Guillermo Rein

Global vulnerability of peatlands to fire and carbon loss

The amount of carbon stored in peats exceeds that stored in vegetation. A synthesis of the literature suggests that smouldering fires in peatlands could become more common as the climate warms, and release old carbon to the air.

Baseline intrinsic flammability of Earth’s ecosystems estimated from paleoatmospheric oxygen over the past 350 million years

Atmospheric oxygen (O2) is estimated to have varied greatly throughout Earth’s history and has been capable of influencing wildfire activity wherever fuel and ignition sources were present. Fires consume huge quantities of biomass in all ecosystems and play an important role in biogeochemical cycles. This means that understanding the influence of O2 on past fire activity has far-reaching consequences for the evolution of life and Earth’s biodiversity over geological timescales. We have used a strong electrical ignition source to ignite smoldering fires, and we measured their self-sustaining propagation in atmospheres of different oxygen concentrations. These data have been used to build a model that we use to estimate the baseline intrinsic flammability of Earth’s ecosystems according to variations in O2 over the past 350 million years (Ma). Our aim is to highlight times in Earth’s history when fire has been capable of influencing the Earth system. We reveal that fire activity would be greatly suppressed below 18.5% O2, entirely switched off below 16% O2, and rapidly enhanced between 19–22% O2. We show that fire activity and, therefore, its influence on the Earth system would have been high during the Carboniferous (350–300 Ma) and Cretaceous (145–65 Ma) periods; intermediate in the Permian (299–251 Ma), Late Triassic (285–201 Ma), and Jurassic (201–145 Ma) periods; and surprisingly low to lacking in the Early–Middle Triassic period between 250–240 Ma. These baseline variations in Earth’s flammability must be factored into our understanding of past vegetation, biodiversity, evolution, and biogeochemical cycles.

Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release w ...

Kinetic and Fuel Property Effects on Forward Smoldering Combustion

In this paper, we present the results from a one-dimensional transient model of forward smoldering. Fuel oxidation and pyrolysis reactions as well as a char oxidation reaction are included in the model. The solid energy, solid species, gas energy, oxygen species (bulk gas and surface), and overall mass conservation equations were discretized in space using finite-difference techniques and were solved using VODE, an ordinary differential equation integrator designed for stiff equations. Local thermal and chemical nonequilibrium are allowed in this model and transfer coefficients are derived from a Nusselt number correlation. A base case is chosen to represent experimental conditions reported in the literature. The effects of inlet gas velocity, kinetic frequency factors, inlet oxygen concentration, and fuel properties such as specific heat, density, conductivity, and pore diameter were studied using this model.

Smouldering natural fires: comparison of burning dynamics in boreal peat and Mediterranean humus

Smouldering of the forest subsurface can be responsible for a large fraction of the total fuel consumed during wildfires. Subsurface fires can take place in organic material stored in shallow forest layers such as duff or humus, and in deeper layers such as peat, landfills and coal seams. These fires play a major role in the global emission to the atmosphere, the destruction of carbon storage in the soil and the damage to the natural environment. Burning dynamics in two different ecosystems affected by smouldering wildfires are studied here; boreal peat and Mediterranean humus. A series of small-scale smouldering experiments have been conducted under laboratory conditions to study the ignition and the severity to the soil. The experimental set-up allowed the temperature and velocity of the fire front to be measured for different fuel moisture contents. The two fuels, peat and humus, were tested and the results are compared.

Self-Sustaining Smoldering Combustion for NAPL Remediation: Laboratory Evaluation of Process Sensitivity to Key Parameters

Smoldering combustion has been introduced recently as a potential remediation strategy for soil contaminated by nonaqueous phase liquids (NAPLs). Published proof-of-concept experiments demonstrated that the process can be self-sustaining (i.e., requires energy input only to start the process) and achieve essentially complete remediation of the contaminated soil. Those initial experiments indicated that the process may be applicable across a broad range of NAPLs and soils. This work presents the results of a series of bench-scale experiments that examine in detail the sensitivity of the process to a range of key parameters, including contaminant concentration, water saturation, soil type, and air flow rates for two contaminants, coal tar and crude oil. Smoldering combustion was observed to be self-sustaining in the range 28,400 to 142,000 mg/kg for coal tar and in the range 31,200 to 104,000 mg/kg for crude oil, for the base case air flux. The process remained self-sustaining and achieved effective remediation across a range of initial water concentrations (0 to 177,000 mg/kg water) despite extended ignition times and decreased temperatures and velocities of the reaction front. The process also exhibited self-sustaining and effective remediation behavior across a range of fine to coarse sand grain sizes up to a threshold maximum value between 6 mm and 10 mm. Propagation velocity is observed to be highly dependent on air flux, and smoldering was observed to be self-sustaining down to an air Darcy flux of at least 0.5 cm/s for both contaminants. The extent of remediation in these cases was determined to be at least 99.5% and 99.9% for crude oil and coal tar, respectively. Moreover, no physical evidence of contamination was detected in the treatment zone for any case where a self-sustaining reaction was achieved. Lateral heat losses to the external environment were observed to significantly affect the smoldering process at the bench scale, suggesting that the field-scale lower bounds on concentration and air flux and upper bound on grain size were not achieved; larger scale experiments and field trials where lateral heat losses are much less significant are necessary to define these process limits for the purposes of field application. This work provides valuable design data for pilot field trials of both in situ and ex situ smoldering remediation applications.

The Effect of Buoyancy on Opposed smoldering

An experimental investigation on the effects of buoyancy on opposed-flow smolder is presented. Tests were conducted on cylindrical samples of open-cell, unretarded polyurethane foams at a range of ambient pressures using the Microgravity Smoldering Combustion (MSC) experimental apparatus. The samples were tested in the opposed configuration, in which the flow of oxidizer is induced in the opposite direction of the propagation of the smolder front. These data were compared with opposed-forced-flow tests conducted aboard STS-69, STS-77, and STS-105 and their ground-based simulations. Thermal measurements were made of the smolder reaction to obtain peak reaction temperatures and smolder velocities as a function of the ambient pressure in the MSC chamber. The smolder reaction was also observed using high-frequency ultrasound pulses as part of the ultrasound imaging system (UIS). The UIS measurements were used as a second means of providing smolder propagation velocities as well as to obtain permeabilities of the reacting samples. Results of forced-flow testing in normal gravity were compared to results in microgravity at a range of ambient pressures and forced flows. Results indicate that a critical oxidizer mass flux of roughly 0.5 to 0.8 g/m2s is required in normal gravity for a self-sustaining propagation in this configuration. In microgravity tests, self-sustained smolder propagation was observed at a significantly lower oxidizer mass flux of 0.30 g/m2s. Analysis suggests that the removal of buoyancy-induced heat losses in microgravity allows for self-sustained propagation at an oxidizer mass flux below the critical value observed in normal-gravity testing. Normal-gravity tests also show that the smolder propagation velocity is linearly dependent on the total oxidizer mass flux in an oxidizer-limited regime. Pressure effects on the chemical kinetics of a smolder reaction are inferred by comparison of normal-gravity and microgravity tests and believed to be only weakly dependent on pressure (∼P 1/3).

Chapter 6 – Infrared Image Analysis as a Tool for Studying the Horizontal Smoldering Propagation of Laboratory Peat Fires

Smoldering fires in peatlands can consume large areas of peat and release important amounts of carbon to the atmosphere as they self-propagate. This chapter focuses on the use of infrared images to characterize the horizontal propagation of smoldering fires in laboratory experiments. In these laboratory experiments an infrared camera takes images of the peat surface at regular intervals during the experiment. We present methods to process and analyze these infrared images that identify the shape and position of the smoldering front, quantify the maximum energy flux, the spread rate and direction of the front and its variability to time. To demonstrate our methods we analyze images from experiments that record the smoldering of dry peats (25% moisture content, mass of water per mass of dry peat) and wet peats (100% moisture content). Infrared images are used to quantify the effect of moisture content upon the smoldering fronts. Our methods demonstrate that smoldering combustion in dry peats has a wider front (6.8 ± 1 cm for the dry peat, 2.4 ± 0.7 cm for the wet peat), a faster spread rate (4.3 ± 1 cm/h for dry peat, 2.6 ± 0.7 cm/h for wet peat), and a lower peak of radiative energy flux (7.1 ± 0.7 kW/m2 for dry peat, 10.51 ± 2.1 kW/m2 for wet peat). Our infrared image analysis is a useful tool to characterize peat fires at an experimental scale. These methods can be applied to peats with different characteristics to identify and compare smoldering propagation dynamics.

The Effect of Model Parameters on the Simulation of Fire Dynamics

Peer-reviewed article published in the Proceedings of the 9th International Symposium on Fire Safety Science, Karlsruhe, 2008.

Thermochemical conversion of biomass in smouldering combustion across scales: The roles of heterogeneous kinetics, oxygen and transport phenomena

The thermochemical conversion of biomass in smouldering combustion is investigated here by combining experiments and modeling at two scales: matter (1mg) and bench (100g) scales. Emphasis is put on the effect of oxygen (0-33vol.%) and oxidation reactions because these are poorly studied in the literature in comparison to pyrolysis. The results are obtained for peat as a representative biomass for which there is high-quality experimental data published previously. Three kinetic schemes are explored, including various steps of drying, pyrolysis and oxidation. The kinetic parameters are found using the Kissinger-Genetic Algorithm method, and then implemented in a one-dimensional model of heat and mass transfer. The predictions are validated with thermogravimetric and bench-scale experiments and then analyzed to unravel the role of heterogeneous reaction. This is the first time that the influence of oxygen on biomass smouldering is explained in terms of both chemistry and transport phenomena across scales.