Summary of Complete Report for USEPA (Nov. 1, 1998)

GREENHOUSE GASES FROM SMALL-SCALE COMBUSTION DEVICES IN DEVELOPING COUNTRIES: Phase III: Charcoal Kilns in Thailand

by

Kirk R. Smith, David M. Pennise, Pojanie Khummongkol, Junfeng Zhang, Winai Panyathanya, R.A. Rasmussen, and M.A.K. Khalil

The charcoal kiln measurements described in this report took place at the Charcoal Research Centre (CRC) in Saraburi, Thailand. This work is part of a multi-year international study of the greenhouse-gas implications of small-scale combustion devices in developing countries. Although individually small, these devices are so numerous and their emission factors per unit output are so significant that, in total, they can have an appreciable influence on global and national inventories of important greenhouse gases.

Reported here is the first phase of a project designed to characterize emissions from the most common charcoal kilns in the developing world. Thailand is the Asian country with the largest charcoal production. Although challenging enough, conditions at the CRC were more controllable than would be the case at a commercial kiln operating in the forest. Thus, it provides a good location for developing and validating methods to be used later in Africa and Latin America.

Introduction

Biomass burning plays important roles in the global carbon cycle. Although complete combustion of biomass produces little more than CO₂ and water, most actual combustion is done in circumstances that result in substantial diversion of biomass carbon into products of incomplete combustion (PIC). After CO₂, the PIC, CH₄, is the most important greenhouse gas (GHG). The other major PIC, CO and TNMOC, indirectly affect global warming through atmospheric chemical reactions that in turn affect GHG levels. A good characterization of biomass burning thus is important for achieving scientific understanding of the potential for human activities to engender global warming, as well as informing the international political/economic discourses about what GHG mitigation measures are warranted and who should pay for them.

Combustion of biomass harvested or naturally regrown on a sustainable basis does not cause a net increase of CO₂ in the atmosphere. Unfortunately, through deforestation and other non-renewable practices, much burned biomass is not replaced. Even with complete recycling of the carbon, however, a biomass fuel cycle can produce a net increase in global warming commitment (GWC) because of the emitted PIC, which have, on average, a higher global warming potential (GWP) per kilogram carbon than CO₂.

Most biomass combustion, whether natural or anthropogenic, is done in circumstances in which access to air is not greatly restricted. Indeed, for most human uses, better combustion efficiency is a distinct advantage. Thus, although not perfect, combustion efficiency is normally relatively high, i.e., only a few percent of fuel carbon is diverted to PIC. There is at least one major exception to this pattern, however, charcoal production, which is done basically by heating the fuelwood in the absence of air, thus creating a higher quality fuel, but at the expense not only of a significant loss of energy in the starting fuelwood, but also significant production of PIC.

Because of its large PIC production, it might be expected that charcoal’s impact on GWC is substantially greater than its share of fuel demand. To date, however, the airborne emissions from charcoal making are poorly characterized in existing greenhouse gas emission databases.

The charcoal kilns used in the developing world are not easily monitored, because they often operate in remote areas over many days or even weeks for a single run. Emissions vary dramatically over the run and may be released from a number of locations on the kiln, which can be nothing more than a carefully constructed mound of earth many meters long.

Airborne emissions were measured during typical operating conditions. The kilns tested were of five types: brick beehive, mud beehive, earth mound, rice husk mound, and single (oil) drum, which are typical of those used in Thailand and elsewhere in the developing world. Emission factors for the production of charcoal were determined for the direct greenhouse gases carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), the indirect greenhouse gases carbon monoxide (CO) and total non-methane hydrocarbons (TNMHC), as well as total suspended particulates (TSP). Charcoal production efficiency (yield) and charcoal and fuelwood composition were determined as well. The data here are the averages of three separate runs for each kiln type.

RESULTS

As is generally known to be the case for charcoal making, the conversion of wood carbon to charcoal carbon was fairly inefficient, ranging from a low of 48% for the earth mound kilns to a maximum of 57% for the more efficient brick beehive kilns (Table 1). Average emission factors, expressed as grams of pollutant per kilogram of charcoal produced, for the three runs of each of the five kiln types ranged from 970-1600 for CO₂, 13-58 for CH₄, 110-340 for CO, 9-95 for TNMHC, 0.017-0.084 for N₂O, and from 0.7-4.2 for TSP. Hence, a substantial fraction of the original fuel carbon was lost as CO₂ and other products of incomplete combustion (PIC). On average, fuelwood carbon is approximately diverted as follows: 52% to charcoal, 24% to CO₂, 10% to PIC, and the remainder as ash, aerosol, and brands (partially carbonized solids). Put another way, even if the charcoal were burned with absolutely no PIC production during its final enduse, nearly one-fifth of the carbon available in the fuelwood has already been released as PIC during charcoal production.

Based on published Global Warming Potential (GWP using a 20-year time horizon) for CH₄ & N₂O only, we estimate that 0.65-1.41 kg C-CO₂ (carbon as carbon dioxide equivalents) is emitted per kg charcoal produced.

One way of looking at the greenhouse-gas implications of charcoal-making can be seen in Figure 1 for the Brick Beehive kiln. Here, two types of global warming commitments (GWC) are applied. The first, called "primary GWC" assumes GWPs only for CO₂ CH₄, and N₂O. The second, "total GWC," applies GWPs also for CO and hydrocarbons, which are less certain. For each, the GWC is calculated under two extreme assumptions: that the kilns rely on a completely renewable wood supply, i.e. all carbon is recycled back into trees, compared to the case in which there is complete deforestation, i.e., no carbon recycling to biomass. In the latter case, of course, the GWC includes the full complement from CO₂ (GWP=1.0). In the former, no contribution from CO₂ is included.

For all the kilns, if the wood is harvested non-renewably (the carbon is not recycled), kiln GWCs vary from 1.9-3.6 times the GWC of the charcoal burned at the stove, assuming that the stove charcoal is completely converted to CO₂. The GWC of a non-renewable charcoal fuel cycle, therefore, is some 3.9-6.2 times that of a fossil fuel cycle producing the same energy (assuming that the fossil-fuel GWC previous to final combustion is 10% of the total for the fossil-fuel cycle). Even if the wood is harvested renewably (complete carbon recycling), the GWC of the charcoal fuel cycle is still some 2-4 times greater than produced by burning an equivalent energy content of liquid or gaseous fossil fuel. (These calculations account for the GWP of CH₄, CO₂, CO and HC. If GWPs of only the first two are counted, the values decrease by about 50%.)

This implies that charcoal fuel cycles are among the most greenhouse-gas-intensive in the world. A natural gas fuel cycle, for example, would have to leak directly into the atmosphere some 12-25% of the CH₄ it delivered for combustion to emit as much GHG per unit delivered energy as a non-renewable charcoal fuel cycle.

These emission factors can be applied to other areas of the world where similar charcoal making methods are used. This will allow for somewhat better global estimates of the inventory of greenhouse gas and air pollutant emissions from the production of charcoal. More localized emissions sampling is necessary, however, for accurate determination of emission factors for two reasons. First, even given the same kiln type, there is great variability globally in both kiln sizes and construction techniques. Second, kiln behavior is largely dependent on operator tending methods, which again vary greatly around the world and can even vary within the same operator across different charcoal making sessions.

Comparison with IPCC Default Values

Table 1 also lists the IPCC default world-average values for CO, CH₄, and hydrocarbons emissions from charcoal kilns for doing GHG inventories where there is no local information. Note the relatively low conversion efficiency assumed (20%), which is only some 60% of the values found in our studies. Nevertheless, the default emissions per kg charcoal produced are within the ranges found in this study, being closest to those for the brick beehive kiln.

Table 1. Summary of the charcoal-making experiments (all values are the averages of three runs for each kiln type)

^a dry basis yield = charcoal mass / dry wood mass

^b carbon yield = charcoal carbon mass / wood carbon mass

^c EF = emission factor, in grams of pollutant per kilogram of charcoal produced

^d PIC = products of incomplete combustion = CO + CH₄ + THMHC + TSP

^e gases +TSP = CO₂ + CO + CH₄ + THMHC + TSP

^f Intergovernmental Panel on Climate Change Guidelines for Greenhouse Gas Inventories, 1997. The IPCC default deficiency of 17% is converted here to a dry basis efficiency.

Index