Decarbonizing the Fertilizers Sector: An Alternative Pathway for Urea and Nitric Acid Production

As natural gas availability decreases and its price rises, there is a growing need to explore renewable alternatives to be used as feedstock for the industrial and chemical sectors. The pulp and paper industry, as a well-established biomass-based sector, has the potential to contribute significantly in this regard. However, this industry faces industrial challenges that make it energy-intensive and responsible for approximately 2.3% of the total annual CO₂ emissions from the global industrial sector [1]. To achieve long-term sustainable growth and enhance both efficiency and profitability, integrating biorefineries into existing facilities is a promising approach [2]. Black liquor (BL), a byproduct of the kraft pulping process, holds significant potential as a feedstock for alternative assessments, as it contains over half of the energy content in the feedstock woody biomass. The syngas resulting from the BL gasification can be either harnessed for electricity generation, or it can be employed in the synthesis of chemicals and biofuels [3].

Ammonia is a highly demanded bulk chemical and, although it can be produced from syngas; currently, almost all synthetic fertilizers production relies on fossil fuel feedstock to obtain the required hydrogen [4]. For this reason, ammonia production is responsible for approximately 2% of the global final energy consumption and contributes to 1.3% of CO₂ emissions from the industrial systems [5]. Approximately 70% of the global ammonia production is dedicated to the manufacturing of fertilizers for the agricultural sector [5]. The production of urea, a key derivative of ammonia, accounts for over 50% of the ammonia demand. Urea is extensively used in fertilizers as a nitrogen source and serves as an important material in the food and chemical industry. Urea synthesis relies on carbon dioxide, making it an interesting route for circular carbon utilization [6]. The demand for ammonia is also driven by the production of nitric acid and ammonium nitrate. Around 75–80% of nitric acid is utilized for fertilizer manufacturing [5]. Consequently, significant efforts are underway to decarbonize the ammonia production supply chain and mitigate the environmental impacts of this sector.

Ammonia production from biomass-derived syngas is emerging more and more as a promising alternative [7]. In fact, many processing steps remain similar to those used in the natural gas-based route. In a comparative study conducted by Florez-Orrego et al. [8], conventional natural gas-based ammonia production plants were compared with a range of facilities using bagasse as a resource. The exergy efficiencies of the natural gas and biomass-based ammonia production plants were found to be 65.8% and 41.3%, respectively. Additionally, the overall emission balance varied from 0.5 t_CO2/t_NH3 to −2.3 t_CO2/t_NH3 for natural gas and biomass-based processes, respectively [8]. The negative values indicated the environmental benefits associated with producing chemicals using alternative energy sources like biomass. According to the authors, the opportunity of importing electricity, if available, could also help reduce the irreversibility of the systems, thus partially offsetting the inefficiencies arising from the additional biomass energy conversion processes. In the case of biomass-based ammonia production, the largest share of indirect emissions (85–100%) comes from the biomass supply, whereas the balance belongs to electricity. Consequently, the carbon footprint in the ammonia supply chain may be influenced by factors such as on-site availability and the distance required for transportation, leading to an increase in indirect emissions. Other works proposed an ammonia and nitric acid production route based on the electricity consumption in a solid oxide electrolyzer [9] and investigated optimal designs in various power supply scenarios with explicit consideration of process dynamics. According to the authors, the incorporation of nitric acid production into renewable power generation improves the operating condition of the solid oxide electrolyzer. The stack utilization increased by 30% and the specific production cost is reduced by 3%. Some works [10] have reviewed the green pathways for urea synthesis in countries with vast renewable resources. The consensus is that the skyrocketing urea prices, due to increasing shipping costs and international geopolitical tensions, may favorably boost the sustainable urea production goals. It may also initiate new business models and diversify the energy resources for those commodities. In fact, given the current and future trends of renewable and green hydrogen cost reduction due to the incentive mechanisms and technical maturity [11], a production cost of much less than US$1000 per ton of green urea could be achieved. Notwithstanding, in view of the reduced efficiency of the biomass-based urea production route, completely green urea production is still a long way ahead from commercial application due to the higher levelized cost [12]. Accordingly, it is necessary to perform techno-economic analyses that include the uncertainty about the cost of the commodities produced and the feedstock consumed. Additionally, an extended exergy consumption analysis, including the direct and indirect, and net and overall emissions balance is necessary to help decision-makers to determine the tradeoff between biomass and natural gas-based fertilizer production routes.

Thus, this work assesses the production of nitrogen fertilizers using syngas derived from the gasification of black liquor, a byproduct of the kraft pulping process. The syngas is used to produce ammonia, which can be further used for synthesizing urea or nitric acid. The performance of these integrated plants is compared to that of conventional kraft mills in terms of thermodynamics and economic indicators, and environmental impact. An optimization framework is implemented to meet the power and heating requirements of the increased utility plant, while minimizing operating costs (OPEX). The extended exergy method, which considers inefficiencies in the upstream supply chains of different energy inputs, is also taken into account. Finally, the novelty of this study also relies on the implementation of an analysis based on the carbon taxes and market uncertainties, and discusses the implications for the decarbonization of nitrogen fertilizers.

Next, a detailed description of the modeled energy systems is presented, and the environmental and thermodynamic performance indicators are defined. The optimization methodology, used to maximize the plant revenues while maximizing the waste heat and mass recovery, is also discussed.

This section describes the modeling and simulation approach used for performing the integration of urea and nitric acid into a kraft pulp mill via upgraded black liquor gasification.

Figure 1 illustrates the integrated flowsheet for pulp and ammonia production via black liquor gasification. This process is modeled in the aspen plus^® software and the detailed description of this integrated system has been thoroughly discussed in Refs. [13–16].

The power and steam demands for the kraft mill are adapted from Refs. [17,18] to produce around 880 air-dried tons of pulp per day. A pulp yield of 46.51% with respect to the total amount of digested biomass is considered [19]. The respective production of black liquor is 1.44 t_BL/t_pulp. The ultimate and proximate analyses for the substance composition, as well as the lower heating values (LHV) of the main inputs and outputs of this process, are reported in Tables 1 and 2, respectively.

First, the weak black liquor coming from the kraft process is dried. In the conventional process, multiple effect evaporators (MEEs) are used, usually with four to six effects [17]. This system has a larger steam consumption, offering the possibility of reducing the costs by lowering the energy requirements [27]. In this regard, Darmawan et al. [28] proposed a mechanical vapor recompression (MVR) system that replaces steam input for BL concentration by an electricity input. In the present study, the conventional multiple-effect evaporator system is also replaced by an MVR system.

Urea synthesis is commercially based on the Basarov reaction (Eq. (2)), which occurs between 125–250 bar and 170–220 °C in the form of two sequential reactions, namely the formation of ammonium carbamate (NH₂COONH₄) (Eq. (3)) and the dehydration of ammonium carbamate, in order to generate urea (CO(NH₂)₂) (Eq. (4)). The ammonium carbamate formation is highly exothermic and fast, and the chemical equilibrium is reached under the operating conditions in the reactor. On the other hand, the dehydration of ammonium carbamate is endothermic and its rate is slow and equilibrium is usually not reached in the reactor. The SR-POLAR thermodynamic model is used for simulating the urea conversion unit (Fig. 2), which is suitable for high-pressure (HP), high-temperature operating conditions. The details of the simulation approach for the urea model are described in Ref. [30].

The CO₂ coming from the ammonia purification unit shown in Fig. 1 is compressed to 141 bar and fed to a CO₂ stripper to contact the urea solution coming from the reactor. In the stripper, ammonium carbamate decomposes, releasing more NH₃ and CO₂ to be stripped out. The outlet liquid solution from the stripper is rich in urea and goes to the low-pressure (LP) urea purification section, obtaining an aqueous urea solution of 77 wt%. The mixture from the top of the urea purification unit is sent to the LP carbamate condenser to compound ammonium carbamate, which is further cooled before it passes through an HP ammonium carbamate circulating pump. The mixture follows to a scrubber where the off-gas is separated and eventually, the reactants go back to the urea reactor to complete the loop.

In the adiabatic urea reactor, an aqueous solution of NH₃ and CO₂, mostly in the form of ammonium carbamate, together with light components, flows upward through the reactor volume to minimize back-mixing and provide sufficient residence time for urea formation. In the reactor, the condensation of the remaining gases occurs along with the decomposition of the carbamate to provide heat for the slightly endothermic conversion reaction of carbamate into urea. The urea solution goes back to the CO₂ stripper, while the unreacted gases follow to the scrubber. In the latter, the recycled carbamate solution absorbs the unreacted NH₃ and CO₂, while inert gases are purged in the off-gas stream. The solution at the bottom of the scrubber is next mixed with the top vapor stream from the CO₂ stripper together with the ammonia feed stream.

Subject to

N is the number of intervals of temperature depending on the supply and target temperatures of the various streams;

Q is the heat exchanged between the process streams (Q_i,r > 0 for the hot stream, Q_i,r < 0 for the cold stream, in kW);

R is the heat flowrate cascaded from higher (r + 1) to lower (r) intervals of temperature (kW);

N_w is the number of units available in the utility systems;

B is the exergy flowrate of the mass and energy streams (kW);

q is the cooling or heating flowrate supplied by the utility systems (kW);

W is the power produced by the utility systems, chemical processes, or exported to/imported from the grid (kW).

The prices of feedstock, consumed/produced electricity, or marketable products (i.e., pulp, nitric acid, or urea) and byproducts (e.g., CO₂) are defined in Table 3 (EUR per kiloWatt hour or per kilogram).

In this section, the performance indicators proposed to compare the conventional kraft pulp mill (multiple effect evaporation and recovery boiler) with the BL integrated gasification systems (i.e., pulp and urea or pulp and nitric acid production plants) are defined.

The relative exergy efficiency indicator is defined in Eq. (13), and it quantifies the deviation from the minimum theoretical exergy consumption necessary to make up the main industrial products, i.e., pulp and chemical fertilizers.

The cash flow calculations are based on a plant lifetime of 20 years. The total CAPEX is divided between the first year (60%) and the second year (40%), with an overhead of 6% allocated to decommissioning costs. The OPEX are determined using the methodology recommended in Ref. [43]. For the kraft pulp mill, the OPEX is assumed to be 4% of the CAPEX, as suggested in Ref. [48]. To account for the associated risk level of the technologies involved, a contingency cost increment of 20% is assumed [43].

When considering investments in new technologies, their attractiveness is determined by the advantages they offer compared to conventional setups. In order to assess the incremental financial impact and identify the conditions under which these gains are most likely to be achieved, an incremental financial analysis is conducted. This analysis utilizes various financial indicators, such as the incremental net present value (INPV), which is calculated using Eq. (19):

A sensitivity analysis was conducted to examine the impact of carbon taxes (0–100 EUR/t_CO2) and interest rates (0–21%) on the INPV. This analysis aims to explore the higher risk perception associated with new technologies and accounts for stricter environmental regulations.

In a preliminary analysis, it is assumed that the costs of feedstock and products remain constant. Subsequently, the incremental financial analysis incorporates the uncertainty associated with the costs; thus, simulating the effects of a volatile market. The Monte Carlo method was employed to simulate the stochastic variation of the commodity prices, assuming normal distributions with mean prices (see Table 3) and a standard deviation of 30%.

By considering the lifetime of the integrated pulp and chemical plants, the INPV could be estimated, taking into account the probability of achieving a negative value, referred to as the “likelihood of loss.” The “likelihood of loss” is influenced by several factors, including interest rates, market price fluctuations, and carbon taxation. These factors represent risk perception, government subsidies, technology readiness levels, and the impact of inflation over time. For the sake of comparison, three scenarios are evaluated in this study, and the assumptions adopted for the variations in carbon taxes, interest rates, and commodity prices in each scenario are summarized in Table 5.

In this section, the performance of the conventional kraft pulp mill with multiple effect evaporators is compared with those of the proposed integrated pulp and chemical plants (nitric acid or urea co-production). The results of the extended exergy analysis are used to demonstrate the effect of biomass conversion integration on the overall performance of the energy systems. Finally, the uncertainty about the prices of the commodities and the cost of the feedstock is analyzed in the light of economic incremental indicators.

Table 6 summarizes the exergy consumption remarks of the conventional kraft mill and the integrated plants producing either nitric acid or urea. In this table, the mixed or autonomous operation modes refer to two cases in which there are either intensive electricity or biomass imports, respectively. Thus, in the second operating mode, the cogeneration system of the integrated pulp and chemical production plants is oversized. As can be seen, the overall exergy consumption of the integrated pulp and chemical production plants increases by around 5–28%, compared to the conventional kraft pulp mill. This increase can be attributed to the additional requirements resulting from the energy integration of the alternative facilities into the existing pulping process. In addition, the integrated production routes with nitric acid or urea plants exhibit higher exergy consumption (16–21%) if an extended exergy consumption analysis is considered. This analysis is based on the reported exergy efficiency of electricity generation (55.68%) by the Brazilian electricity grid, as well as the reported exergy efficiencies of the supply chains of oil (95.20%) and biomass (86.13%) [30,31].

In the case of the alternative production routes with nitric acid and urea production, the heating requirements are substantially lower, compared to the standalone kraft pulp mill based on the conventional multiple-effect evaporation system. This fact is a consequence of the enhanced energy integration and the capitalization on the waste heat available in the chemical production steps. In this regard, the cooling requirements also increase in the case of the nitric acid-based solution, which could be further used to supply the heat to a nearby district heating. The marketable CO₂ produced could be also regarded as a source of income for the integrated chemical complexes, with the twofold advantage of its high purity (i.e., coming from the syngas purification unit) and its biogenic origin. This available CO₂ could be even used in CO₂ management systems to store renewable energy from solar [49,50] or wind energy after reacting electrolysis-derived hydrogen with the CO₂ to produce synthetic natural gas [51].

The integrated composite curves of the conventional and the integrated plants are presented in Fig. 4. In the conventional case (i.e., standalone kraft pulp mill based on multiple effect evaporators), the heating requirements are fully supplied by the combustion of the residual bark and strong black liquor in the recovery boiler. This renders the pulp production system self-sufficient in terms of both steam and power demands, and it even has the potential to export excess electricity. Figures 4(a)–4(e) show the alterations in the integrated composite curves after the installation of an MVR unit. The superheated steam raised using an MVR unit serves the purpose of concentrating the black liquor, leading to a relatively larger extent of heat exchange, at the expense of the consumption of only a fraction of power (due to a high MVR COP >= 10).

Looking at the integrated curves of the integrated chemical production plants (Figs. 4(b)–4(e)), the integrated curves change their shape as soon as the plant is operating either under the mixed mode (i.e., more electricity import) or autonomous mode (i.e., no electricity import). For the autonomous operating modes, a large amount of chips must be burned to balance the energy requirements, resulting in an increased amount of exergy destruction. On the other hand, in the mixed operating modes, the waste heat recovery is maximized due to the import of most of the power consumed in the plant, leading to a reduction of the irreversibility. Clearly, this is a consequence of the features of the Brazilian electricity mix; thus, it cannot be readily extrapolated to other geographical scenarios. In any case, this is a proof of concept about the need for increasing the efficiency of the energy conversion systems that supply electricity, so that the overall performance of the other economic sectors can be improved.

Moreover, it can be observed from Fig. 5, that there is a significant reduction of the plant-wide exergy efficiency for all the studied scenarios (14% up to 18%) when the extended exergy efficiencies are considered. This is a consequence of the inclusion of additional sources of irreversibility to import biomass and electricity at the gate of the plant.

Additionally, Fig. 6 evidences that the integrated pulp and chemicals production configurations have higher operating revenues, thanks to the value-added products. Even with a higher yearly investment cost of the integrated pulp and fertilizers plants, their net plant revenues are more attractive compared to the conventional kraft pulp mill.

The direct and indirect CO₂ emissions (i.e., related to the supply chains of the electricity, oil, and biomass imports), as well as the overall and net CO₂ emissions balance, are shown in Fig. 7. The significant indirect contributions to the overall and net atmospheric CO₂ emissions bring to light the environmental issues that may go unnoticed if the imported electricity or biomass is considered as virtually renewable, emission-free inputs. In fact, the large amount of biomass imported, mainly in the autonomous operating mode, proportionally increases the indirect CO₂ emissions produced. The avoided CO₂ emissions are related to the carbon capture system in the syngas purification section of the ammonia production plant. For the integrated pulp and urea facility, part of this CO₂ captured is used to synthesize urea; thus, impacting the overall CO₂ emissions balance. Clearly, as soon as the urea is used in the soil, it releases the CO₂ back to the atmosphere. On the other hand, negative CO₂ emissions in the nitric acid production represent an opportunity for issuing carbon credits and, thus, increasing the financial attractiveness of the integrated chemical system.

If the direct biomass-derived emissions are considered as neutral (i.e., circular) emissions, all the integrated setups achieve net negative CO₂ emissions values: between −1.08 t_CO2/t _PULP and −1.09 t_CO2/t _PULP for the nitric acid co-production plant operating under the mixed and autonomous modes, respectively; and −0.70 t_CO2/t_Pulp and −0.72 t_CO2/t_Pulp for the urea co-production plant operating under the mixed and autonomous modes, respectively. The negative values indicate an overall positive impact toward the depletion of the CO₂ present in the atmosphere.

According to Fig. 8, an extensive import from a highly renewable electricity mix brings about positive values of INPV regardless of the carbon tax and the chemical production route. The economic results for the pulp and urea co-production route are shown in Fig. 8(a), while plots presented in Fig. 8(c) are for the pulp and nitric acid co-production routes. These results contrast with the performance of the co-production routes based on the autonomous operation mode, in which the inefficient combined heat and power generation in the cogeneration plants (without electricity import) may lead to conditions of negative INPV, especially for low carbon taxes and high-interest rates. Meanwhile, in the case of the autonomous operating mode, interest rates above 13% do not favor any more positive INPVs. Certainly, carbon pricing could be a measure for pushing the economic transformation and improve the waste recovery, but the effects of its application must be carefully evaluated as it can have a strong effect on the assets and their economic feasibility [52].

The heat maps given in Tables 7–10 summarize the results for the scenario (i) DCTIR_SC defined in Table 5. For the urea (Tables 7 and 8) and the nitric acid (Tables 9 and 10) co-production routes, the results suggest favorable scenarios for both operation modes (mixed and autonomous), as positive INPVs are likely probably to be obtained for almost all the range of carbon taxes evaluated, but still depending on the interest rates.

Meanwhile, the heat maps shown in Tables 11 and 12 summarize the results for scenarios (ii) LCT_DIR_SC and (iii) SCTC_DIR, respectively, defined in Table 5. As it can be seen, the integrated pulp and chemical fertilizers plants show more favorable results in terms of likelihood of loss by operating under the mixed operation mode. This fact advocates for the fertilizers co-production in scenarios with a diversified energy matrix based on renewable resources, with electricity mixes mainly relying on renewable resources. Undoubtedly, carbon pricing along with broader fiscal reforms will be necessary to drive a sustainable transition that could lead to the achievement of short-term and long-term net zero goals [52].

In this work, the advantage of the energy integration of a kraft pulp mill and fertilizers production plants (i.e., nitric acid or urea) via black liquor gasification process is compared to the performance of a conventional kraft pulp mill. The accounting of the extended exergy consumption and the extended exergy efficiency of the upstream supply chains allowed to assess the cradle-to-gate impact of the integrated pulp and chemical production routes. An increase between 16% and 21% in overall exergy consumption was observed when these supply chain inefficiencies are considered. The integrated composite curves of the integrated pulp and chemical plants were compared to the curves of the standalone conventional kraft pulp mill, which suggested potential improvements and also challenges in terms of exergy destruction and waste heat recovery. Two different operating modes (mixed and autonomous modes) can impact waste heat recovery. The overall emissions balance was found to be as much as 1.97 t_CO2/t_Pulp for the conventional kraft pulp mill; whereas for the integrated pulp and urea or pulp and nitric acid co-production plants, this balance varied from −0.79 t_CO2/t_Pulp to 0.60 t_CO2/t_Pulp. Despite the increased investment cost, the operating revenues of the alternative configurations are not considerably impacted in comparison with the conventional kraft pulp mill, due to the commercialization of the newly produced value-added CO₂ and fertilizers. In any case, the results proved to be strongly dependent on the indirect emissions, the energy resources consumed (e.g., chips or electricity) and the market prices adopted for the resources and the products of the integrated plants. Finally, the incremental financial analysis under uncertainty of the feedstock costs and carbon taxes allowed to understand the behavior of the integrated systems proposed by considering different market fluctuations. As a result, for the nitric acid and the urea production, positive INPVs are likely in most of the range of carbon taxes considered, provided that a mixed import of biomass and electricity is accomplished. In fact, even when the systems are subject to linear-increasing or stochastic carbon taxes, the scenarios operating under the mixed mode (intensive electricity import) showed a larger potential for decarbonizing the pulp and fertilizer sector with a more robust economic performance.

The first and second authors thank Suisse Energie for the funding of the project “Continuous training program: process engineering for decarbonization of the Swiss industry.” The first author thanks the Brazilian National Research Council for Scientific and Technological Development, CNPq (Grant 142148/2019-9) and the Swiss Government Excellence Scholarship (Ref. 2021.0235). The second author would like to thank the Colombian Administrative Department of Science, Technology and Innovation—MinCiencias (COLCIENCIAS Grant 1.128.416.066-646/2014), and the Swiss Federal Office of Energy, Grant SI/502336-01 “Integration of High Temperature Heat Pumps in Swiss Industrial Processes.”

There are no conflicts of interest.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.

f =

load factor

q =

heating or cooling duty from utility systems (kW)

r =

indirect CO₂ emissions factor (⁠$gCO2$/kJ_fuel)

y =

existence optimization factor

B =

specific chemical exergy (kJ/kg)

B =

chemical exergy (kW)

C =

carbon mass fraction (%)

H =

hydrogen mass fraction (%)

M =

molecular weight (kg/kmol)

N =

number of utility unit or temperature intervals

R =

cascaded heat (kW)

T =

temperature (°C)

W =

power (kW)

FC =

fixed carbon (%)

LHV =

lower heating value (MJ/kg)

 φ =

ratio of the chemical exergy to the lower heating value

ω =

process or utility unit