₂). By integration,, we first calculate the number of moles of carbon remaining in the fluidized bed, we can then estimate the conversion rate of carbon into char:

$${\text{X}}_{\text{cC}} = \frac{{\mathop \smallint \nolimits_{0}^{t} \mathop \sum \nolimits_{\text{i}} ({\text{N}}_{\text{i}} \cdot \aleph_{\text{i}}^{\text{C}} ){\text{dt}}}}{{{\raise0.7ex\hbox{${{\text{F}}_{{{\text{daf}},{\text{B}}}} }$} \!\mathord{\left/ {\vphantom {{{\text{F}}_{{{\text{daf}},{\text{B}}}} } {{\text{M}}_{\text{B}} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${{\text{M}}_{\text{B}} }$}}}}$$

(6)

From these results and by knowing the chemical formula of char, we calculate the mass of char produced (not converted) per kg of dry ash free fuel.

• The carbon conversion rate into tars, ${\text{X}}_{\text{cT}}$, defined as the ratio between the number of moles of carbon in the tars and the number of carbon introduced in the reactor as fuel. It is estimated by difference:

$${\text{X}}_{\text{cT}} = 1 - {\text{X}}_{\text{cG}} - {\text{X}}_{\text{cC}}$$

(7)

• The mass production yield of the pyrolysis products expressed in g/kg of fuel daf, _B is calculated as follows: ${\text{P}}_{\text{Gi}} *\frac{{M_{i} }}{{V_{m} }}$ where ${\text{P}}_{\text{Gi}}$ is the production yield of i constituent (Nm³/kg _daf,B), $M_{i}$ is the molar mass of the i constituent and $V_{m}$ is the molar volume (= 0,0224 Nm³/mol).

• The low heating value is expressed as follows:

$$LHV = \sum LHV_{i} \cdot y_{i}^{syngas}$$

(8)

$LHV_{i}$ represents the low heating value of i constituent in the syngas, and $y_{i}^{syngas}$ is its molar fraction in steady state.

• The production yield of tars and pyrolytic water, expressed in g/kg_daf,B, were determined as follows:

For a controlled flow rate and sampling time (1 h), the mass variation of the isopropanol solution (used in the tar protocol) is measured. This mass variation corresponds to the quantity of pyrolytic liquid (water and tars) captured. Knowing the flow rate of the incondensable gas, measured using a rotameter placed at the outlet of the vacuum pump, and the sampling time, we can calculate the production yield of the pyrolytic liquid in g/Nm³ of dry gas. The dry gas included nitrogen (tracer) and the syngas.

By measuring the mass fraction of the water in the sample (isopropanol + pyrolytic liquid), by Karl Fischer analysis, we can then calculate the water and tars contents in g/Nm³ of dry gas.

The measurement of the volume fraction of nitrogen in the dry gas allows us to express these condensable species quantities in g/Nm³ of dry syngas. Furthermore, they can also be represented in g/kg of fuel daf, _B by multiplying by the value of syngas production yield (Nm³/kg_daf,B). Note that the water amount measured included the water produced by pyrolysis (pyrolytic water) and that introduced by the moisture in the fuel. By subtracting the fuel moisture, we can get the real value of the pyrolytic water yield.

Appendix 3

See Figs. 19 and 20.

Fig. 19

Evolution of H₂, CO and CO₂ molar percentages in syngas versus the temperature during the pyrolysis of sewage sludge (Figure on the right: sand; figure on the left: olivine)

Fig. 20

Evolution of CH₄ and C₂H_X molar percentages in syngas versus the temperature during the pyrolysis of sewage sludge (Figure on the right: sand; figure on the left: olivine)

Appendix 4

See Table 9.

Table 9 Influence of temperature and fuel nature on tars production yields (SS700, SS850, GW850, PIN850)