Air pollutant emissions from rice straw open field burning in India, Thailand and the Philippines

Abstract

Rice is a widely grown crop in Asia. China (30%) and India (21%) contribute to about half of the world's total rice production. In this study, three major rice-producing countries in Asia are considered, India, Thailand and the Philippines (the later two contributing 4% and 2% of the world's rice production). Rice straw is one of the main field based residues produced along with this commodity and its applications vary widely in the region. Although rice production practises vary from one country to another, open burning of straw is a common practice in these countries.

In this study, an approach was followed aiming at (a) determining the quantity of rice straw being subject to open field burning in those countries, (b) congregating pollutant specific emissions factors for rice straw burning, and (c) quantifying the resulting air pollutant emissions. Uncertainties in the results obtained as compared to a global approach are also discussed.

Introduction

As a general definition, open field burning is the burning of living and dead vegetation (Koppmann et al., 2005). Streets et al. (2003) estimated that on an annual average basis 730 Tg of biomass are being burnt in Asia out of which 250 Tg come from agricultural burning. Open field burning of crop residues is a human initiated activity to prepare the field for the next crop, remove residues, control weeds and release nutrients for the next crop cycle. The field burning of rice straw is commonly practiced in the region when there is a short duration to prepare the field for the next crop.

Field burning is a process of uncontrolled combustion during which carbon dioxide (CO₂), the principal product of the combustion, is emitted into the atmosphere along with carbon monoxide (CO), un-burnt carbon (as well as traces of methane i.e. CH₄), nitrogen oxides (NO_x) and comparatively less amount of sulphur dioxide (SO₂). In Asia, based on the results of a study from Streets et al. (2003), the annual contribution from open burning⁴ of biomass is estimated to emitting 0.37 Tg of SO₂, 2.8 Tg of NO_x, 1100 Tg of CO₂, 67 Tg of CO and 3.1 Tg of methane (CH₄). The annual contribution from crop residues burning alone is calculated to be 0.10 Tg of SO₂, 0.96 Tg of NO_x, 379 Tg of CO₂, 23 Tg of CO and 0.68 Tg of CH₄.

From the perspective of current applications, although the quantity used is not significant, rice straw is used as a fuel in domestic cook stoves especially in rural areas. In this process, control over the combustion of the biomass is approximate and leads to harmful air pollutants emissions. It is difficult to estimate the quantity of residue used, which varies largely from year to year and is therefore characterised by great uncertainty.

Part of the rice straw remaining uncollected in the field and not burnt is subsequently ploughed into the soil to serve as a fertiliser for the next crop. The rate of its anaerobic decomposition is influenced by soil moisture content or soil wetness conditions during the next crop, which directly affects the amount of CH₄ released from this process. Although soil incorporation of rice straw can provide a source of nutrients for the next crop, it has also shown to be conducive to crop diseases (Hrynchuk, 1998) and to often affect rough rice⁵ yield due to short-term negative effect of nitrogen immobilisation (Buresh and Sayre, 2007). This is one of the reasons why open field burning is often practiced for disposal of rice residues.

In recent years, it has been observed that open burning of crop residues also contributes to emissions of harmful air pollutants, which can cause severe impacts on human health, including polycyclic aromatic hydrocarbons (PAHs) (Korenaga et al., 2001), as well as polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs), referred to as dioxins (Gullett and Touati, 2003, Lin et al., 2007). These air pollutants have significant toxicological properties and are notably potential carcinogens. Air pollution not only affects human health and the environment, but also indirectly the economy of a country (Meesubkwang, 2007).

In order to contribute addressing the above mentioned issues, this paper aims at estimating the quantity of rice straw subject to open field burning in India, Thailand and the Philippines through an adequate calculation methodology and the quantitative contribution of these burning practices on air pollution using appropriate emission factors. The obtained results are compared and discussed with those reported in the literature.