Wheat Straw Open Burning: Emissions and Impact on Climate Change Wheat Straw Open Burning: Emissions and Impact on Climate Change

The state of Baja California, Mexico, is the second national wheat producer. Mexicali, the capital of Baja California, is the primary wheat producer, and it represents the most significant crop in the valley, with 90,609 ha of a cultivated surface by 2015; it leads to a wheat production of 585,334 t and a generation of 661,446 t of wheat straw as agricultural residue. The 15% of this waste has various uses. The 85% of wheat straw is open burnt in situ to prepare the farmland for the next agricultural cycle. Through the development of an emissions and energy model on iThink ® , the emissions of 6,185 t of PM, 35,983 t of CO, and 1,125 t of CH 4 considering a headfire burning or 3,373 t of PM, 30,360 t of CO, and 731 t of CH 4 by backfire burning were estimated. Also, the wheat straw wasted energy was estimated at 8.15 PJ by 2015, with a lower heating value of 14.50 MJ/kg determined experimentally. The results highlight that for each hectare of harvested wheat, 6.205 t of wheat straw are generated and burnt. It represents the emission of pollutants and 89,972.50 MJ of wasted energy.


Introduction
Agriculture is the oldest economic sector in the world, and it is more reliant on fertile soils and stable climate than any other type of trade [1]. Nowadays, wheat is one of the key cereals cultivated in the world, with an annual production of 733 million tons by 2015 [2]. In the same year, the harvested surface dedicated to wheat production was 819,928 ha in Mexico [3]. The wheat varieties Triticum aestivum and T. durum are the most common. In the fall-winter season, 90% of wheat production is obtained, and the remaining 10% in the spring-summer season. The harvest season is performed predominantly in May and June [4].
The Mexicali Valley is one of the most important agricultural areas of the northwest of Mexico, and it has one of the most extensive surfaces dedicated to wheat production nationally. This valley is located on the state of Baja California and shares the atmospheric basin with the Imperial Valley, USA (Figure 1). Its principal crop is wheat, with an average productivity of 6.46 t/ha, of the T. aestivum variety. Apart from the favorable climate conditions for this crop in the region, the use of improved varieties of a high productive potential and the experience of the producer in the application of the technological innovations for its management had been determinants to achieve this level of production [5]. After harvesting, it is necessary to dispose large amounts of straw generated as agricultural waste, with a rate of 7.3 t/ha [6]. Usually, 85% of this waste is burnt in situ in the open air with the objective of preparing the fields for double-cropping or the next agricultural cycle, and the remaining percentage has various applications [7]. Some wheat producers of the Mexicali Valley that conduct this practice argue that the burning represents a traditional practice and that the incineration of agricultural waste is necessary since it eliminates perennial weeds, diseases, and pests (Figure 2). Other producers ensure that for burning wheat straw, the use of machinery is not a requirement, saving money in machinery, diesel, and the tractor's operator and that it gives more time with the purpose of preparing the fields for the next cycle. However, contrary to the producers' assumptions, the burning calcine nitrogen, phosphorus, and the soil organic matter, as well as generating additional costs and a drop in yield and, in consequence, shrinkages on the utility in obtaining less volumes by wheat hectare between cycle and cycle [8], has been demonstrated.
The in situ burning of wheat straw implies the emissions of large quantities of PM, CO, and CH 4 that impact the environment, causing deterioration in the air quality of the Valley, including the city of Mexicali, as well as respiratory diseases for the population (Figure 3). In this sense, it is relevant to mention that Mexicali is one of the cities of Mexico with a higher level of morbidity from acute respiratory infections.
The emissions caused by the open burning of wheat straw affect the climate. Consequently, it has an impact on crop growth and yields are negatively affected by suboptimal water supply and abnormal temperatures due to physical damages, physiological disruptions, and biochemical changes [9,10]. The use of conditional promoters driving gene expression at specific developmental stages, in response to specific environmental cues, will make possible the generation of transgenic crops able to grow under various abiotic stresses with minimal yield losses [11].
Also, when the wheat straw is open burnt, the energy contained in the same is wasted. The wheat straw could be valorized and reconverted into biofuels or directly used in electric generation.
The utilization of bioenergy has significant environmental, and also economic, benefits because the biomass waste is valorized as biofuel. The use of wheat straw as raw material for any productive process presents diverse factors that must be considered. Among those factors are the low density of biomass, handling and high transportation cost, an attractive heating value, and the physicochemical characterization [12].
In this chapter, the emissions caused by the headfire or backfire burning of wheat straw T. aestivum in Baja California, Mexico, for the period 1987-2015, were estimated through the development of a model on the iThink ® dynamic simulator [13]. Also, the energy emitted by wheat straw burning was calculated considering its significant heating value of 14.50 MJ/kg determined experimentally [14], and it was included in this model.

Materials and methods
The emissions and energy associated with the agricultural burnings depend on many parameters; for that, those supported by current and reliable information were selected. The settings used to feed the model are the following:

Historical series of the wheat-harvested surface
Wheat straw is a waste generated in large quantities during wheat harvesting. To estimate its generation in the Mexicali Valley, information on the annual wheat harvested surface on the 1987-2015 period was used and is presented in Table 1 [15,16].

Wheat straw generation index and lower heating value
To estimate the quantity of wheat straw generated by agricultural cycle, a generation index of 7.3 t/ha was considered [6]. The lower heating value of the wheat straw was considered as 14.50 MJ/kg, which was experimentally determined. The tests were realized with the T. aestivum wheat variety from Baja California, Mexico [14].

PM, CO, and CH 4 emission factors by agricultural burning technique
To estimate the PM, CO, and CH 4 emissions, generated by wheat straw burnt in situ in the open air, the factors reported by the EPA AP-42 [17], enlisted in Table 2, were used. Such report clusters the emission factors according to the incineration technique used by the farmers. It is important to note that in the Mexicali Valley case, both techniques are used by producers, for which the calculations were made considering the two of them. The incinerating techniques according to the EPA are described as follows: • Headfire: Burning technique where the fire advances in the wind direction; • Backfire: Burning technique in which the fire advances to the opposite direction of the wind. Figure 4 displays the sequence and relationships between the parameters used in the emissions and energy model.

Emissions and energy model
Based on the selected parameters and with the purpose of facilitating the analysis of the emissions associated with wheat straw burning during the 1987-2015 period, a dynamic model was developed on iThink ® , whose simplified version is illustrated in Figure 5. The development of the model allows to establish and observe practically and graphically the interrelations of the different variables used to estimate the emissions corresponding to wheat straw burning and the quantity of energy generated during the combustion of the agricultural waste under study and associated emissions.

Discussion and results
The simulation results indicate that for headfire burning, the annual emissions (PM, CO, and The decrease of emission in backfire burning is due to a more significant interaction generated between the wheat straw and the oxygen present in the air because the incineration occurs against the wind which promotes the slow burning of wheat straw and better combustion. The annual average of discarded energy in the 1987-2015 period was of 6.51 PJ, which represents the 1.81% of the biomass energy in Mexico, 2015 [18]. However, the use of this wasted energy presents some challenges and opportunities that must be taken into consideration, which implies evaluating the technical and economic feasibility of any process. Figure 9 displays the matter and energy balance corresponding to one wheat hectare harvested in the Mexicali Valley, where the index of wheat production by hectare is of 6.46 t and the generation of wheat straw is 7.3 t. The 15% of wheat straw generated has many applications such as incorporation in agricultural soil, cattle food, construction material elaboration, among others.

Conclusions
Wheat cultivation is an intensive activity of great importance for the economic development of Baja California, Mexico. It also means the generation of vast amounts of wheat straw that is burnt in situ and emits large quantities of PM, CO, and CH 4 annually, contaminants that affect the air quality of Mexicali and its valley.
Since 1987 until 2015, the sown surface of wheat has incremented in the Mexicali Valley, resulting in an increase in the polluting emissions and the wasted energy.   Also, the total available energy estimated, draw from wheat straw incineration, for the 1987-2015 period is 188.81 PJ, which represents a high energy potential that can be exploited in productive processes.
Through the development of the model on iThink ® , the emissions and the wasted energy, as a result of wheat straw burning, were estimated in the period under study. It demonstrates the severity of the problem and justifies the necessity of promoting sustainable alternatives for the disposal of wheat straw, with a lower environmental impact, among the farmers of the region.