CEMS vs PEMS: Choosing the Right Emissions Monitoring System

Choosing between **CEMS** and **PEMS** is one of the highest-impact decisions for a Saudi industrial plant. The wrong choice can lead to **non-compliance with NCEC and RCER-2025**, oversized capex, or unreliable data that fails QAL2 audits. [image:1] ## What is a CEMS? A **CEMS** uses physical analyzers (NDIR, FTIR, chemiluminescence) to measure stack gas concentrations in real time. Includes sample probe, conditioning, gas analyzers (NOx, SO₂, CO, O₂), DAHS, and calibration gases. Recognised as the **reference method** under NCEC, RCER, US EPA, and EU IED. ## What is a PEMS? A **PEMS** is a software model that predicts emissions from process variables (fuel flow, air-fuel ratio, ambient T/P). Reduces hardware but requires **rigorous validation**. [image:2] ## Side-by-Side Comparison ### Regulatory Acceptance - **CEMS** — accepted for all categories, mandatory under RCER-2025 for many sources. - **PEMS** — accepted for **gas turbines, boilers, reciprocating engines**; **not accepted** for cement kilns, refineries' main stacks. ### Accuracy - CEMS: ±2 to 5% under QAL2 - PEMS: ±10% for NOx on well-modelled gas turbines ### Capex (USD) - CEMS: $250k to $600k per stack - PEMS: $80k to $180k per source ### Opex (annual) - CEMS: $40k to $90k - PEMS: $15k to $35k [callout: PEMS is not "cheaper" by default. 10-year TCO narrows when including model revalidation, sensor drift, and risk of regulatory rejection.] ## When to Choose CEMS - **Cement kilns, refinery main stacks, incinerators** - Sources >100 MWth - Variable fuel sources - ISO 14001, IED, World Bank financing ## When to Choose PEMS - Mid-size **gas turbines and boilers** with stable operation - Severe space constraints - Backup or redundant monitoring [image:3] ## QAL2 and QAL3 — Make-or-Break Tests Data must pass **QAL2 (initial validation)** and **QAL3 (drift checks)** under EN 14181. OzoneCo runs ~60 QAL2 campaigns per year. Top three failures: 1. Sample line condensation causing low NOx 2. Non-representative probe locations 3. Span gas drift from incorrect cylinder storage [video:https://www.youtube.com/watch?v=x1SgmFa0r04] ## How OzoneCo Helps You Get Monitoring Right OzoneCo is one of Saudi Arabia's leading suppliers of stack monitoring solutions, partnered with ABB, SICK, Siemens, Yokogawa, Emerson: - **Feasibility and TCO study** for CEMS vs PEMS - **Engineering, supply, installation, commissioning** - **QAL1, QAL2, annual QAL3** by accredited lab - **DAHS and eRCER / NCEC telemetry** - **AMC** with 24/7 SLA - **Operator training** and audit support [callout: OzoneCo CEMS installations have maintained an average **>97% data availability** across 200+ stacks — well above the 90% RCER-2025 threshold.]